BGP Configuration Guide for Cisco 8000 Series Routers, IOS XR Release 7.10.x
Bias-Free Language
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Border Gateway Protocol (BGP) is an Exterior Gateway Protocol (EGP) that allows you to create loop-free interdomain routing
between autonomous systems. An autonomous system is a set of routers under a single technical administration. Routers in an autonomous system can use multiple Interior Gateway
Protocols (IGPs) to exchange routing information inside the autonomous system and an EGP to route packets outside the autonomous
system.
This module provides conceptual and configuration information on BGP.
Tip
You can programmatically configure BGP and retrieve operational data using openconfig-network-instance.yang OpenConfig data model. To get started with using data models, see the
Programmability Configuration Guide for Cisco 8000 Series Routers.
Prerequisites for Implementing BGP
You must be in a user group associated with a task group that includes the proper task IDs. The command reference guides include
the task IDs required for each command. If you suspect user group assignment is preventing you from using a command, contact
your AAA administrator for assistance.
The current Internet BGP table contains approximately 1.1 million IPv4 routes and 200,000 IPv6 routes. With an average of
two paths per route, the BGP process typically requires around 5.5 GB of RAM to manage the full Internet BGP table. As the
IPv6 Internet table continues to expand, the memory requirements for the BGP process are expected to increase. Therefore,
Cisco recommends using the Service Edge (SE) version of Route Processor (RP) or Route Switch Processor (RSP) cards, or fixed
chassis, on routers that will maintain a full BGP table
BGP Functional Overview
BGP uses TCP as its transport protocol. Two BGP routers form a TCP connection between one another (peer routers) and exchange
messages to open and confirm the connection parameters.
BGP routers exchange network reachability information. This information is mainly an indication of the full paths (BGP autonomous
system numbers) that a route should take to reach the destination network. This information helps construct a graph that shows
which autonomous systems are loop free and where routing policies can be applied to enforce restrictions on routing behavior.
Any two routers forming a TCP connection to exchange BGP routing information are called peers or neighbors. BGP peers initially
exchange their full BGP routing tables. After this exchange, incremental updates are sent as the routing table changes. BGP
keeps a version number of the BGP table, which is the same for all of its BGP peers. The version number changes whenever BGP
updates the table due to routing information changes. Keepalive packets are sent to ensure that the connection is alive between
the BGP peers and notification packets are sent in response to error or special conditions.
Note
ASN change for BGP process is not currently supported via commit replace.
BGP Router Identifier
For BGP sessions between neighbors to be established, BGP must be assigned a router ID. The router ID is sent to BGP peers
in the OPEN message when a BGP session is established.
BGP attempts to obtain a router ID in the following ways (in order of preference):
By means of the address configured using the bgp router-id command in router configuration mode.
By using the highest IPv4 address on a loopback interface in the system if the router is booted with saved loopback address
configuration.
By using the primary IPv4 address of the first loopback address that gets configured if there are not any in the saved configuration.
If none of these methods for obtaining a router ID succeeds, BGP does not have a router ID and cannot establish any peering
sessions with BGP neighbors. In such an instance, an error message is entered in the system log, and the show bgp summary command displays a router ID of 0.0.0.0.
After BGP has obtained a router ID, it continues to use it even if a better router ID becomes available. This usage avoids
unnecessary flapping for all BGP sessions. However, if the router ID currently in use becomes invalid (because the interface
goes down or its configuration is changed), BGP selects a new router ID (using the rules described) and all established peering
sessions are reset.
Note
We strongly recommend that the bgp router-id command is configured to prevent unnecessary changes to the router ID (and consequent flapping of BGP sessions).
BGP Route Distinguisher
In network design solutions where customer equipment is dual-homed and Fast Reroute is required, such as in EVPN and BGP PIC
Edge solutions, the Route Distinguisher (RD) associated with each VRF must be unique per Provider Edge (PE) router. In other
design scenarios, while it isn’t mandatory for the RD to be unique per PE, it is highly recommended to make it unique. This
practice facilitates easier transitions to dual-homed solutions in the future.
There are few available options to keep unique RD per device:
Manual configuration: You must manually assign a unique value per device in the network. For example, in this scenario:
Leaf (ToR) = RD 1
Edge DCI Gateway = RD 2
Remote PE = RD 3
Use rd auto command under VRF. To assign a unique route distinguisher for each router, you must ensure that each router has a unique
BGP router-id. If so, the rd auto command assigns a Type 1 route distinguisher to the VRF using the following format: ip-address:number. The IP address is specified by the BGP router-id statement and the number (which is derived as an unused index in the 0
to 65535 range) is unique across the VRFs.
Note
In a DCI deployment, for route re-originate with stitching-rt for a particular VRF, using the same Route Distinguisher (RD)
between edge DCI gateway and MPLS-VPN PE or same RD between edge DCI gateway and Leaf (ToR) is not supported.
BGP Maximum Prefix - Discard Extra Paths
IOS XR BGP maximum-prefix feature imposes a maximum limit on the number of prefixes that are received from a neighbor for
a given address family. Whenever the number of prefixes received exceeds the maximum number configured, the BGP session is
terminated, which is the default behavior, after sending a cease notification to the neighbor. The session is down until a
manual clear is performed by the user. The session can be resumed by using the clear bgp command. It is possible to configure a period after which the session can be automatically brought up by using the maximum-prefix command with the restart keyword. The maximum prefix limit can be configured by the user.
Note
Starting IOS-XR Release 7.3.1, the router does not apply default limits if the user does not configure the maximum number
of prefixes for the address family.
Discard Extra Paths
An option to discard extra paths is added to the maximum-prefix configuration. Configuring the discard extra paths option
drops all excess prefixes received from the neighbor when the prefixes exceed the configured maximum value. This drop does
not, however, result in session flap.
The benefits of discard extra paths option are:
Limits the memory footstamp of BGP.
Stops the flapping of the peer if the paths exceed the set limit.
When the discard extra paths configuration is removed, BGP sends a route-refresh message to the neighbor if it supports the
refresh capability; otherwise the session is flapped.
On the same lines, the following describes the actions when the maximum prefix value is changed:
If the maximum value alone is changed, a route-refresh message is sourced, if applicable.
If the new maximum value is greater than the current prefix count state, the new prefix states are saved.
If the new maximum value is less than the current prefix count state, then some existing prefixes are deleted to match the
new configured state value.
There is currently no way to control which prefixes are deleted.
Configure Discard Extra Paths
The discard extra paths option in the maximum-prefix configuration allows you to drop all excess prefixes received from the
neighbor when the prefixes exceed the configured maximum value. This drop does not, however, result in session flap.
The benefits of discard extra paths option are:
Limits the memory footstamp of BGP.
Stops the flapping of the peer if the paths exceed the set limit.
When the discard extra paths configuration is removed, BGP sends a route-refresh message to the neighbor if it supports the
refresh capability; otherwise the session is flapped.
Note
When the router drops prefixes, it is inconsistent with the rest of the network, resulting in possible routing loops.
If prefixes are dropped, the standby and active BGP sessions may drop different prefixes. Consequently, an NSR switchover
results in inconsistent BGP tables.
The discard extra paths configuration cannot co-exist with the soft reconfig configuration.
When the system runs out of physical memory, bgp process exits and you must manually restart bpm. To manually restart, use
the process restart bpm command.
Perform this task to configure BGP maximum-prefix discard extra paths.
SUMMARY STEPS
configure
router bgpas-number
neighborip-address
address-family {ipv4 | ipv6} unicast
maximum-prefixmaximumdiscard-extra-paths
Use the
commit or
end command.
DETAILED STEPS
Step 1
configure
Example:
RP/0/RP0/CPU0:router# configure
Enters XR Config mode.
Step 2
router bgpas-number
Example:
RP/0/RP0/CPU0:router(config)# router bgp 10
Specifies the autonomous system number and enters the BGP configuration mode, allowing you to configure the BGP routing process.
The following screen output shows details about the discard extra paths option:
RP/0//CPU0:ios# show bgp neighbor 10.0.0.1
BGP neighbor is 10.0.0.1
Remote AS 10, local AS 10, internal link
Remote router ID 0.0.0.0
BGP state = Idle (No best local address found)
Last read 00:00:00, Last read before reset 00:00:00
Hold time is 180, keepalive interval is 60 seconds
Configured hold time: 180, keepalive: 60, min acceptable hold time: 3
Last write 00:00:00, attempted 0, written 0
Second last write 00:00:00, attempted 0, written 0
Last write before reset 00:00:00, attempted 0, written 0
Second last write before reset 00:00:00, attempted 0, written 0
Last write pulse rcvd not set last full not set pulse count 0
Last write pulse rcvd before reset 00:00:00
Socket not armed for io, not armed for read, not armed for write
Last write thread event before reset 00:00:00, second last 00:00:00
Last KA expiry before reset 00:00:00, second last 00:00:00
Last KA error before reset 00:00:00, KA not sent 00:00:00
Last KA start before reset 00:00:00, second last 00:00:00
Precedence: internet
Multi-protocol capability not received
Received 0 messages, 0 notifications, 0 in queue
Sent 0 messages, 0 notifications, 0 in queue
Minimum time between advertisement runs is 0 secs
For Address Family: IPv4 Unicast
BGP neighbor version 0
Update group: 0.1 Filter-group: 0.0 No Refresh request being processed
Route refresh request: received 0, sent 0
0 accepted prefixes, 0 are bestpaths
Cumulative no. of prefixes denied: 0.
Prefix advertised 0, suppressed 0, withdrawn 0
Maximum prefixes allowed 10 (discard-extra-paths) <<<<<<<<<<<<<<<<<<<<<
Threshold for warning message 75%, restart interval 0 min
AIGP is enabled
An EoR was not received during read-only mode
Last ack version 1, Last synced ack version 0
Outstanding version objects: current 0, max 0
Additional-paths operation: None
Send Multicast Attributes
Connections established 0; dropped 0
Local host: 0.0.0.0, Local port: 0, IF Handle: 0x00000000
Foreign host: 10.0.0.1, Foreign port: 0
Last reset 00:00:00
Restrictions
These restrictions apply to the discard extra paths feature:
When the router drops prefixes, it is inconsistent with the rest of the network, resulting in possible routing loops.
If prefixes are dropped, the standby and active BGP sessions may drop different prefixes. Consequently, an NSR switchover
results in inconsistent BGP tables.
The discard extra paths configuration cannot co-exist with the soft reconfig configuration.
BGP Labeled Unicast
The BGP Labeled Unicast (LU) feature, also known as unified MPLS, provides MPLS transport
between Provider Edge (PE) routers that are separated by either many IGP boundaries
(intra-AS) or by many autonomous systems (inter-AS). Using autonomous systems border
routers (ASBRs), you can advertise loopback prefixes of PEs and their MPLS label
bindings: iBGP between area border routers (ABRs) and eBGP between autonomous system
border routers. You can use Multihop eBGP between the PEs if they are in different
autonomous systems (ASes) to exchange the VPN routes. You can run 6PE and other services
between the PEs that have BGP LU connectivity.
The BGP LU feature lowers the IGP labeled prefix scale and adjacency scale values. If the
router is not being configured with BGP LU, it is necessary to prevent lowering of scale
values. Hence it is mandatory to configure the hw-module command before you enable the
BGP LU feature. Restart the router for the hw-module command configuration to take
effect.
Restrictions
Cisco 8000 supports only per-vrf label mode.
You can use LDP or Segment Routing (SR) as the transport underlay. You cannot
use TE as the transport underlay.
BGP PIC edge feature is not supported.
L3VPN and 6VPE over BGP LU feature is not supported.
BGP PIC core feature is supported.
The label-allocation-mode is deprecated from release 7.4.1. The function of this command can be carried out using label mode command under configured
address-family.
Supported features
The following features are supported:
BGP LU with inter-AS option C
6PE over MPLS transport using LDP or Segment Routing.
BGP PIC core
Topology
Figure 1. BGP Labeled Unicast (Intra-Autonomous System) Control Plane and Data
Plane
The above diagram explains how PE1 is connected with PE2 through MPLS connectivity.
PE1 and PE2 are separated by many areas within the same AS. Consider three network
areas OSPF1, OSPF2, and OSPF3. Each of these areas is running separate OSPFs. LDP
acts as transport between each of these areas. To establish a connection between the
Provider Edge routers PE1 and PE2, send iBGP from PE2 to PE1 through P3, ASBR2, P1
and ASBR1, P2. PE1 must learn the loopback address of PE2 to establish a connection
between the loopback address of PE1 and the loopback address of PE2.
The loopback address of PE2 which is 10.1.1.7 advertises a BGP label through iBGP to
ASBR2. This address is advertised as an implicit null label. The ASBR2 allocates a
local label 14003 for the loopback address 10.1.1.7 and sends it to ASBR1. ASBR1
allocates its own label 14005 to the loopback address 10.1.1.7 and sends it to PE1.
PE1 has learnt the prefix of loopback address 10.1.1.7 and the BGP label 14005. The
BGP next hop for PE1 is ASBR1. When PE1 sends traffic to PE2, PE1 adds two labels:
the BGP-LU label and transport LDP label. The transport LDP label 24000, is above
the BGP-LU label 14005. PE1 imposes the transport LDP label and the BGP-LU label
when PE1 transmits an IP packet destined to the loopback address 10.1.1.7. The
transport LDP label carries the packet to ASBR1. ASBR1 receives the IP packet. It
contains only the BGP-LU label, 14005. ASBR1 swaps the BGP-LU label from 14005 to
14003 and imposes transport LDP label 24001 and sends the IP packet to ASBR2. ASBR2
receives the packet. The BGP-LU label for the loopback address 10.1.1.7 in ASBR2 is
implicit null. Only the transport label is pushed to 24002. ASBR2 transmits the
transport label that carries the transport to PE2.
Figure 2. BGP Labeled Unicast (Intra-Autonomous System Option C) Control Plane and Data
Plane
ASBR2 prefers IGP MPLS path over BGP path 10.1.1.7. It advertises LDP local label as
BGP label to ASBR1. A LDP swap operation takes place on ASBR2.
The above figure explains how PE1 is connected with PE2 through MPLS connectivity
using eBGP. In the above-mentioned scenario, eBGP exists between ASBR1 and ASBR2.
PE2 advertises the BGP-LU label which has a value of implicit null to ASBR2 through
iBGP. The loopback address is known to ASBR2 through the IGP. ASBR2 prefers the IGP
path with ldp label 24002. ASBR2 allocates local label 24004 to loopback 10.1.1.7.
It advertises the local label 24004 to ASBR1. ASBR1 creates a local label 14005 and
advertises it to PE1. Now, PE1 is aware of the loopback address 10.1.1.7. The IP
packet has two labels: the BGP label 14005 and the transport label 24000. PE1
transmits the IP packet to ASBR1. The IP packet received by ASBR1 has only the BGP
LU label 14005. ASBR1 swaps BGP-LU label from 14005 to 24004. The IP packet reaches
ASBR2 where LDP label 24002 is pushed and transmits the packet to PE2.
Figure 3. 6PE over BGP LU (Inter-AS Option C) Control Plane and Data Plane
The above illustration explains how PE1 is connected with PE2 through MPLS
connectivity using Multihop eBGP between multiple ASes. Multihop BGP exists between
PE1 and PE2. PE1 and PE2 can exchange 6PE routes on the multihop eBGP with the
labels. The label value for 6PE is v6 explicit null. When PE2 advertises v6 prefix
10::2/128, the label is always the explicit null label. The BGP label and LDP label
constitute the top two labels. The 6PE label constitutes the bottom label which is
v6 explicit null. The v6 packet reaches PE1 with destination IP 10:2. The label
imposition takes place here. The 6PE label of value 2 is imposed first, the BGP
label 14005 is imposed next, and then the next hop LDP label 14005 for the BGP LU
next hop is imposed. ASBR1 swaps BGP-LU label from 14005 to 24004 and forwards the
packet to ASBR2. ASBR2 adds LDP label on top of 6PE label 2 and forwards it to P3
where LDP label is POPed, so PE2 receives packet with 6PE explicit null label only.
PE2 performs a v6 lookup and forwards the packet.
Configure BGP Labeled Unicast
Router(config)# hw-module profile cef bgplu enable
Router(config)# router bgp 1
Router(config-bgp)# bgp router-id 2001:DB8::1
Router(config-bgp)# address-family ipv6 unicast
Router(config-bgp-af)# redistribute connected route-policy set-lbl-idx
Router(config-bgp-af)# allocate-label all
Router(config-bgp-af)# exit
Router(config-bgp)# neighbor 2001:DB8::2
Router(config-bgp)# remote-as 1
Router(config-bgp)# update-source Loopback 0
Router(config-bgp)# address-family ipv6 labeled-unicast
Router(config-bgp)# route-policy pass-all in
Router(config-bgp)# route-policy pass-all out
/* Note: Restart the router for the hw-module command configuration to take effect. */
Router # show bgp ipv6 unicast labels
Network Next Hop Rcvd Label Local Label
Router# show bgp ipv6 unicast labels
Network Next Hop Rcvd Label Local Label
Exclusion of Label Allocation for Non-Advertised Routes
Table 1. Feature History Table
Feature Name
Release Information
Feature Description
Exclusion of Label Allocation for Non-Advertised Routes
Release 7.10.1
We have enabled better label space management and hardware resource utilization by making MPLS label allocation more flexible.
This flexibility means you can now assign these labels to only those routes that are advertised to their peer routes, ensuring
better label space management and hardware resource utilization.
Prior to this release, label allocation was done regardless of whether the routes being advertised. This resulted in inefficient
use of label space.
The functionality to control label allocation to the routes which are not advertised to peers is introduced. You can now choose
to assign labels to the routes which are advertised to the peers.
Provider Edge (PE) routers works as autonomous systems border routers (ASBRs) where this feature is configured.
You can set the community attribute to either no-advertise or no-export in route-policy configuration mode to the routes which are not going to be advertised to peers. Once the community attribute in the route-policy is updated, the router doesn’t allocate any label to those routes.
Note
no-export is only for eBGP and no-advertise can be used for both eBGP and iBGP.
How to exclude label allocation for non-advertised routes
Configuration Example
This example shows how to set the community parameter to no-advertise for the routes which are not going to be advertised to any peer routes.
/*Configure the community set*/
Router(config)#community-set no-advertise
Router(config-comm)#no-advertise
Router(config-comm)#end-set
/*Configure the route policy*/
Router(config)#route-policy set-no-advertise
Router(config-rpl)#set community no-advertise additive
Router(config-rpl)#end-policy
Router(config-bgp-af)#route-policy pass_all
Router(config-rpl)# pass
Router(config-rpl)#end-policy
Router(config)#route-policy pass_all
Router(config-rpl)# pass
Router(config-rpl)#end-policy
/*Apply the route policy as inbound route policy*/
Router(config)#router bgp 1
Router(config-bgp)# neighbor 192.0.2.1
Router(config-bgp-nbr)# remote-as 1
Router(config-bgp-nbr)# update-source Loopback0
Router(config-bgp-nbr)# address-family ipv4 unicast
Router(config-bgp-nbr-af)# route-policy set-no-advertise in
Router(config-bgp-nbr-af)# route-policy pass_all out
Router(config-bgp-nbr-af)#commit
Running Configuration
community-set no-advertise
no-advertise
end-set
!
!
route-policy set-no-advertise
set community no-advertise additive
end-policy
!
!
route-policy pass_all
pass
end-policy
!
Verification
Use show bgp vpnv6 unicast rd command to verify the community parameter is set to no-advertised.
Router(config)# show bgp vpnv6 unicast rd 2001:DB8:0:ABCD::1
BGP routing table entry for 0:ABCD::1 Route Distinguisher: 2001:DB8
Versions:
Process bRIB/RIB SendTblVer
Speaker 19207 19207
Paths: (1 available, best #1, not advertised to any peer)
Not advertised to any peer
Path #1: Received by speaker 0
Not advertised to any peer
Local, (Received from a RR-client)
192.0.2.254 from 192.0.2.1 (192.0.2.1)
Received Label 16
Origin IGP, metric 3, localpref 3, aigp metric 3, valid, internal, best, group-best, import-candidate, not-in-vrf
Received Path ID 0, Local Path ID 1, version 19207
Community: 1:1 no-advertise
Extended community: Color:3333 RT:2001:DB8
AIGP set by inbound policy metric
Total AIGP metric 3
EIBGP Policy-Based Multipath with Equal Cost Multipath
Table 2. Feature History Table
Feature Name
Release Name
Description
EIBGP Policy-Based Multipath with Equal Cost Multipath
Release 7.10.1
You can gain control over traffic distribution and load-balancing capabilities by including policy-based multipath selection
across various BGP variations, including iBGP, eBGP, and eiBGP. This is achieved through the utilization of BGP communities,
nexthops, and path types.
Additionally, by employing the equal cost multipath (ECMP) option in eiBGP, this feature provides the capability to select
ECMP across the iBGP paths chosen for eiBGP.
The feature introduces these changes:
CLI:
The keywords
route-policy and equal-cost are added to the command:
The enhanced policy-based multipath selection in BGP operates now at the default Virtual Routing and Forwarding (VRF) level
for variations of BGP, such as iBGP, eBGP and eiBGP. To improve this functionality, the policy-based multipath selection is
now extended to include iBGP, eBGP and eiBGP by utilizing communities as the underlying mechanism. By utilizing communities,
the selection of multiple paths based on specific policy criteria becomes more elaborate. It enables better control over the
routing decisions within the BGP network.
eiBGP traditionally implements the unequal-cost mutipath (UCMP) capability to enable the use of both iBGP and eBGP paths.
This feature, utilizing the equal-cost multipath option (ECMP), ensures that the nexthop IGP metric remains consistent across
the chosen iBGP paths. Hence the metric evaluation is not performed between eBGP and iBGP paths because they have distinct
path types.
Topology
This topology illustrates a network comprising BGP peers denoted as R1 through R6. Consider a scenario, there is specific
need wherein you are in the process of transitioning from utilizing eBGP multipaths to iBGP multipaths. Throughout this transition,
you require the simultaneous operation of both eBGP and iBGP to facilitate a seamless migration.
Topology Setup
This topology showcases distinct path types, where eBGP paths are visually depicted using a red-colored line labeled as 1,
and the iBGP paths are visually illustrated using a green-colored line labeled as 2.
Expected Behavior
In the context of CE routers (CEI, CE2, CE3, CE4, C5, and C6), the preferred path for prefixes will be from eBGP, specifically
from the R4 router. Although there might be paths from R5 and R6 routers and also from RI and R2 routers through iBGP, the
selection of best paths will prioritize eBGP multipaths from R4. This is the classic behavior. In classic eiBGP, unequal-cost
paths are employed, leading to the disregard of metrics. However, you rely on the IGP metric for optimal performance.
After Implementing This Feature
The iBGP paths with the shortest AS-PATH length are chosen for R5 and R6 router paths. The same iBGP multipath selection process
applies to paths from R1 and R2 routers. As a result, the R1 and R2 routers establishes an iBGP peering session with the R3
router. Therefore, a combination of eBGP and iBGP paths, referred to as eiBGP, is now available for prefixes advertised to
hosts beyond the CE devices. The CE routers require load balancing of prefixes to R3 router and R4 router. However, it is
necessary to exclude paths originating from R5 and R6 routers and R1 and R2 routers. Therefore, you must configure additive
community on the R1 router and R2 routers towards the R5 and R6 routers.
With the setup depicted in the topology, you can establish the coexistence of both eBGP and iBGP, thus enabling seamless transition
from utilizing eBGP multipaths to iBGP multipaths. By including the default VRF in policy-based multipath selection, you apply
route policies to control how traffic is distributed within your network. By leveraging the BGP attributes such as BGP communities,
nexthops, and path types within these route policies, you determine path selection. For example, you can use BGP communities
to prioritize certain routes or manipulate nexthops to direct traffic over specific paths. This enables you to optimize routing
decisions based on your specific requirements and goals, allowing you to gain control over traffic distribution and load-balancing
capabilities across various BGP variations within your network.
By enabling ECMP, you allow a router to distribute traffic evenly across multiple equal-cost paths. This ensures that each
path carries a portion of the traffic load, preventing any single path from becoming overwhelmed. By enabling the ECMP option
in eiBGP, you allow the router to consider multiple iBGP paths with equal costs as viable options for traffic distribution.
These paths are treated as equal-cost paths. This enhances load balancing in your network.
Benefits
This feature, with the inclusion of policy-based multipath selection, enables you to gain control over traffic distribution
and load-balancing capabilities across various BGP variations, including iBGP, eBGP, and eiBGP. This is achieved through the
utilization of BGP communities, nexthops, and path types.
Neglecting the utilization of BGP communities, nexthops, and path types within the default VRF during policy-based multipath
selection can lead to limited control over traffic routing. The absence of BGP communities hinders the ability to apply specific
policies to route updates, while ignoring nexthops and path types diminishes the accuracy of path selection decisions. This
may result in suboptimal traffic distribution and load balancing.
Not applying ECMP within eiBGP can make the router to depend on its default path selection procedure to designate a singular
optimal route from the accessible iBGP paths. This approach does not yield the load balancing and traffic distribution advantages
offered by ECMP.
Restrictions for EIBGP Policy-Based Multipath with Equal Cost Multipath
The following are the restricions:
Configuring eiBGP along with either eBGP or iBGP is not allowed.
The maximum-paths route policy allows for checks on community, nexthop, and path type only.
The usage of the Accumulated Interior Gateway Protocol (AIGP) metric attribute is restricted only to equal-cost EIBGP scenarios.
The OpenConfig model is not supported.
When configuring eBGP and iBGP multipath together, it is possible to assign distinct or identical route policies to each of
them. However, the selection of the policy to be applied between eBGP and iBGP is determined by the bestpath path type of
the prefixes. If a prefix is determined to have a better path via iBGP, the iBGP route policy will be applied, while for prefixes
where eBGP is deemed better, the eBGP route policy will be applied.
Configure EIBGP Policy-Based Multipath with Equal Cost Multipath
Configuration Example
Perform the following steps to configure EIBGP Policy-Based Multipath with Equal Cost Multipath:
Configure the community, path-type, or nexthop.
Configure the route-policy with the multipath selection and equal-cost multipath for eiBGP.
Configure the community-set from the R1 and R2 routers
Configure the route-policy and equal-cost multipath option for eiBGP
The route-policy EIBGP is configured on R1 and R2 routers. This route-policy examines the BGP communities associated with
BGP routes and takes specific actions based on the community values. If the community matches “ABC”, the route is not selected
for multipath. For all the other cases, the router selects a path for multipath if it matches the best-path's metric and has
the same path-type (i.e., iBGP or EBGP). If the path-type is different from the best path-type, it must be the best among
the other path types. In addition to community, you also use path-type or next-hop as a route-policy option.
Router(config)# route-policy EIBGP
Router(config-rpl)# if community matches-any ABC then
Router(config-rpl-if)# pass
Router(config-rpl-if)# else
Router(config-rpl-else)# drop
Router(config-rpl-else)# endif
Router(config-rpl)# end-policy
Router(config)# router bgp 100
Router(config-bgp)# address-family ipv4 unicast
Router(config-bgp-af)# maximum-paths eibgp 32 equal-cost route-policy EIBGP
Router(config-bgp-af)# commit
Running Configuration
community-set ABC
2:1
end-set
!
route-policy EIBGP
if community matches-any ABC then
pass
else
drop
endif
end-policy router bgp 100
address-family ipv4 unicast
maximum-paths eibgp 32 equal-cost route-policy EIBGP
!
Verification
Verify that the router supports eiBGP multipath for this destination, and the route entries has been successfully received
and processed.
Router# show bgp 203.0.113.99/32
BGP routing table entry for 203.0.113.99/32
Versions:
Process bRIB/RIB SendTblVer
Speaker 27 27
Last Modified: Feb 23 16:08:54.000 for 04:12:23
Paths: (7 available, best #2)
Advertised IPv4 Unicast paths to update-groups (with more than one peer):
0.1 0.4
Path #1: Received by speaker 0
Not advertised to any peer
200 300
209.165.200.11 from 209.165.200.11 (192.168.0.3), -> From R4
Origin IGP, localpref 100, valid, external, multipath
Received Path ID 0, Local Path ID 0, version 0
Community: 2:1
Origin-AS validity: (disabled)
Path #2: Received by speaker 0
Advertised IPv4 Unicast paths to update-groups (with more than one peer):
0.1 0.4
200 300
209.165.201.1 from 209.165.201.1 (209.165.201.1) -> From R4
Origin IGP, localpref 100, valid, external, best, group-best, multipath
Received Path ID 0, Local Path ID 1, version 27
Community: 2:1
Origin-AS validity: (disabled)
Path #3: Received by speaker 0
Not advertised to any peer
200 300, (Received from a RR-client)
192.168.2.6 (metric 2) from 198.51.100.1 (198.51.100.1) -> From R3
Origin IGP, localpref 100, valid, internal, multipath, backup, add-path
Received Path ID 0, Local Path ID 2, version 6
Community: 2:1
Path #4: Received by speaker 0
Not advertised to any peer
200 300, (Received from a RR-client)
192.168.0.6 (metric 2) from 192.0.2.1 (192.0.2.1) -> From R5
Origin IGP, localpref 100, valid, internal
Received Path ID 0, Local Path ID 0, version 0
Community: 11:11 99:99
Path #5: Received by speaker 0
Not advertised to any peer
200 300, (Received from a RR-client)
192.168.0.2 (metric 5) from 192.168.0.2 (192.168.0.2) -> From R2
Origin IGP, localpref 100, valid, internal
Received Path ID 0, Local Path ID 0, version 0
Community: 2:1 99:99
/* The router does not select Path 5, even though it satisfies the route-policy community constraint, because it has a higher metric (i.e., metric 5) than the best path of its path type (i.e., iBGP metric 2). */
Path #6: Received by speaker 0
Not advertised to any peer
200 300, (Received from a RR-client)
192.168.0.4 (metric 2) from 192.168.0.4 (192.168.0.4) -> From R5
Origin IGP, localpref 100, valid, internal
Received Path ID 0, Local Path ID 0, version 0
Community: 11:11 99:99
Path #7: Received by speaker 0
Not advertised to any peer
100 300, (Received from a RR-client)
192.168.0.5 (metric 2) from 192.168.0.5 (192.168.0.5) -> From R3
Origin IGP, localpref 100, valid, internal, multipath
Received Path ID 0, Local Path ID 0, version 0
Community: 2:1
Protection of Directly Connected EBGP Neighbors through Interface-Based LPTS Identifier
Table 3. Feature History Table
Feature Name
Release Name
Description
Protection of Directly Connected EBGP Neighbors through Interface-Based LPTS Identifier
Release 7.10.1
We have enhanced the network security for directly connected eBGP neighbors by ensuring that only packets originating from
designated eBGP neighbors can traverse through a single interface, thus preventing IP spoofing. This is made possible because
we've now added an interface identifier for Local Packet Transport Services (LPTS). LPTS filters and polices the packets based
on the type of flow rate you configure.
Local Packet Transport Services (LPTS) maintains tables describing all packet flows destined for the secure domain router
(SDR), making sure that packets are delivered to their intended destinations.
With respect to BGP sessions, LPTS bindings can be categorized as follows:
BGP Known: These LPTS entries correspond to BGP sessions with established neighbors.
BGP Configured Peer: LPTS entries in this category are designated to receive the initial packets (TCP SYN and 3rd ACK) from
specifically configured BGP neighbors.
BGP Default Entries: This category encompasses LPTS entries that capture all packets originating from un-configured BGP neighbors.
An attacker who spoofs a packet using the exact combination of source IP, destination IP, source port, and destination port,
and then floods these packets from another interface within the same VRF, will cause the packet to match the BGP known LPTS
entry. As a result, the packet will traverse up to the TCP layer and potentially be dropped at that level. All BGP known LPTS
entries share a common LPTS policer, which means that packets arriving through any of these entries will be policed at the
specified rate.
However, if the attacker sends these packets at a rate exceeding the policer's defined rate, this will lead to congestion
in this flow, adversely impacting BGP established peers. As a result, these BGP sessions may experience instability, which
could lead to flapping.
This feature enables you to protect your network by adding an interface identifier for LPTS in directly connected eBGP neighbors.
LPTS filters and polices the packets based on the type of flow rate you configure. This feature ensures that only packets
originating from designated eBGP neighbors can traverse through a single interface, thus preventing IP spoofing. The interface
identifier that is added will be passed to the LPTS and TCP only when the below-mentioned criteria are met:
The BGP peer is configured to be external.
The Fast External Failover (FEF) is not disabled.
The BGP peer is direclty connected.
The BGP peer is not a dynamic peer.
eBGP multihop is not enabled.
The default eBGP TTL is used.
The "ignore connected" option is not configured.
A non-link local IPv6 neighbor address is configured.
In the LPTS binding process through the LPTS socket option, BGP generates a tuple for the interface identifier for every directly
configured eBGP neighbor.
The configured BGP LPTS entry will only match an incoming connection (TCP SYN packet) if it is received from the programmed
interface.
The BGP default entry handles incoming connections, or any other packets, received on interfaces other than the specified
ones. These packets are subjected to rigorous policing and forwarded to TCP for reset generation. As a result, any spoofed
packets arriving from non-desired interfaces will not affect the BGP configured peer LPTS entries.
Upon receiving a passive connection from the programmed interface and establishing it at the TCP level, TCP will inherit the
same interface for the BGP known LPTS entry, which will be created for this specific connection.
Packets that match the source IP, destination IP, source port, destination port, and VRF information of an established connection
, but are received from a different interface, will not be matched to the LPTS entry. As a result, these packets will be directed
to the BGP default entry. This mechanism ensures that spoofed packets originating from non-desired interfaces will not affect
the BGP known peer LPTS entries.
During the bind process for an active connection, BGP will also furnish the interface identifier. TCP will incorporate this
interface information into the LPTS entry corresponding to the active connection, effectively safeguarding BGP known LPTS
entries against spoofed packets that might match this connection but originate from a different interface.
Configure Protection of Directly Connected EBGP Neighbors through Interface-Based LPTS Identifier
To enable Local Packet Transport Services (LPTS) secure binding, perform the following steps:
Router# show bgp process | in LPTS
Wed Dec 14 14:28:33.779 PST
LPTS secure binding is enabled
Verify that the status of the connected interface identifier in LPTS is active:
Router# show bgp neighbor 192.0.2.3, detail | in Connected
Wed Dec 14 14:28:51.814 PST
Connected IFH: 0x1000080, IFH in LPTS 0x1000080
Convergence for BGP Labeled Unicast PIC Edge
Table 4. Feature History Table
Feature Name
Release Information
Feature Description
Convergence for BGP Labeled Unicast PIC Edge
Release 7.7.1
This feature improves the convergence time of BGP labeled unicast (LU) routes to subseconds when an ingress provider edge
router fails or loses PE router connectivity, and another PE router needs to be connected. This feature minimizes traffic
drops when the primary paths fail for the BGP LU routes.
BGP Labeled Unicast (LU) PIC Edge feature enables you to create and store both the primary and backup path in the Routing
Information Base (RIB), Forwarding Information Base (FIB), and Cisco Express Forwarding. When the router detects a failure,
the backup or alternate path immediately takes over, thus this feature enables fast failover and convergence in subseconds.
For BGP LU PIC Edge to work, the edge iBGP devices, such as ingress PEs and Autonomous System Border Router (ASBR), must support
BGP PIC and must receive backup BGP next hop.
The topology diagram given below illustrates the Convergence for BGP Labeled Unicast PIC Edge feature. The topology is explained
as follows:
The BGP LU PIC Edge feature is enabled on a provider edge router, PE1.
PE1 learns the BGP LU prefix from the remote PE router, PE2.
PE1 routes traffic through the Area Border Routers, ABR1, ABR2 and ABR3. If one of them fails, the preprogrammed backup of
the failed ABR routes the traffic.
PE1 routes traffic through the Area Border Routers, ABR1, ABR2 and ABR3.
PE2 is marked as the backup or alternate next hop and is programmed into the FIB of PE1.
When PE1 learns PE2 is not reachable through ABR1, it immediately changes the BGP next hop for the PE1's prefix to ABR2.
The switchover occurs in less than a second regardless of the number of prefixes.
Subsecond convergence occurs although updates to multiple BGP prefixes are pending.
Topology
Figure 4. BGP LU PIC Edge
Guidelines and Limitations
This feature supports BGP multipaths that allows the router to install multiple internal BGP paths and multiple external BGP
paths to the forwarding table. The multiple paths enable BGP to load balance traffic across multiple links.
The convergence time is independent of the BGP LU route scale.
Configure Convergence for BGP Labeled Unicast PIC Edge
Perform the following steps to configure Convergence for BGP Labeled Unicast PIC Edge:
Configure BGP labeled unicast and attach route-policy to BGP address families.
Configure BGP labeled unicast multipath and attach route-policy to BGP address families
Router# show cef 192.0.2.1/32
192.168.0.0/32, version 31, internal 0x5000001 0x40 (ptr 0x901d2370) [1], 0x0 (0x90d2beb8), 0xa08 (0x91c74378)
Prefix Len 32, traffic index 0, precedence n/a, priority 4
via 203.0.113.1/32, 3 dependencies, recursive [flags 0x6000] << Primary Path
path-idx 0 NHID 0x0 [0x90319650 0x0]
recursion-via-/32
next hop 192.51.100.1/32 via 24006/0/21
next hop 209.165.200.225/32 Hu0/0/0/25 labels imposed {24002 24000}
next hop 10.0.0.1/32 Hu0/0/0/26 labels imposed {24002 24000}
via 203.0.113.2/32, 2 dependencies, recursive, backup [flags 0x6100] << Backup Path
path-idx 1 NHID 0x0 [0x903197b8 0x0]
recursion-via-/32
next hop 209.165.200.225/32 via 24005/0/21
next hop 192.51.100.1/32 Hu0/0/0/25 labels imposed {24001 24000}
next hop 10.0.0.1/32 Hu0/0/0/26 labels imposed {24001 24000}
Black Box Monitoring
Table 5. Feature History Table
Feature Name
Release Information
Feature Description
Black Box Monitoring
Release 7.3.2
This feature enables you to set up forwarding path on the router that
you can use to probe customer circuits for system metrics specific
to the network devices. Such monitoring helps you to keep up the
service level agreements with your customers.
This feature uses a technique whereby a dummy BGP session is established across the GRE
encapsulation and decapsulation infrastructure. To terminate the dummy BGP session, the
router peers to an address that is configured on the peering fabric which is peering to
itself.
The router must peer to an address which is configured on the PF, peering to itself in
essence. The only way to make this work is by plugging two interfaces into one another
with a physical cable. After two interfaces are connected to one another place one of
them into a VRF so that the BGP session is brought up. A router does not attempt to
establish a BGP session to itself normally, so you must separate the routing table using
a VRF. On the other interface it is a 'normal' interface in the global vrf with the same
configuration that is typically on a PF peering interface.
Configuration Example
Perform the following steps to configure BGP and GRE tunnel..
/* Configure the Local Proxy ARP on the Bundle-Ether interfaces.*/
Router(config)# interface Bundle-Ether1.1
Router(config-if)# ipv4 address 10.1.1.1 255.255.255.240
Router(config-if)# local-proxy-arp
Router(config-if)# encapsulation dot1q 12
Router(config-if)# ipv4 access-group acl-aa ingress
Router(config-if)# exit
Router(config)# interface Bundle-Ether2.1
Router(config-if)# vrf aa
Router(config-if-vrf)# ipv4 address 10.1.1.2 255.255.255.240
Router(config-if-vrf)# local-proxy-arp
Router(config-if-vrf)# encapsulation dot1q 12
/* Configure a bundle on FortyGigE interfaces.*/
Router(config)# interface FortyGigE 0/0/0/46
Router(config-if)# bundle id 1 mode on
Router(config-if)# exit
Router(config)# interface FortyGigE0/0/0/47
Router(config-if)# bundle id 2 mode on
/* Configure the access list.*/
Router(config-if)# ipv4 access-list acl-aa
Router(config-if)# 1 permit icmp any host 10.1.1.1 echo-reply
Router(config-if)# 2 permit ipv4 any any nexthop1 ipv4 100.100.2.2
Router(config-if)# 10 permit tcp any eq bgp any
Router(config-if)# 20 permit tcp any any eq bgp
/* Configure BGP.*/
Router(config)# router bgp 100
Router(config-bgp)# bgp router-id 10.10.10.10
Router(config-bgp)# bgp log neighbor changes detail
Router(config-bgp)# address-family ipv4 unicast
Router(config-bgp)# maximum-paths ebgp 64
Router(config-bgp)# maximum-paths ibgp 64
/* Apply route policy. */
Router(config)# address-family vpnv4 unicast
Router(config-af)# vrf aa
Router(config-af)# rd auto
Router(config-af)# exitexit
Router(config)# address-family ipv4 unicast
Router(config)# exit
Router(config)# neighbor 10.1.1.1
Router(config-nbr)# remote-as 200
Router(config-nbr)# ebgp-multihop 4
Router(config-nbr)# exit
Router(config)# address-family ipv4 unicast
Router(config-af)#send-community-ebgp
Router(config-af)# route-policy pass-all in
Router(config-af)# route-policy pass-all out
/* Configure loopback interfaces. */
Router(config)# interface Loopback1001
Router(config-if)# ipv4 address 10.10.10.10 255.255.255.255
Router(config)# exit
Router(config)# interface Loopback1002
Router(config-if)# vrf aa
Router(config-if-vrf)# ipv4 address 10.10.10.10 255.255.255.255
/* Configure a class map. */
Router(config)# class-map type traffic match-all aa
Router(config-cmap)# match protocol gre
Router(config-cmap)# match destination-address ipv4 10.10.10.10 255.255.255.255
Router(config-cmap)# end-class-map
/* Configure a policy map. */
Router(config)# policy-map type pbr pmap1
Router(config-pmap)# class type traffic aa
Router(config-pmap-c)# decapsulate gre
Router(config-pmap-c)# class type traffic class-default
Router(config-pmap-c)# end-policy-map
/* Configure VRF policy. */
Router(config)# vrf-policy
Router(config-vrf)# vrf default address-family ipv4 policy type pbr input pmap1
Router(config)# interface tunnel-ip 1100
Router(config-if)#ipv4 unnumbered Loopback1001
Router(config-if)#tunnel mode gre ipv4 encap
Router(config-if)#tunnel source Loopback1001
Router(config-if)#tunnel destination 200.1.2.1
Router(config-if)#logging events link-status
Running Configuration
interface Bundle-Ether1.1
ipv4 address 10.1.1.1 255.255.255.240
local-proxy-arp
encapsulation dot1q 12
ipv4 access-group aa-acl ingress
interface Bundle-Ether2.1
vrf aa
ipv4 address 10.1.1.2 255.255.255.240
local-proxy-arp
encapsulation dot1q 12
interface FortyGigE0/0/0/46
bundle id 1 mode on
interface FortyGigE0/0/0/47
bundle id 2 mode on
ipv4 access-list aa-acl
1 permit icmp any host 10.1.1.1 echo-reply
2 permit ipv4 any any nexthop1 ipv4 100.100.2.2
10 permit tcp any eq bgp any
20 permit tcp any any eq bgp
router bgp 100
bgp router-id 10.10.10.10
bgp log neighbor changes detail
address-family ipv4 unicast
maximum-paths ebgp 64
maximum-paths ibgp 64
!
address-family vpnv4 unicast
!
vrf aa
rd auto
address-family ipv4 unicast
!
neighbor 10.1.1.1
remote-as 200
ebgp-multihop 4
address-family ipv4 unicast
send-community-ebgp
route-policy pass-all in
route-policy pass-all out
interface Loopback1001
ipv4 address 10.10.10.10 255.255.255.255
RP/0/RP0/CPU0:SF-DD#sh run int loopback 1002
interface Loopback1002
vrf aa
ipv4 address 10.10.10.10 255.255.255.255
class-map type traffic match-all aa
match protocol gre
match destination-address ipv4 10.10.10.10 255.255.255.255
end-class-map
policy-map type pbr pmap1
class type traffic aa
decapsulate gre
class type traffic class-default
end-policy-map
!
vrf-policy
vrf default address-family ipv4 policy type pbr input pmap1
interface tunnel-ip1100
ipv4 unnumbered Loopback1001
tunnel mode gre ipv4 encap
tunnel source Loopback1001
tunnel destination 200.1.2.1
logging events link-status
Verification
Verify the configuration of black box monitoring.
Router# show bgp vrf aa neighbors
BGP neighbor is 10.1.1.1, vrf aa
Remote AS 200, local AS 100, external link
Remote router ID 200.1.2.1
BGP state = Established, up for 00:12:35
NSR State: None
Last read 00:00:30, Last read before reset 00:00:00
Hold time is 180, keepalive interval is 60 seconds
Configured hold time: 180, keepalive: 60, min acceptable hold time: 3
Last write 00:00:30, attempted 19, written 19
Second last write 00:01:30, attempted 19, written 19
Last write before reset 00:00:00, attempted 0, written 0
Second last write before reset 00:00:00, attempted 0, written 0
Last write pulse rcvd Sep 29 05:50:49.983 last full not set pulse count 30
Last write pulse rcvd before reset 00:00:00
Connections established 1; dropped 0
Local host: 10.1.1.2, Local port: 52660, IF Handle: 0x00000000
Foreign host: 10.1.1.1, Foreign port: 179
Last reset 00:00:00
External BGP neighbor may be up to 4 hops away.
BGP Labeled Unicast Version 6
Table 6. Feature History Table
Feature Name
Release Information
Feature Description
BGP Labeled Unicast Version 6
Release 7.3.16
This feature extends the BGP Labeled Unicast (LU) functionality over
IPv6. This feature provides connectivity between PEs to run
services, such as L3VPN and 6PVE. This feature allows the PEs to
transport traffic across autonomous systems (AS) boundaries.
BGP LU allows you to transport MPLS traffic across IGP boundaries. By
advertising loopbacks and label bindings across IGP boundaries
routers communicate with other routers in remote areas that do not
share the same local IGP.
Overview of BGP Labeled Unicast
The BGP Labeled Unicast (LU) feature, also known as unified MPLS, provides MPLS
transport between Provider Edge (PE) routers that are separated by either many IGP
boundaries (intra-AS) or by many autonomous systems (inter-AS). Using autonomous
systems border routers (ASBRs), you can advertise loopback prefixes of PEs and their
MPLS label bindings: iBGP between area border routers (ABRs) and eBGP between
autonomous system border routers. You can use Multihop eBGP between the PEs if they
are in different autonomous systems (ASes) to exchange the VPN routes. You can run
6PE and other services between the PEs that have BGP LU connectivity.
The BGP LU feature lowers the IGP labeled prefix scale and adjacency scale values. If
the router is not being configured with BGP LU, it is necessary to prevent lowering
of scale values. Hence it is mandatory to configure the hw-module command before you
enable the BGP LU feature. Restart the router for the hw-module command
configuration to take effect.
The BGP Labeled Unicast Version 6 (BGP LU v6) feature extends the BGP Labeled Unicast
(LU) functionality over IPv6.
Restrictions
6VPE over BGP LU feature is not supported.
Inter-AFI is not supported.
BGP PIC core feature is not supported.
Coexistence of 6PE with the same neighbor is not supported.
Coexistence of BGP LU version 6 IPv6 unicast-address family is not
supported.
VPNV6 over BGP LU v6 is not supported.
Link-local addresses are not supported.
Rewrite cases, in which BGP LU is itself the transport, is not supported.
Carrier Supporting Carrier Version 6 is not supported.
Inter-AS Option-C with BGP LU Version 6 is not supported.
Router# show hw-module profile cef
Thu Jun 17 00:06:32.974 UTC
------------------------------------------------------------------------------------
Knob Status Applied Action
------------------------------------------------------------------------------------
BGPLU Configured Yes None
LPTS ACL Unconfigured Yes None
Dark Bandwidth Unconfigured Yes None
MPLS Per Path Stats Unconfigured Yes None
Tunnel TTL Decrement Unconfigured Yes None
High-Scale No-LDP-Over-TE Unconfigured Yes None
IPv6 Hop-limit Punt Unconfigured Yes None
IP Redirect Punt Unconfigured Yes None
Verify the details of route paths along with the BGP and transport label
information.
Router# show cef ipv6 192:168:9::80/128
Wed Jun 16 07:42:04.789 UTC
192:168:9::80/128, version 27, internal 0x5000001 0x40 (ptr 0x93f2d478) [1], 0x0 (0x93ef6cc0), 0xa08 (0x9460a8a8)
Updated Jun 16 07:36:00.189
Prefix Len 128, traffic index 0, precedence n/a, priority 4, encap-id 0x1001000000001
via 10:0:1::51/128, 3 dependencies, recursive [flags 0x6000]
path-idx 0 NHID 0x0 [0x94720660 0x0]
recursion-via-/128
next hop 10:0:1::51/128 via 16061/0/21
next hop fe80::7af8:c2ff:fee4:20c0/128 Hu0/0/0/27 labels imposed {16061 25001}
/*
16061 - Transport Label
25001 – BGP Label
*/
Verify the BGP LU version 6 routes and BGP label information in BGP process.
Router# show bgp ipv6 unicast labels
Wed Jun 16 07:34:58.968 UTC
BGP router identifier 10.0.1.50, local AS number 1
BGP generic scan interval 60 secs
Non-stop routing is enabled
BGP table state: Active
Table ID: 0xe0800000 RD version: 6
BGP main routing table version 6
BGP NSR Initial initsync version 3 (Reached)
BGP NSR/ISSU Sync-Group versions 0/0
BGP scan interval 60 secs
Status codes: s suppressed, d damped, h history, * valid, > best
i - internal, r RIB-failure, S stale, N Nexthop-discard
Origin codes: i - IGP, e - EGP, ? - incomplete
Network Next Hop Rcvd Label Local Label
*> 192:168::/64 192:168:1::70 nolabel 24006
*>i192:168:9::80/128 10:0:1::51 25001 nolabel
Processed 2 prefixes, 2 paths
BGP Next Hop Tracking
BGP receives notifications from the Routing Information Base (RIB) when next-hop information changes (event-driven notifications).
BGP obtains next-hop information from the RIB to:
Determine whether a next hop is reachable.
Find the fully recursed IGP metric to the next hop (used in the best-path calculation).
Validate the received next hops.
Calculate the outgoing next hops.
Verify the reachability and connectedness of neighbors.
BGP is notified when any of the following events occurs:
Next hop becomes unreachable
Next hop becomes reachable
Fully recursed IGP metric to the next hop changes
First hop IP address or first hop interface change
Next hop becomes connected
Next hop becomes unconnected
Next hop becomes a local address
Next hop becomes a nonlocal address
Note
Reachability and recursed metric events trigger a best-path recalculation.
Event notifications from the RIB are classified as critical and noncritical. Notifications for critical and noncritical events
are sent in separate batches. However, a noncritical event is sent along with the critical events if the noncritical event
is pending and there is a request to read the critical events.
Critical events are related to the reachability (reachable and unreachable), connectivity (connected and unconnected), and
locality (local and nonlocal) of the next hops. Notifications for these events are not delayed.
Noncritical events include only the IGP metric changes. These events are sent at an interval of 3 seconds. A metric change
event is batched and sent 3 seconds after the last one was sent.
The next-hop trigger delay for critical and noncritical events can be configured to specify a minimum batching interval for
critical and noncritical events using the nexthop trigger-delay command. The trigger delay is address family dependent.
The BGP next-hop tracking feature allows you to specify that BGP routes are resolved using only next hops whose routes have
the following characteristics:
To avoid the aggregate routes, the prefix length must be greater than a specified value.
The source protocol must be from a selected list, ensuring that BGP routes are not used to resolve next hops that could lead
to oscillation.
This route policy filtering is possible because RIB identifies the source protocol of route that resolved a next hop as well
as the mask length associated with the route. The nexthop route-policy command is used to specify the route-policy.
Next Hop as the IPv6 Address of Peering Interface
BGP can carry IPv6 prefixes over an IPv4 session. The next hop for the IPv6 prefixes can be set through a nexthop policy.
In the event that the policy is not configured, the nexthops are set as the IPv6 address of the peering interface (IPv6 neighbor
interface or IPv6 update source interface, if any one of the interfaces is configured).
If the nexthop policy is not configured and neither the IPv6 neighbor interface nor the IPv6 update source interface is configured,
the next hop is the IPv4 mapped IPv6 address.
IPv6 Multiprotocol BGP Peering Using a Global Address
When all ECMP links are shutdown except any one of the interfaces, the next-hop is changed from global address to link-local
address which leads to traffic loss of all flows for a few seconds transient time.
You can then configure the set next-hop ipv6-global command under the BGP table-policy to avoid traffic loss over an undisturbed path.
BGP installs global ipv6 address nexthop for multipath routes and install linklocal and ifhandle for single path route to connect ebgp neighbor directly. You can configure the set next-hop ipv6-global command under the BGP table-policy as follows to set the global ipv6 address nexthop:
route-policy RESILIENT-HASH-V6
if destination in (1000:1000::/32 le 128) or destination in (2000:1000::/32 le 128) then
set load-balance ecmp-consistent
set next-hop ipv6-global
pass
endif
pass
end-policy
Scoped IPv4 Table Walk
To determine which address family to process, a next-hop notification is received by first de-referencing the gateway context
associated with the next hop, then looking into the gateway context to determine which address families are using the gateway
context. The IPv4 unicast address families share the same gateway context, because they are registered with the IPv4 unicast
table in the RIB. As a result, the global IPv4 unicast table processed when an IPv4 unicast next-hop notification is received
from the RIB. A mask is maintained in the next hop, indicating the next hop belongs to IPv4 unicast. This scoped table walk
localizes the processing in the appropriate address family table.
Reordered Address Family Processing
The software walks address family tables based on the numeric value of the address family. When a next-hop notification batch
is received, the order of address family processing is reordered to the following order:
IPv4 tunnel
VPNv4 unicast
IPv4 labeled unicast
IPv4 unicast
IPv4 multicast
IPv6 unicast
New Thread for Next-Hop Processing
The critical-event thread in the spkr process handles only next-hop, Bidirectional Forwarding Detection (BFD), and fast-external-failover
(FEF) notifications. This critical-event thread ensures that BGP convergence is not adversely impacted by other events that
may take a significant amount of time.
show, clear, and debug Commands
The show bgp nexthops command provides statistical information about next-hop notifications, the amount of time spent in processing those notifications,
and details about each next hop registered with the RIB. The clear bgp nexthop performance-statistics command ensures that the cumulative statistics associated with the processing part of the next-hop show command can be cleared to help in monitoring. The clear bgp nexthop registration command performs an asynchronous registration of the next hop with the RIB.
The debug bgp nexthop command displays information on next-hop processing. The out keyword provides debug information only about BGP registration of next hops with RIB. The in keyword displays debug information about next-hop notifications received from RIB. The out keyword displays debug information about next-hop notifications sent to the RIB.
BGP Configuration
BGP in Cisco IOS XR software follows a neighbor-based configuration model that requires that all configurations for a particular
neighbor be grouped in one place under the neighbor configuration. Peer groups are not supported for either sharing configuration
between neighbors or for sharing update messages. The concept of peer group has been replaced by a set of configuration groups
to be used as templates in BGP configuration and automatically generated update groups to share update messages between neighbors.
Configuration Modes
BGP configurations are grouped into modes. The following sections show how to enter some of the BGP configuration modes. From
a mode, you can enter the ? command to display the commands available in that mode.
Router Configuration Mode
The following example shows how to enter router configuration mode:
Cisco IOS XR BGP uses a neighbor submode to make it possible to enter configurations without having to prefix every configuration
with the neighbor keyword and the neighbor address:
Cisco IOS XR software has a submode available for neighbors in which it is not necessary for every command to have a “neighbor
x.x.x.x” prefix:
In Cisco IOS XR software, the configuration is as follows:
An address family configuration submode inside the neighbor configuration submode is available for entering address family-specific
neighbor configurations. In the Cisco IOS XR software, the configuration is as follows:
Router(config-bgp)# neighbor 2002::2
Router(config-bgp-nbr)# remote-as 2023
Router(config-bgp-nbr)# address-family ipv6 unicast
Router(config-bgp-nbr-af)# next-hop-self
Router(config-bgp-nbr-af)# route-policy one in
Configuration Templates
The af-group, session-group, and neighbor-group configuration commands provide template support for the neighbor configuration in Cisco IOS XR software.
The af-group command is used to group address family-specific neighbor commands within an IPv4, IPv6, address family. Neighbors that have
the same address family configuration are able to use the address family group (af-group) name for their address family-specific
configuration. A neighbor inherits the configuration from an address family group by way of the use command. If a neighbor is configured to use an address family group, the neighbor (by default) inherits the entire configuration
from the address family group. However, a neighbor does not inherit all of the configuration from the address family group
if items are explicitly configured for the neighbor. The address family group configuration is entered under the BGP router
configuration mode. The following example shows how to enter address family group configuration mode
The session-group command allows you to create a session group from which neighbors can inherit address family-independent configuration. A
neighbor inherits the configuration from a session group by way of the use command. If a neighbor is configured to use a session group, the neighbor (by default) inherits the entire configuration
of the session group. A neighbor does not inherit all of the configuration from a session group if a configuration is done
directly on that neighbor. The following example shows how to enter session group configuration mode:
The neighbor-group command helps you apply the same configuration to one or more neighbors. Neighbor groups can include session groups and address
family groups and can comprise the complete configuration for a neighbor. After a neighbor group is configured, a neighbor
can inherit the configuration of the group using the use command. If a neighbor is configured to use a neighbor group, the neighbor inherits the entire BGP configuration of the neighbor
group.
The following example shows how to enter neighbor group configuration mode:
However, a neighbor does not inherit all of the configuration from the neighbor group if items are explicitly configured for
the neighbor. In addition, some part of the configuration of the neighbor group could be hidden if a session group or address
family group was also being used.
Configuration grouping has the following effects in Cisco IOS XR software:
Commands entered at the session group level define address family-independent commands (the same commands as in the neighbor
submode).
Commands entered at the address family group level define address family-dependent commands for a specified address family
(the same commands as in the neighbor-address family configuration submode).
Commands entered at the neighbor group level define address family-independent commands and address family-dependent commands
for each address family (the same as all available neighbor commands), and define the use command for the address family group and session group commands.
Template Inheritance Rules
In Cisco IOS XR software, BGP neighbors or groups inherit configuration from other configuration groups.
For address family-independent configurations:
Neighbors can inherit from session groups and neighbor groups.
Neighbor groups can inherit from session groups and other neighbor groups.
Session groups can inherit from other session groups.
If a neighbor uses a session group and a neighbor group, the configurations in the session group are preferred over the global
address family configurations in the neighbor group.
For address family-dependent configurations:
Address family groups can inherit from other address family groups.
Neighbor groups can inherit from address family groups and other neighbor groups.
Neighbors can inherit from address family groups and neighbor groups.
Configuration group inheritance rules are numbered in order of precedence as follows:
If the item is configured directly on the neighbor, that value is used. In the example that follows, the advertisement interval
is configured both on the neighbor group and neighbor configuration and the advertisement interval being used is from the
neighbor configuration:
The following output from the show bgp neighbors command shows that the advertisement interval used is 20 seconds:
Router# show bgp neighbors 10.1.1.1
BGP neighbor is 10.1.1.1, remote AS 1, local AS 140, external link
Remote router ID 0.0.0.0
BGP state = Idle
Last read 00:00:00, hold time is 180, keepalive interval is 60 seconds
Received 0 messages, 0 notifications, 0 in queue
Sent 0 messages, 0 notifications, 0 in queue
Minimum time between advertisement runs is 20 seconds
For Address Family: IPv4 Unicast
BGP neighbor version 0
Update group: 0.1
eBGP neighbor with no inbound or outbound policy; defaults to 'drop'
Route refresh request: received 0, sent 0
0 accepted prefixes
Prefix advertised 0, suppressed 0, withdrawn 0, maximum limit 524288
Threshold for warning message 75%
Connections established 0; dropped 0
Last reset 00:00:14, due to BGP neighbor initialized
External BGP neighbor not directly connected.
Otherwise, if an item is configured to be inherited from a session-group or neighbor-group and on the neighbor directly, then
the configuration on the neighbor is used. If a neighbor is configured to be inherited from session-group or af-group, but
no directly configured value, then the value in the session-group or af-group is used. In the example that follows, the advertisement
interval is configured on a neighbor group and a session group and the advertisement interval value being used is from the
session group:
The following output from the show bgp neighbors command shows that the advertisement interval used is 15 seconds:
Router# show bgp neighbors 192.168.0.1
BGP neighbor is 192.168.0.1, remote AS 1, local AS 140, external link
Remote router ID 0.0.0.0
BGP state = Idle
Last read 00:00:00, hold time is 180, keepalive interval is 60 seconds
Received 0 messages, 0 notifications, 0 in queue
Sent 0 messages, 0 notifications, 0 in queue
Minimum time between advertisement runs is 15 seconds
For Address Family: IPv4 Unicast
BGP neighbor version 0
Update group: 0.1
eBGP neighbor with no inbound or outbound policy; defaults to 'drop'
Route refresh request: received 0, sent 0
0 accepted prefixes
Prefix advertised 0, suppressed 0, withdrawn 0, maximum limit 524288
Threshold for warning message 75%
Connections established 0; dropped 0
Last reset 00:03:23, due to BGP neighbor initialized
External BGP neighbor not directly connected.
Otherwise, if the neighbor uses a neighbor group and does not use a session group or address family group, the configuration
value can be obtained from the neighbor group either directly or through inheritance. In the example that follows, the advertisement
interval from the neighbor group is used because it is not configured directly on the neighbor and no session group is used:
The following output from the show bgp neighbors command shows that the advertisement interval used is 15 seconds:
Router# show bgp neighbors 192.168.1.1
BGP neighbor is 192.168.2.2, remote AS 1, local AS 140, external link
Remote router ID 0.0.0.0
BGP state = Idle
Last read 00:00:00, hold time is 180, keepalive interval is 60 seconds
Received 0 messages, 0 notifications, 0 in queue
Sent 0 messages, 0 notifications, 0 in queue
Minimum time between advertisement runs is 15 seconds
For Address Family: IPv4 Unicast
BGP neighbor version 0
Update group: 0.1
eBGP neighbor with no outbound policy; defaults to 'drop'
Route refresh request: received 0, sent 0
Inbound path policy configured
Policy for incoming advertisements is POLICY_1
0 accepted prefixes
Prefix advertised 0, suppressed 0, withdrawn 0, maximum limit 524288
Threshold for warning message 75%
Connections established 0; dropped 0
Last reset 00:01:14, due to BGP neighbor initialized
External BGP neighbor not directly connected.
To illustrate the same rule, the following example shows how to set the advertisement interval to 15 (from the session group)
and 25 (from the neighbor group). The advertisement interval set in the session group overrides the one set in the neighbor
group. The inbound policy is set to POLICY_1 from the neighbor group.
The following output from the show bgp neighbors command shows that the advertisement interval used is 15 seconds:
Router# show bgp neighbors 192.168.2.2
BGP neighbor is 192.168.2.2, remote AS 1, local AS 140, external link
Remote router ID 0.0.0.0
BGP state = Idle
Last read 00:00:00, hold time is 180, keepalive interval is 60 seconds
Received 0 messages, 0 notifications, 0 in queue
Sent 0 messages, 0 notifications, 0 in queue
Minimum time between advertisement runs is 15 seconds
For Address Family: IPv4 Unicast
BGP neighbor version 0
Update group: 0.1
eBGP neighbor with no inbound or outbound policy; defaults to 'drop'
Route refresh request: received 0, sent 0
0 accepted prefixes
Prefix advertised 0, suppressed 0, withdrawn 0, maximum limit 524288
Threshold for warning message 75%
Connections established 0; dropped 0
Last reset 00:02:03, due to BGP neighbor initialized
External BGP neighbor not directly connected.
Otherwise, the default value is used. In the example that follows, neighbor 10.0.101.5 has the minimum time between advertisement
runs set to 30 seconds (default) because the neighbor is not configured to use the neighbor configuration or the neighbor
group configuration:
The following output from the show bgp neighbors command shows that the advertisement interval used is 30 seconds:
Router# show bgp neighbors 10.0.101.5
BGP neighbor is 10.0.101.5, remote AS 1, local AS 140, external link
Remote router ID 0.0.0.0
BGP state = Idle
Last read 00:00:00, hold time is 180, keepalive interval is 60 seconds
Received 0 messages, 0 notifications, 0 in queue
Sent 0 messages, 0 notifications, 0 in queue
Minimum time between advertisement runs is 30 seconds
For Address Family: IPv4 Unicast
BGP neighbor version 0
Update group: 0.2
eBGP neighbor with no inbound or outbound policy; defaults to 'drop'
Route refresh request: received 0, sent 0
0 accepted prefixes
Prefix advertised 0, suppressed 0, withdrawn 0, maximum limit 524288
Threshold for warning message 75%
Connections established 0; dropped 0
Last reset 00:00:25, due to BGP neighbor initialized
External BGP neighbor not directly connected.
The inheritance rules used when groups are inheriting configuration from other groups are the same as the rules given for
neighbors inheriting from groups.
Viewing Inherited Configurations
You can use the following show commands to view BGP inherited configurations:
show bgp neighbors
Use the show bgp neighbors command to display information about the BGP configuration for neighbors.
Use the configuration keyword to display the effective configuration for the neighbor, including any settings that have been inherited from session
groups, neighbor groups, or address family groups used by this neighbor.
Use the inheritance keyword to display the session groups, neighbor groups, and address family groups from which this neighbor is capable of
inheriting configuration.
The show bgp neighbors command examples that follow are based on this sample configuration:
Use the show bgp neighbors command to display information about the BGP configuration for neighbors.
Use the configuration keyword to display the effective configuration for the neighbor, including any settings that have been inherited from session
groups, neighbor groups, or address family groups used by this neighbor.
Use the inheritance keyword to display the session groups, neighbor groups, and address family groups from which this neighbor is capable of
inheriting configuration.
The show bgp neighbors command examples that follow are based on this sample configuration:
Use the show bgp af-group command to display address family groups:
Use the configuration keyword to display the effective configuration for the address family group, including any settings that have been inherited
from address family groups used by this address family group.
Use the inheritance keyword to display the address family groups from which this address family group is capable of inheriting configuration.
Use the users keyword to display the neighbors, neighbor groups, and address family groups that inherit configuration from this address
family group.
The show bgp af-group sample commands that follow are based on this sample configuration:
The following example displays sample output from the show bgp af-group command using the configuration keyword. This example shows from where each configuration item was inherited. The default-originate command was configured directly on this address family group (indicated by [ ]). The remove-private-as command was inherited from address family group GROUP_2, which in turn inherited from address family group GROUP_3:
The following example displays sample output from the show bgp af-group command using the users keyword:
Router# show bgp af-group GROUP_2 users
IPv4 Unicast: a:GROUP_1
The following example displays sample output from the show bgp af-group command using the inheritance keyword. This shows that the specified address family group GROUP_1 directly uses the GROUP_2 address family group, which
in turn uses the GROUP_3 address family group:
Router# show bgp af-group GROUP_1 inheritance
IPv4 Unicast: a:GROUP_2 a:GROUP_3
show bgp session-group
Use the show bgp session-group command to display session groups:
Use the configuration keyword to display the effective configuration for the session group, including any settings that have been inherited from
session groups used by this session group.
Use the inheritance keyword to display the session groups from which this session group is capable of inheriting configuration.
Use the users keyword to display the session groups, neighbor groups, and neighbors that inherit configuration from this session group.
The output from the show bgp session-group command is based on the following session group configuration:
The following is sample output from the show bgp session-group command with the inheritance keyword showing that the GROUP_1 session group inherits session parameters from the GROUP_3 and GROUP_2 session groups:
Router# show bgp session-group GROUP_1 inheritance
Session: s:GROUP_2 s:GROUP_3
The following is sample output from the show bgp session-group command with the users keyword showing that both the GROUP_1 and GROUP_2 session groups inherit session parameters from the GROUP_3 session group:
Router# show bgp session-group GROUP_3 users
Session: s:GROUP_1 s:GROUP_2
show bgp session-group
Use the show bgp session-group command to display session groups:
Use the configuration keyword to display the effective configuration for the session group, including any settings that have been inherited from
session groups used by this session group.
Use the inheritance keyword to display the session groups from which this session group is capable of inheriting configuration.
Use the users keyword to display the session groups, neighbor groups, and neighbors that inherit configuration from this session group.
The output from the show bgp session-group command is based on the following session group configuration:
The following is sample output from the show bgp session-group command with the inheritance keyword showing that the GROUP_1 session group inherits session parameters from the GROUP_3 and GROUP_2 session groups:
Router# show bgp session-group GROUP_1 inheritance
Session: s:GROUP_2 s:GROUP_3
The following is sample output from the show bgp session-group command with the users keyword showing that both the GROUP_1 and GROUP_2 session groups inherit session parameters from the GROUP_3 session group:
Router# show bgp session-group GROUP_3 users
Session: s:GROUP_1 s:GROUP_2
show bgp neighbor-group
Use the show bgp neighbor-group command to display neighbor groups:
Use the configuration keyword to display the effective configuration for the neighbor group, including any settings that have been inherited from
neighbor groups used by this neighbor group.
Use the inheritance keyword to display the address family groups, session groups, and neighbor groups from which this neighbor group is capable
of inheriting configuration.
Use the users keyword to display the neighbors and neighbor groups that inherit configuration from this neighbor group.
The examples are based on the following group configuration:
The following is sample output from the show bgp neighbor-group command with the configuration keyword. The configuration setting source is shown to the right of each command. In the output shown previously, the remote
autonomous system is configured directly on neighbor group GROUP_1, and the send community setting is inherited from neighbor
group GROUP_2, which in turn inherits the setting from address family group GROUP_3:
The following is sample output from the show bgp neighbor-group command with the inheritance keyword. This output shows that the specified neighbor group GROUP_1 inherits session (address family-independent) configuration
parameters from neighbor group GROUP_2. Neighbor group GROUP_2 inherits its session parameters from session group GROUP_3.
It also shows that the GROUP_1 neighbor group inherits IPv4 unicast configuration parameters from the GROUP_2 neighbor group,
which in turn inherits them from the GROUP_2 address family group, which itself inherits them from the GROUP_3 address family
group:
The following is sample output from the show bgp neighbor-group command with the users keyword. This output shows that the GROUP_1 neighbor group inherits session (address family-independent) configuration parameters
from the GROUP_2 neighbor group. The GROUP_1 neighbor group also inherits IPv4 unicast configuration parameters from the GROUP_2
neighbor group:
BGP does not support the concept of a default address family. An address family must be explicitly configured under the BGP
router configuration for the address family to be activated in BGP. Similarly, an address family must be explicitly configured
under a neighbor for the BGP session to be activated under that address family. It is not required to have any address family
configured under the BGP router configuration level for a neighbor to be configured. However, it is a requirement to have
an address family configured at the BGP router configuration level for the address family to be configured under a neighbor.
Neighbor Address Family Combinations
For default VRF, both IPv4 Unicast and IPv4 Labeled-unicast address families are supported under the same neighbor.
For non-default VRF, both IPv4 Unicast and IPv4 Labeled-unicast address families are not supported under the same neighbor.
However, the configuration is accepted on the router with the following error:
bgp[1051]: %ROUTING-BGP-4-INCOMPATIBLE_AFI : IPv4 Unicast and IPv4 Labeled-unicast Address families together are not supported under the same neighbor.
When one BGP session has both IPv4 unicast and IPv4 labeled-unicast AFI/SAF, then the routing behavior is nondeterministic.
Therefore, the prefixes may not be correctly advertised. Incorrect prefix advertisement results in reachability issues. In
order to avoid such reachability issues, you must explicitly configure a route policy to advertise prefixes either through
IPv4 unicast or through IPv4 labeled-unicast address families.
Routing Policy Enforcement
External BGP (eBGP) neighbors must have an inbound and outbound policy configured. If no policy is configured, no routes are
accepted from the neighbor, nor are any routes advertised to it. This added security measure ensures that routes cannot accidentally
be accepted or advertised in the case of a configuration omission error.
Note
This enforcement affects only eBGP neighbors (neighbors in a different autonomous system than this router). For internal BGP
(iBGP) neighbors (neighbors in the same autonomous system), all routes are accepted or advertised if there is no policy.
Table Policy
The table policy feature in BGP allows you to configure traffic index values on routes as they are installed in the global
routing table. This feature is enabled using the table-policy command and supports the BGP policy accounting feature.
BGP policy accounting uses traffic indices that are set on BGP routes to track various counters.
Table policy also provides the ability to drop routes from the RIB based on match criteria. This feature can be useful in
certain applications and should be used with caution as it can easily create a routing ‘black hole’ where BGP advertises routes
to neighbors that BGP does not install in its global routing table and forwarding table.
BGP Update Group
When a change to the configuration occurs, the router automatically recalculates update group memberships and applies the
changes.
For the best optimization of BGP update group generation, we recommend that the network operator keeps outbound routing policy
the same for neighbors that have similar outbound policies. This feature contains commands for monitoring BGP update groups.
BGP Update Generation and Update Groups
The BGP Update Groups feature separates BGP update generation from neighbor configuration. The BGP Update Groups feature introduces
an algorithm that dynamically calculates BGP update group membership based on outbound routing policies. This feature does
not require any configuration by the network operator. Update group-based message generation occurs automatically and independently.
BGP Cost Community
The BGP cost community is a nontransitive extended community attribute that is passed to internal BGP (iBGP) and confederation
peers but not to external BGP (eBGP) peers. The cost community feature allows you to customize the local route preference
and influence the best-path selection process by assigning cost values to specific routes. The extended community format defines
generic points of insertion (POI) that influence the best-path decision at different points in the best-path algorithm.
How BGP Cost Community Influences the Best Path Selection Process
The cost community attribute influences the BGP best-path selection process at the point of insertion (POI). By default, the
POI follows the Interior Gateway Protocol (IGP) metric comparison. When BGP receives multiple paths to the same destination,
it uses the best-path selection process to determine which path is the best path. BGP automatically makes the decision and
installs the best path in the routing table. The POI allows you to assign a preference to a specific path when multiple equal
cost paths are available. If the POI is not valid for local best-path selection, the cost community attribute is silently
ignored.
Cost communities are sorted first by POI then by community ID. Multiple paths can be configured with the cost community attribute
for the same POI. The path with the lowest cost community ID is considered first. In other words, all cost community paths
for a specific POI are considered, starting with the one with the lowest cost community. Paths that do not contain the cost
community cost (for the POI and community ID being evaluated) are assigned the default community cost value (2147483647).
If the cost community values are equal, then cost community comparison proceeds to the next lowest community ID for this POI.
To select the path with the lower cost community, simultaneously walk through the cost communities of both paths. This is
done by maintaining two pointers to the cost community chain, one for each path, and advancing both pointers to the next applicable
cost community at each step of the walk for the given POI, in order of community ID, and stop when a best path is chosen or
the comparison is a tie. At each step of the walk, the following checks are done:
If neither pointer refers to a cost community,
Declare a tie;
Elseif a cost community is found for one path but not for the other,
Choose the path with cost community as best path;
Elseif the Community ID from one path is less than the other,
Choose the path with the lesser Community ID as best path;
Elseif the Cost from one path is less than the other,
Choose the path with the lesser Cost as best path;
Else Continue.
Note
Paths that are not configured with the cost community attribute are considered by the best-path selection process to have
the default cost value (half of the maximum value [4294967295] or 2147483647).
Applying the cost community attribute at the POI allows you to assign a value to a path originated or learned by a peer in
any part of the local autonomous system or confederation. The cost community can be used as a “tie breaker” during the best-path
selection process. Multiple instances of the cost community can be configured for separate equal cost paths within the same
autonomous system or confederation. For example, a lower cost community value can be applied to a specific exit path in a
network with multiple equal cost exit points, and the specific exit path is preferred by the BGP best-path selection process.
.
Note
The cost community comparison in BGP is enabled by default. Use the bgp bestpath cost-community ignore command to disable the comparison.
Cost Community Support for Aggregate Routes and Multipaths
The BGP cost community feature supports aggregate routes and multipaths. The cost community attribute can be applied to either
type of route. The cost community attribute is passed to the aggregate or multipath route from component routes that carry
the cost community attribute. Only unique IDs are passed, and only the highest cost of any individual component route is applied
to the aggregate for each ID. If multiple component routes contain the same ID, the highest configured cost is applied to
the route. For example, the following two component routes are configured with the cost community attribute using an inbound
route policy:
10.0.0.1
POI=IGP
cost community ID=1
cost number=100
192.168.0.1
POI=IGP
cost community ID=1
cost number=200
If these component routes are aggregated or configured as a multipath, the cost value 200 is advertised, because it has the
highest cost.
If one or more component routes do not carry the cost community attribute or the component routes are configured with different
IDs, then the default value (2147483647) is advertised for the aggregate or multipath route. For example, the following three
component routes are configured with the cost community attribute using an inbound route policy. However, the component routes
are configured with two different IDs.
10.0.0.1
POI=IGP
cost community ID=1
cost number=100
172.16.0.1
POI=IGP
cost community ID=2
cost number=100
192.168.0.1
POI=IGP
cost community ID=1
cost number=200
The single advertised path includes the aggregate cost communities as follows:
Influencing Route Preference in a Multiexit IGP Network
This figure shows an IGP network with two autonomous system boundary routers (ASBRs) on the edge. Each ASBR has an equal cost
path to network 10.8/16.
Figure 5. Multiexit Point IGP Network
Both paths are considered to be equal by BGP. If multipath loadsharing is configured, both paths to the routing table are
installed and are used to balance the load of traffic. If multipath load balancing is not configured, the BGP selects the
path that was learned first as the best path and installs this path to the routing table. This behavior may not be desirable
under some conditions. For example, the path is learned from ISP1 PE2 first, but the link between ISP1 PE2 and ASBR1 is a
low-speed link.
The configuration of the cost community attribute can be used to influence the BGP best-path selection process by applying
a lower-cost community value to the path learned by ASBR2. For example, the following configuration is applied to ASBR2:
Router(config)# route-policy ISP2_PE1
Router(config-rpl)# set extcommunity cost (1:1)
The preceding route policy applies a cost community number of 1 to the 10.8.0.0 route. By default, the path learned from ASBR1
is assigned a cost community number of 2147483647. Because the path learned from ASBR2 has a lower-cost community number,
the path is preferred.
Adding Routes to the Routing Information Base
If a nonsourced path becomes the best path after the best-path calculation, BGP adds the route to the Routing Information
Base (RIB) and passes the cost communities along with the other IGP extended communities.
When a route with paths is added to the RIB by a protocol, RIB checks the current best paths for the route and the added paths
for cost extended communities. If cost-extended communities are found, the RIB compares the set of cost communities. If the
comparison does not result in a tie, the appropriate best path is chosen. If the comparison results in a tie, the RIB proceeds
with the remaining steps of the best-path algorithm. If a cost community is not present in either the current best paths or
added paths, then the RIB continues with the remaining steps of the best-path algorithm.
BGP DMZ Aggregate Bandwidth
Table 7. Feature History Table
Feature Name
Release Information
Feature Description
Removal of Link-Bandwidth Extended Community to iBGP Peers
Release 7.3.2
The demilitarized zone (DMZ) link-bandwidth extended community allows BGP to send traffic
over multiple internal BGP (iBGP) learned paths. The traffic that is
sent is proportional to the bandwidth of the links that are used to
exit the autonomous system. By default, iBGP propagates DMZ
link-bandwidth community. This feature minimizes the risk of
exposure of the community parameters, which are used to control the
routing policy in the service provider network, to networks zones
where they are not recognized or not required.
BGP supports aggregating dmz-link bandwidth values of external BGP (eBGP)
multipaths when advertising the route to interior BGP (iBGP) peer.
There is no explicit command to aggregate bandwidth. The bandwidth is
aggregated if following conditions are met:
The network has multipaths and all the multipaths have link-bandwidth values.
The next-hop attribute set to next-hop-self. The next-hop attribute for all
routes advertised to the specified neighbor to the address of the local router.
There is no out-bound policy configured that might change the dmz-link bandwidth
value.
If the dmz-link bandwidth value is not known for any one of the
multipaths (eBGP or iBGP), the dmz-link value for all multipaths
including the best path is not downloaded to routing information base (RIB).
The dmz-link bandwidth value of iBGP multipath is not considered
during aggregation.
The route that is advertised with aggregate value can be best path or add-path.
Add-path does not qualify for DMZ link bandwidth aggregation as next hop is
preserved. Configuring next-hop-self for add-path is not supported.
For VPNv4 and VPNv6 afi, if dmz link-bandwidth value is configured
using outbound route-policy, specify the route table or use the
additive keyword. Else, this will lead to
routes not imported on the receiving end of the peer.
Removal of Link-Bandwidth Extended Community to iBGP Peers
The demilitarized zone (DMZ) link-bandwidth extended community allows BGP to send traffic over
multiple internal BGP (iBGP) learned paths. The traffic that is sent is proportional
to the bandwidth of the links that are used to exit the autonomous system. By
default, iBGP propagates DMZ link-bandwidth community. The Removal of Link-Bandwidth
Extended Community to iBGP Peers feature provides the flexibility to remove the DMZ
link-bandwidth community to minimize the risk of exposure of the community
parameters to networks zones where they are not recognized or unnecessary.
Configuration Example
Perform the following steps to allow users to be able to configure route-policy to
remove the extended communities.
/* Delete all the extended communities. */
Router(config)# route-policy dmz_del_all
Router(config-rpl)# delete extcommunity bandwidth all
Router(config-rpl)# pass
Router(config-rpl)# end-policy
/* Delete only the extended communities that match an extended community mentioned in the list. */
Router(config)# route-policy dmz_CE1_del_non_match
Router(config-rpl)# if destination in (10.9.9.9/32) then
Router(config-rpl-if)# delete extcommunity bandwidth in (10:7000)
Router(config-rpl-if)# endif
Router(config-rpl)# pass
Router(config-rpl)# end-policy
/* Delete all the extended communities. */
Router(config)# route-policy dmz_del_param2($a,$b)
Router(config-rpl)# if destination in (10.9.9.9/32) then
Router(config-rpl-if)# delete extcommunity bandwidth in ($a:$b)
Router(config-rpl-if)# endif
Router(config-rpl)# pass
Router(config-rpl)# end-policy
Verification
Verify the configuration that allows the user to remove a particular extended
community.
Router# show bgp 10.9.9.9/32
Fri Aug 27 13:15:05.833 EDT
BGP routing table entry for 10.9.9.9/32
Versions:
Process bRIB/RIB SendTblVer
Speaker 15 15
Last Modified: Aug 27 13:06:45.000 for 00:08:21
Paths: (3 available, best #1)
Advertised IPv4 Unicast paths to peers (in unique update groups):
13.13.13.5
Path #1: Received by speaker 0
Advertised IPv4 Unicast paths to peers (in unique update groups):
13.13.13.5
10
10.10.10.1 from 10.10.10.1 (192.168.0.1)
Origin incomplete, metric 0, localpref 100, valid, external, best, group-best, multipath
Received Path ID 0, Local Path ID 1, version 15
Extended community: LB:10:48
Origin-AS validity: (disabled)
Path #2: Received by speaker 0
Not advertised to any peer
10
11.11.11.3 from 11.11.11.3 (192.168.0.3)
Origin incomplete, metric 0, localpref 100, valid, external, multipath
Received Path ID 0, Local Path ID 0, version 0
Extended community: LB:10:48
Origin-AS validity: (disabled)
Path #3: Received by speaker 0
Not advertised to any peer
10
12.12.12.4 from 12.12.12.4 (192.168.0.4)
Origin incomplete, metric 0, localpref 100, valid, external, multipath
Received Path ID 0, Local Path ID 0, version 0
Extended community: LB:10:48
Origin-AS validity: (disabled)
22:35 30-09-2021
Configuring BGP DMZ Aggregate Bandwidth: Example
This is a sample configuration for Border Gateway Protocol Demilitarized Zone (BGP DMZ) link bandwidth. Consider the topology,
R1---(iBGP)---R2---(iBGP)---R3:
On R1:
bgp: prefix p/n has:
path 1(bestpath) with LB value 100
path 2(ebgp multipath) with LB value 30
path 3(ebgp multipath) with LB value 50
When best path is advertised to R2, send aggregated dmz-link bandwidth value of 180; aggregated value of paths 1, 2 and 3.
On R2:
bgp: prefix p/n has:
path 1(bestpath) with LB value 60
path 2(ebgp multipath) with LB value 200
path 3(ebgp multipath) with LB value 50
When best path is advertised to R3, send aggregated dmz-link bandwidth value of 310; aggregated value of paths 1, 2 and 3.
On R3:
bgp: prefix p/n has:
path 1(bestpath) with LB 180 {learned from R1}
path 2(ibgp multipath) with LB 310 {learned from R2}
Configuring Policy-based Link Bandwidth: Example
This is a sample configuration for policy-based DMZ link bandwidth. The link-bandwidth ext-community can be set on a per-path basis either at the neighbor-in or neighbor-out policy attach-points. The dmz-link-bandwidth knob is configured under eBGP neighbor configuration mode. All paths received from that particular neighbor will be marked
with the link-bandwidth extended community when sent to iBGP peers.
neighbor 10.0.101.2
remote-as 1001
dmz-link-bandwidth <<< Under neighbor.
address-family ipv4 unicast
route-policy pass in
route-policy pass out
!
64-ECMP Support for BGP
IOS XR supports configuration of up to 64 equal cost multipath (ECMP) next hops for BGP. 64-ECMP is required in networks,
where overloaded routers can load balance the traffic over as many as 64 LSPs.
BGP Best Path Algorithm
BGP routers typically receive multiple paths to the same destination. The BGP best-path algorithm determines the best path
to install in the IP routing table and to use for forwarding traffic. This section describes the Cisco IOS XR software implementation
of BGP best-path algorithm, as specified in Section 9.1 of the Internet Engineering Task Force (IETF) Network Working Group
draft-ietf-idr-bgp4-24.txt document.
The BGP best-path algorithm implementation is in three parts:
Part 1—Compares two paths to determine which is better.
Part 2—Iterates over all paths and determines which order to compare the paths to select the overall best path.
Part 3—Determines whether the old and new best paths differ enough so that the new best path should be used.
Note
The order of comparison determined by Part 2 is important because the comparison operation is not transitive; that is, if
three paths, A, B, and C exist, such that when A and B are compared, A is better, and when B and C are compared, B is better,
it is not necessarily the case that when A and C are compared, A is better. This nontransitivity arises because the multi
exit discriminator (MED) is compared only among paths from the same neighboring autonomous system (AS) and not among all paths.
Comparing Pairs of Paths
Perform the following steps to compare two paths and determine the better path:
If either path is invalid (for example, a path has the maximum possible MED value or it has an unreachable next hop), then
the other path is chosen (provided that the path is valid).
If the paths have unequal pre-bestpath cost communities, the path with the lower pre-bestpath cost community is selected as
the best path.
If the paths have unequal weights, the path with the highest weight is chosen.
Note
The weight is entirely local to the router, and can be set with the weight command or using a routing policy.
If the paths have unequal local preferences, the path with the higher local preference is chosen.
Note
If a local preference attribute was received with the path or was set by a routing policy, then that value is used in this
comparison. Otherwise, the default local preference value of 100 is used. The default value can be changed using the bgp default local-preference command.
If one of the paths is a redistributed path, which results from a redistribute or network command, then it is chosen. Otherwise, if one of the paths is a locally generated aggregate, which results from an aggregate-address command, it is chosen.
Note
Step 1 through Step 4 implement the “Path Selection with BGP”of RFC 1268.
If the paths have unequal AS path lengths, the path with the shorter AS path is chosen. This step is skipped if bgp bestpath as-path ignore command is configured.
Note
When calculating the length of the AS path, confederation segments are ignored, and AS sets count as 1.
Note
eiBGP specifies internal and external BGP multipath peers. eiBGP allows simultaneous use of internal and external paths.
If the paths have different origins, the path with the lower origin is selected. Interior Gateway Protocol (IGP) is considered
lower than EGP, which is considered lower than INCOMPLETE.
If appropriate, the MED of the paths is compared. If they are unequal, the path with the lower MED is chosen.
A number of configuration options exist that affect whether or not this step is performed. In general, the MED is compared
if both paths were received from neighbors in the same AS; otherwise the MED comparison is skipped. However, this behavior
is modified by certain configuration options, and there are also some corner cases to consider.
If the bgp bestpath med always command is configured, then the MED comparison is always performed, regardless of neighbor AS in the paths. Otherwise, MED
comparison depends on the AS paths of the two paths being compared, as follows:
If a path has no AS path or the AS path starts with an AS_SET, then the path is considered to be internal, and the MED is
compared with other internal paths.
If the AS path starts with an AS_SEQUENCE, then the neighbor AS is the first AS number in the sequence, and the MED is compared
with other paths that have the same neighbor AS.
If the AS path contains only confederation segments or starts with confederation segments followed by an AS_SET, then the
MED is not compared with any other path unless the bgp bestpath med confed command is configured. In that case, the path is considered internal and the MED is compared with other internal paths.
If the AS path starts with confederation segments followed by an AS_SEQUENCE, then the neighbor AS is the first AS number
in the AS_SEQUENCE, and the MED is compared with other paths that have the same neighbor AS.
Note
If no MED attribute was received with the path, then the MED is considered to be 0 unless the bgp bestpath med missing-as-worst command is configured. In that case, if no MED attribute was received, the MED is considered to be the highest possible value.
If one path is received from an external peer and the other is received from an internal (or confederation) peer, the path
from the external peer is chosen.
If the paths have different IGP metrics to their next hops, the path with the lower IGP metric is chosen.
If the paths have unequal IP cost communities, the path with the lower IP cost community is selected as the best path.
If all path parameters in Step 1 through Step 10 are the same, then the router IDs are compared. If the path was received
with an originator attribute, then that is used as the router ID to compare; otherwise, the router ID of the neighbor from
which the path was received is used. If the paths have different router IDs, the path with the lower router ID is chosen.
Note
Where the originator is used as the router ID, it is possible to have two paths with the same router ID. It is also possible
to have two BGP sessions with the same peer router, and therefore receive two paths with the same router ID.
If the paths have different cluster lengths, the path with the shorter cluster length is selected. If a path was not received
with a cluster list attribute, it is considered to have a cluster length of 0.
Finally, the path received from the neighbor with the lower IP address is chosen. Locally generated paths (for example, redistributed
paths) are considered to have a neighbor IP address of 0.
Order of Comparisons
The second part of the BGP best-path algorithm implementation determines the order in which the paths should be compared.
The order of comparison is determined as follows:
The paths are partitioned into groups such that within each group the MED can be compared among all paths. The same rules
as in Comparing Paths section are used to determine whether MED can be compared between any two paths. Normally, this comparison results in one
group for each neighbor AS. If the bgp bestpath med always command is configured, then there is just one group containing all the paths.
The best path in each group is determined. Determining the best path is achieved by iterating through all paths in the group
and keeping track of the best one seen so far. Each path is compared with the best-so-far, and if it is better, it becomes
the new best-so-far and is compared with the next path in the group.
A set of paths is formed containing the best path selected from each group in Step 2. The overall best path is selected from
this set of paths, by iterating through them as in Step 2.
Best Path Change Suppression
The third part of the implementation is to determine whether the best-path change can be suppressed or not—whether the new
best path should be used, or continue using the existing best path. The existing best path can continue to be used if the
new one is identical to the point at which the best-path selection algorithm becomes arbitrary (if the router-id is the same).
Continuing to use the existing best path can avoid churn in the network.
Note
This suppression behavior does not comply with the IETF Networking Working Group draft-ietf-idr-bgp4-24.txt document, but
is specified in the IETF Networking Working Group draft-ietf-idr-avoid-transition-00.txt document.
The suppression behavior can be turned off by configuring the bgp bestpath compare-routerid command. If this command is configured, the new best path is always preferred to the existing one.
Otherwise, the following steps are used to determine whether the best-path change can be suppressed:
If the existing best path is no longer valid, the change cannot be suppressed.
If either the existing or new best paths were received from internal (or confederation) peers or were locally generated (for
example, by redistribution), then the change cannot be suppressed. That is, suppression is possible only if both paths were
received from external peers.
If the paths were received from the same peer (the paths would have the same router-id), the change cannot be suppressed.
The router ID is calculated using rules in Comparing Pairs of Paths section.
If the paths have different weights, local preferences, origins, or IGP metrics to their next hops, then the change cannot
be suppressed. Note that all these values are calculated using the rules in Comparing Pairs of Paths section..
If the paths have different-length AS paths and the bgp bestpath as-path ignore command is not configured, then the change cannot be suppressed. Again, the AS path length is calculated using the rules in
Comparing Pairs of Paths section.
If the MED of the paths can be compared and the MEDs are different, then the change cannot be suppressed. The decision as
to whether the MEDs can be compared is exactly the same as the rules in Comparing Pairs of Paths section, as is the calculation of the MED value.
If all path parameters in Step 1 through Step 6 do not apply, the change can be suppressed.
Administrative Distance
An administrative distance is a rating of the trustworthiness of a routing information source. In general, the higher the
value, the lower the trust rating.
Normally, a route can be learned through more than one protocol. Administrative distance is used to discriminate between routes
learned from more than one protocol. The route with the lowest administrative distance is installed in the IP routing table.
By default, BGP uses the administrative distances shown in BGP Default Administrative Distances section.
Table 8. BGP Default Administrative Distances
Distance
Default Value
Function
External
20
Applied to routes learned from eBGP.
Internal
200
Applied to routes learned from iBGP.
Local
200
Applied to routes originated by the router.
Note
Distance does not influence the BGP path selection algorithm, but it does influence whether BGP-learned routes are installed
in the IP routing table.
In most cases, when a route is learned through eBGP, it is installed in the IP routing table because of its distance (20).
Sometimes, however, two ASs have an IGP-learned back-door route and an eBGP-learned route. Their policy might be to use the
IGP-learned path as the preferred path and to use the eBGP-learned path when the IGP path is down.
Figure 6. Back Door Example
In Back Door Example section, Routers A and C and Routers B and C are running eBGP. Routers A and B are running an IGP (such as Routing Information
Protocol [RIP], Interior Gateway Routing Protocol [IGRP], Enhanced IGRP, or Open Shortest Path First [OSPF]). The default
distances for RIP, IGRP, Enhanced IGRP, and OSPF are 120, 100, 90, and 110, respectively. All these distances are higher than
the default distance of eBGP, which is 20. Usually, the route with the lowest distance is preferred.
Router A receives updates about 160.10.0.0 from two routing protocols: eBGP and IGP. Because the default distance for eBGP
is lower than the default distance of the IGP, Router A chooses the eBGP-learned route from Router C. If you want Router A
to learn about 160.10.0.0 from Router B (IGP), establish a BGP back door. See .
In the following example, a network back-door is configured:
Router A treats the eBGP-learned route as local and installs it in the IP routing table with a distance of 200. The network
is also learned through Enhanced IGRP (with a distance of 90), so the Enhanced IGRP route is successfully installed in the
IP routing table and is used to forward traffic. If the Enhanced IGRP-learned route goes down, the eBGP-learned route is installed
in the IP routing table and is used to forward traffic.
Although BGP treats network 160.10.0.0 as a local entry, it does not advertise network 160.10.0.0 as it normally would advertise
a local entry.
Route Dampening
Route dampening is a BGP feature that minimizes the propagation of flapping routes across an internetwork. A route is considered
to be flapping when it is repeatedly available, then unavailable, then available, then unavailable, and so on.
For example, consider a network with three BGP autonomous systems: autonomous system 1, autonomous system 2, and autonomous
system 3. Suppose the route to network A in autonomous system 1 flaps (it becomes unavailable). Under circumstances without
route dampening, the eBGP neighbor of autonomous system 1 to autonomous system 2 sends a withdraw message to autonomous system 2.
The border router in autonomous system 2, in turn, propagates the withdrawal message to autonomous system 3. When the route
to network A reappears, autonomous system 1 sends an advertisement message to autonomous system 2, which sends it to autonomous
system 3. If the route to network A repeatedly becomes unavailable, then available, many withdrawal and advertisement messages
are sent. Route flapping is a problem in an internetwork connected to the Internet, because a route flap in the Internet backbone
usually involves many routes.
Minimize Flapping
The route dampening feature minimizes the flapping problem as follows. Suppose again that the route to network A flaps. The
router in autonomous system 2 (in which route dampening is enabled) assigns network A a penalty of 1000 and moves it to history
state. The router in autonomous system 2 continues to advertise the status of the route to neighbors. The penalties are cumulative.
When the route flaps so often that the penalty exceeds a configurable suppression limit, the router stops advertising the
route to network A, regardless of how many times it flaps. Thus, the route is dampened.
The penalty placed on network A is decayed until the reuse limit is reached, upon which the route is once again advertised.
At half of the reuse limit, the dampening information for the route to network A is removed.
Note
No penalty is applied to a BGP peer reset when route dampening is enabled, even though the reset withdraws the route.
BGP Routing Domain Confederation
One way to reduce the iBGP mesh is to divide an autonomous system into multiple sub-autonomous systems and group them into
a single confederation. To the outside world, the confederation looks like a single autonomous system. Each autonomous system
is fully meshed within itself and has a few connections to other autonomous systems in the same confederation. Although the
peers in different autonomous systems have eBGP sessions, they exchange routing information as if they were iBGP peers. Specifically,
the next hop, MED, and local preference information is preserved. This feature allows you to retain a single IGP for all of
the autonomous systems.
BGP Optimal Route Reflector
BGP-ORR (optimal route reflector) enables virtual route reflector (vRR) to calculate the best path from a route reflector
(RR) client's point of view.
BGP ORR calculates the best path by:
Running SPF multiple times in the context of its RR clients or RR clusters (set of RR clients)
Saving the result of different SPF runs in separate databases
Using these databases to manipulate BGP best path decision and thereby allowing BGP to use and announce best path that is
optimal from the client’s point of view
Note
Enabling the ORR feature increases the memory footprint of BGP and RIB. With increased number of vRR configured in the network,
ORR adversely impacts convergence for BGP.
In an autonomous system, a BGP route reflector acts as a focal point and advertises routes to its peers (RR clients) along
with the RR's computed best path. Since the best path advertised by the RR is computed from the RR's point of view, the RR's
placement becomes an important deployment consideration.
With network function virtualization (NFV) becoming a dominant technology, service providers (SPs) are hosting virtual RR
functionality in a cloud using servers. A vRR can run on a control plane device and can be placed anywhere in the topology
or in a SP data center. Cisco IOS XRv 9000 Router can be implemented as vRR over a NFV platform in a SP data center. vRR allows
SPs to scale memory and CPU usage of RR deployments significantly. Moving a RR out of its optimal placement requires vRRs
to implement ORR functionality that calculates the best path from a RR client's point of view.
BGP ORR offers these benefits:
Calculates the bestpath from the point of view of a RR client.
Enables vRR to be placed anywhere in the topology or in a SP data center.
Allows SPs to scale memory and CPU usage of RR deployments.
Use Case
Consider a BGP Route Reflector topology where:
Router R1, R2, R3, R4, R5 and R6 are route reflector clients
Router R1 and R4 advertise 6/8 prefix to vRR
Figure 7. BGP-ORR Topology
vRR receives prefix 6/8 from R1 and R4. Without BGP ORR configured in the network, the vRR selects R4 as the closest exit
point for RR clients R2, R3, R5, and R6, and reflects the 6/8 prefix learned from R4 to these RR clients R2, R3, R5, and R6.
From the topology, it is evident that for R2 the best path is R1 and not R4. This is because the vRR calculates best path
from the RR's point of view.
When the BGP ORR is configured in the network, the vRR calculates the shortest exit point in the network from R2’s point of
view (ORR Root: R2) and determines that R1 is the closest exit point to R2. vRR then reflects the 6/8 prefix learned from
R1 to R2.
Configuring BGP ORR includes:
enabling ORR on the RR for the client whose shortest exit point is to be determined
applying the ORR configuration to the neighbor
Enabling ORR on vRR for R2 (RR client)
For example to determine shortest exit point for R2; configure ORR on vRR with an IP address of R2 that is 192.0.2.2. Use
6500 as AS number and g1 as orr (root) policy name:
Next, apply the ORR policy to BGP neighbor R2 (this enables RR to advertise best path calculated using the root IP address,
192.0.2.2, configured in orr (root) policy g1 to R2):
Configuring MPLS Traffic-Engineering on Root Router
The root routers advertise the Multi Protocol Label Switching (MPLS) TE router-ID that matches with the configured root address
on the RR. So, you must configure the root router with a minimal MPLS TE configuration to advertise this MPLS TE router-ID.
The minimal set of commands that you need to configure depends on the operating system of the root router.
The following is a sample configuration on the root router:
To verify whether R2 received the best exit, execute the show bgp <prefix> command (from R2) in EXEC mode. In the above example, R1 and R4 advertise the 6/8 prefix; run the show bgp 6.0.0.0/8 command:
R2# show bgp 6.0.0.0/8
Tue Apr 5 20:21:58.509 UTC
BGP routing table entry for 6.0.0.0/8
Versions:
Process bRIB/RIB SendTblVer
Speaker 8 8
Last Modified: Apr 5 20:00:44.022 for 00:21:14
Paths: (1 available, best #1)
Not advertised to any peer
Path #1: Received by speaker 0
Not advertised to any peer
Local
192.0.2.1 (metric 20) from 203.0.113.1 (192.0.2.1)
Origin incomplete, metric 0, localpref 100, valid, internal, best, group-best
Received Path ID 0, Local Path ID 1, version 8
Originator: 192.0.2.1, Cluster list: 203.0.113.1
The above show output states that the best path for R2 is through R1, whose IP address is 192.0.2.1 and the metric of the
path is 20.
Execute the show bgp command from the vRR to determine the best path calculated for R2 by ORR. R2 has its own update-group because it has a different
best path (or different policy configured) than those of other peers:
VRR#show bgp 6.0.0.0/8
Thu Apr 28 13:36:42.744 UTC
BGP routing table entry for 6.0.0.0/8
Versions:
Process bRIB/RIB SendTblVer
Speaker 13 13
Last Modified: Apr 28 13:36:26.909 for 00:00:15
Paths: (2 available, best #2)
Advertised to update-groups (with more than one peer):
0.2
Path #1: Received by speaker 0
ORR bestpath for update-groups (with more than one peer):
0.1
Local, (Received from a RR-client)
192.0.2.1 (metric 30) from 192.0.2.1 (192.0.2.1)
Origin incomplete, metric 0, localpref 100, valid, internal, add-path
Received Path ID 0, Local Path ID 2, version 13
Path #2: Received by speaker 0
Advertised to update-groups (with more than one peer):
0.2
ORR addpath for update-groups (with more than one peer):
0.1
Local, (Received from a RR-client)
192.0.2.4 (metric 20) from 192.0.2.4 (192.0.2.4)
Origin incomplete, metric 0, localpref 100, valid, internal, best, group-best
Received Path ID 0, Local Path ID 1, version 13
Note
Path #1 is advertised to update-group 0.1. R2 is in update-group 0.1.
Execute the show bgp command for update-group 0.1 verify whether R2 is in update-group 0.1.
VRR#show bgp update-group 0.1
Thu Apr 28 13:38:18.517 UTC
Update group for IPv4 Unicast, index 0.1:
Attributes:
Neighbor sessions are IPv4
Internal
Common admin
First neighbor AS: 65000
Send communities
Send GSHUT community if originated
Send extended communities
Route Reflector Client
ORR root (configured): g1; Index: 0
4-byte AS capable
Non-labeled address-family capable
Send AIGP
Send multicast attributes
Minimum advertisement interval: 0 secs
Update group desynchronized: 0
Sub-groups merged: 0
Number of refresh subgroups: 0
Messages formatted: 5, replicated: 5
All neighbors are assigned to sub-group(s)
Neighbors in sub-group: 0.2, Filter-Groups num:1
Neighbors in filter-group: 0.2(RT num: 0)
192.0.2.2
For further verification, check the contents of the table created on vRR as a result of configuring the g1 policy. From R2’s
point of view, the cost of reaching R1 is 20 and the cost of reaching R4 is 30. Therefore, the closest and best exit for R2
is through R1:
Border Gateway Protocol (BGP) routers receive multiple paths to the same destination. As a standard, by default the BGP best
path algorithm decides the best path to install in IP routing table. This is used for traffic forwarding.
BGP assigns the first valid path as the current best path. It then compares the best path with the next in the list. This
process continues, until BGP reaches the end of the list of valid paths. This contains all rules used to determine the best
path. When there are multiple paths for a given address prefix, BGP:
Selects one of the paths as the best path as per the best-path selection rules.
Installs the best path in its forwarding table. Each BGP speaker advertises only the best-path to its peers.
Note
The advertisement rule of sending only the best path does not convey the full routing state of a destination, present on a
BGP speaker to its peers.
After the BGP speaker receives a path from one of its peers; the path is used by the peer for forwarding packets. All other
peers receive the same path from this peer. This leads to a consistent routing in a BGP network. To improve the link bandwidth
utilization, most BGP implementations choose additional paths satisfy certain conditions, as multi-path, and install them
in the forwarding table. Incoming packets for such are load-balanced across the best-path and the multi-path(s). You can install
the paths in the forwarding table that are not advertised to the peers. The RR route reflector finds out the best-path and
multi-path. This way the route reflector uses different communities for best-path and multi-path. This feature allows BGP
to signal the local decision done by RR or Border Router. With this new feature, selected by RR using community-string (if
is-best-path then community 100:100). The controller checks which best path is sent to all R's. Border Gateway Protocol routers
receive multiple paths to the same destination. While carrying out best path computation there will be one best path, sometimes
equal and few non-equal paths. Thus, the requirement for a best-path and is-equal-best-path.
The BGP best path algorithm decides the best path in the IP routing table and used for forwarding traffic. This enhancement
within the RPL allows creating policy to take decisions. Adding community-string for local selection of best path. With introduction
of BGP Additional Path (Add Path), BGP now signals more than the best Path. BGP can signal the best path and the entire path
equivalent to the best path. This is in accordance to the BGP multi-path rules and all backup paths.
Remotely Triggered Blackhole Filtering with RPL Next-hop Discard Configuration
Remotely triggered black hole (RTBH) filtering is a technique that provides the ability to drop undesirable traffic before
it enters a protected network. RTBH filtering provides a method for quickly dropping undesirable traffic at the edge of the
network, based on either source addresses or destination addresses by forwarding it to a null0 interface. RTBH filtering based
on a destination address is commonly known as Destination-based RTBH filtering. Whereas, RTBH filtering based on a source
address is known as Source-based RTBH filtering.
RTBH filtering is one of the many techniques in the security toolkit that can be used together to enhance network security
in the following ways:
Effectively mitigate DDoS and worm attacks
Quarantine all traffic destined for the target under attack
Enforce blocklist filtering
Configure Destination-based RTBH Filtering
RTBH is implemented by defining a route policy (RPL) to discard undesirable traffic at next-hop using set next-hop discard command.
RTBH filtering sets the next-hop of the victim's prefix to the null interface. The traffic destined to the victim is dropped
at the ingress.
The set next-hop discard configuration is used in the neighbor inbound policy. When this config is applied to a path, though the primary next-hop
is associated with the actual path but the RIB is updated with next-hop set to Null0. Even if the primary received next-hop
is unreachable, the RTBH path is considered reachable and will be a candidate in the bestpath selection process. The RTBH
path is readvertised to other peers with either the received next-hop or nexthop-self based on normal BGP advertisement rules.
A typical deployment scenario for RTBH filtering would require running internal Border Gateway Protocol (iBGP) at the access
and aggregation points and configuring a separate device in the network operations center (NOC) to act as a trigger. The triggering
device sends iBGP updates to the edge, that cause undesirable traffic to be forwarded to a null0 interface and dropped.
Consider below topology, where a rogue router is sending traffic to a border router.
Figure 8. Topology to Implement RTBH Filtering
Configurations applied on the Trigger Router
Configure a static route redistribution policy that sets a community on static routes marked with a special tag, and apply
it in BGP:
route-policy RTBH-trigger
if tag is 777 then
set community (1234:4321, no-export) additive
pass
else
pass
endif
end-policy
router bgp 65001
address-family ipv4 unicast
redistribute static route-policy RTBH-trigger
!
neighbor 192.168.102.1
remote-as 65001
address-family ipv4 unicast
route-policy bgp_all in
route-policy bgp_all out
Configure a static route with the special tag for the source prefix that has to be block-holed:
router static
address-family ipv4 unicast
10.7.7.7/32 Null0 tag 777
Configurations applied on the Border Router
Configure a route policy that matches the community set on the trigger router and configure set next-hop discard:
route-policy RTBH
if community matches-any (1234:4321) then
set next-hop discard
else
pass
endif
end-policy
Apply the route policy on the iBGP peers:
router bgp 65001
address-family ipv4 unicast
!
neighbor 192.168.102.2
remote-as 65001
address-family ipv4 unicast
route-policy RTBH in
route-policy bgp_all out
Default Address Family for show Commands
Most of the show commands provide address family (AFI) and subaddress family (SAFI) arguments (see RFC 1700 and RFC 2858 for information on
AFI and SAFI). The Cisco IOS XR software parser provides the ability to set the afi and safi so that it is not necessary to
specify them while running a show command. The parser commands are:
set default-afi {ipv4 | ipv6 | all}
set default-safi{unicast | multicast | all}
The parser automatically sets the default afi value to ipv4 and default safi value to unicast. It is necessary to use only the parser commands to change the default afi value from ipv4 or default safi value from unicast. Any afi or safi keyword specified in a show command overrides the values set using the parser commands. Use the following show default-afi-safi-vrf command to check the currently set value of the afi and safi.
TCP Maximum Segment Size
Maximum Segment Size (MSS) is the largest amount of data that a computer or a communication device can receive in a single,
unfragmented TCP segment. All TCP sessions are bounded by a limit on the number of bytes that can be transported in a single
packet; this limit is MSS. TCP breaks up packets into chunks in a transmit queue before passing packets down to the IP layer.
The TCP MSS value is dependent on the maximum transmission unit (MTU) of an interface, which is the maximum length of data
that can be transmitted by a protocol at one instance. The maximum TCP packet length is determined by both the MTU of the
outbound interface on the source device and the MSS announced by the destination device during the TCP setup process. The
closer the MSS is to the MTU, the more efficient is the transfer of BGP messages. Each direction of data flow can use a different
MSS value.
Per Neighbor TCP MSS
The per neighbor TCP MSS feature allows you to create unique TCP MSS profiles for each neighbor. Per neighbor TCP MSS is supported
in two modes: neighbor group and session group. Before, TCP MSS configuration was available only at the global level in the
BGP configuration.
The per neighbor TCP MSS feature allows you to:
Enable per neighbor TCP MSS configuration.
Disable TCP MSS for a particular neighbor in the neighbor group or session group using the inheritance-disable command.
Unconfigure TCP MSS value. On unconfiguration, TCP MSS value in the protocol control block (PCB) is set to the default value.
Note
The default TCP MSS value is 536 (in octets) or 1460 (in bytes). The MSS default of 1460 means that TCP segments the data
in the transmit queue into 1460-byte chunks before passing the packets to the IP layer.
To configure per neighbor TCP MSS, use the tcp mss command under per neighbor, neighbor group or session group configuration.
For detailed configuration steps, see the Configuring Per Neighbor TCP MSS section.
For detailed steps to disable per neighbor TCP MSS, see the Disabling Per Neighbor TCP MSS section.
BGP Keychains
BGP keychains enable keychain authentication between two BGP peers. The BGP endpoints must both comply with draft-bonica-tcp-auth-05.txt
and a keychain on one endpoint and a password on the other endpoint does not work.
BGP is able to use the keychain to implement hitless key rollover for authentication. Key rollover specification is time based,
and in the event of clock skew between the peers, the rollover process is impacted. The configurable tolerance specification
allows for the accept window to be extended (before and after) by that margin. This accept window facilitates a hitless key
rollover for applications (for example, routing and management protocols).
The key rollover does not impact the BGP session, unless there is a keychain configuration mismatch at the endpoints resulting
in no common keys for the session traffic (send or accept).
BGP Nonstop Routing
The Border Gateway Protocol (BGP) Nonstop Routing (NSR) with Stateful Switchover (SSO) feature enables all bgp peerings to
maintain the BGP state and ensure continuous packet forwarding during events that could interrupt service. Under NSR, events
that might potentially interrupt service are not visible to peer routers. Protocol sessions are not interrupted and routing
states are maintained across process restarts and switchovers.
BGP NSR provides nonstop routing during the following events:
Route processor switchover
Process crash or process failure of BGP or TCP
Note
BGP NSR is enabled by default. Use the nsr disable command to turn off BGP NSR. The no nsr disable command can also be used to turn BGP NSR back on if it has been disabled.
In case of process crash or process failure, NSR will be maintained only if nsr process-failures switchover command is configured. In the event of process failures of active instances, the nsr process-failures switchover configures failover as a recovery action and switches over to a standby route processor (RP) or a standby distributed route
processor (DRP) thereby maintaining NSR. An example of the configuration command is RP/0/RSP0/CPU0:router(config) # nsr process-failures switchover
The nsr process-failures switchover command maintains both the NSR and BGP sessions in the event of a BGP or TCP process crash. Without this configuration, BGP
neighbor sessions flap in case of a BGP or TCP process crash. This configuration does not help if the BGP or TCP process is
restarted in which case the BGP neighbors are expected to flap.
When the l2vpn_mgr process is restarted, the NSR client (te-control) flaps between the Ready and Not Ready state. This is the expected behavior and there is no traffic loss.
During route processor switchover and In-Service System Upgrade (ISSU), NSR is achieved by stateful switchover (SSO) of both
TCP and BGP.
NSR does not force any software upgrades on other routers in the network, and peer routers are not required to support NSR.
When a route processor switchover occurs due to a fault, the TCP connections and the BGP sessions are migrated transparently
to the standby route processor, and the standby route processor becomes active. The existing protocol state is maintained
on the standby route processor when it becomes active, and the protocol state does not need to be refreshed by peers.
Events such as soft reconfiguration and policy modifications can trigger the BGP internal state to change. To ensure state
consistency between active and standby BGP processes during such events, the concept of post-it is introduced that act as
synchronization points.
BGP NSR provides the following features:
NSR-related alarms and notifications
Configured and operational NSR states are tracked separately
NSR statistics collection
NSR statistics display using show commands
XML schema support
Auditing mechanisms to verify state synchronization between active and standby instances
CLI commands to enable and disable NSR
Support for 5000 NSR sessions
BGP Best-External Path
The best–external path functionality supports advertisement of the best–external path to the iBGP and Route Reflector peers
when a locally selected bestpath is from an internal peer. BGP selects one best path and one backup path to every destination.
By default, selects one best path . Additionally, BGP selects another bestpath from among the remaining external paths for
a prefix. Only a single path is chosen as the best–external path and is sent to other PEs as the backup path. BGP calculates
the best–external path only when the best path is an iBGP path. If the best path is an eBGP path, then best–external path
calculation is not required.
The procedure to determine the best–external path is as follows:
Determine the best path from the entire set of paths available for a prefix.
Eliminate the current best path.
Eliminate all the internal paths for the prefix.
From the remaining paths, eliminate all the paths that have the same next hop as that of the current best path.
Rerun the best path algorithm on the remaining set of paths to determine the best–external path.
BGP considers the external and confederations BGP paths for a prefix to calculate the best–external path. BGP advertises the
best path and the best–external path as follows:
On the primary PE—advertises the best path for a prefix to both its internal and external peers
On the backup PE—advertises the best path selected for a prefix to the external peers and advertises the best–external path
selected for that prefix to the internal peers
BGP Prefix Independent Convergence
BGP Prefix Independent Convergence (PIC) feature enables the activation of a backup path in the event of the primary path
failure.
Networks use Fast reroute (FRR) to calculate the next best path (backup path) and store it in BGP and IP Routing Information
Bases (RIBs). The RIBs share the backup path information with the Forwarding Information Base (FIB). BGP PIC feature uses
the backup path information in the FIB to quickly switch to this path during network failure, provided the line cards are
enabled for PIC.
Drawbacks of Using Prefix-Dependent Convergence
In a standard BGP network, a BGP router advertises only its best path to a destination prefix. Hence, in an autonomous system,
routers running BGP are not aware of all the possible paths to a destination prefix. In the event of a link or network failure
that causes the best path to fail, the following process takes place:
The affected BGP router advertising the failed best path, announces a withdrawal of the path.
The BGP routers receiving the best path withdrawal from the affected BGP router, withdraw their own best paths, and recalculate
their best paths to the destination prefix.
The BGP routers advertise their recalculated best paths to all neighboring routers.
Each BGP router that receives a new best path from its neighboring BGP router, again evaluates its own best path, and possibly
withdraws and recalculates its best path.
The BGP routers that recalculate their best paths, again advertise the new paths in the network.
Because this process repeats until all the BGP routers have the best path to the destination prefix, convergence of the network
takes a lot of time. This form of convergence is known as prefix-dependent convergence. If route reflectors are configured
in the network, then convergence takes even longer.
Benefits of Using Prefix-Independent Convergence
When prefix-independent convergence is configured in a BGP network, all BGP routers advertise their best external paths to
a destination prefix. This indicates that all BGP routers are aware of multiple best external paths to a destination prefix.
Each BGP router selects a backup path from the available best external paths, and downloads it to its FIB. Hence, the FIB
on each BGP router contains a best path and a best external path to a destination prefix. In the event of a link or network
failure that causes the best path to fail, the FIB on the affected BGP router can switch all its routes using the failed path
to the best external path, in a single operation. Because this form of convergence takes minimal time, it is preferred in
large scale network deployments.
Using Prefix-Independent Convergence with Route Reflectors
For traffic from the customer edge router to a remote provider edge router, the BGP local-pref attribute is used to select the primary path (from a primary PE) and the backup path (from the backup PE). Even though the
remote provider edge router receives the backup (best external) path from the backup PE, when the backup PE receives the iBGP
best path from the primary PE, it withdraws the backup path from the core network. Hence, the primary and backup (best external)
paths must be pre-programmed in the network for PIC to work.
When the primary path fails, the delay in convergence is because of the following process that takes place:
The primary PE sends a request to the provider core network for withdrawing the primary path.
The backup PE advertises the backup (best external) path as the new primary (best) path.
The remote PE recalculates its primary paths on receiving the withdrawal request from the primary PE, and the new primary
path from the backup PE.
Traffic resumes in the network after all prefixes in the FIB are updated with the new primary path.
Hence, convergence is slow because it depends on prefixes advertised by the PE routers.
By introducing prefix-independent convergence, the following changes take place:
Primary and backup paths are pre-programmed in the RIB and FIB.
All provider edge routers receive the backup path from the FIB.
In the event of primary path failure, the FIB modifies LDIs to include the backup path and instantly divert traffic along
this route.
Note
To use BGP PIC feature with route reflectors, the provider edge routers must be configured with unique route distinguishers
(RDs) within the context of a VRF. Else, the paths from different PEs are considered to be belonging to the same network,
and the route reflector cannot accurately calculate the best backup path.
Backup Path Selection Process
Use the following procedure to identify the best backup path to be programmed in the RIB and FIB.
Use the best path algorithm to identify the best path from the available set of paths for a prefix.
Eliminate the best path.
Eliminate all paths that have the same next hop as the best path.
Rerun the best path algorithm on the remaining set of paths to identify the best backup path.
Configure BGP PIC in Provider Edge Networks
This section describes the procedure to configure BGP PIC for provider edge networks.
Topology
Consider the topology shown in the following illustration.
Figure 9. Prefix Independent Convergence in Provider Edge Networks
For traffic from the customer edge router CE to the provider edge router PE3, the BGP local-pref attribute is used to select CE-PE1-PE3 as the primary path, and CE-PE2-PE3 as the backup path. PE1-P-PE2 is the best internal
path for the provider core network.
Before you Begin
Before you can configure the BGP PIC feature, ensure that you have configured the following:
The loopback and network interfaces as per the topology.
The VRFs for the provider core network.
Configuration
Use the configuration in this section to configure BGP PIC feature for the illustrated topology.
Router PE1
For traffic from Router CE to Router PE3, the eBGP path from Router CE is stored as the primary path on Router PE1.
Configure Router PE1 to install the backup (best external) path advertised by Router PE2, and the period for which the local
label must be retained on convergence, as shown.
Run the following commands on Router PE3 to verify the BGP PIC feature in operation.
Verify the presence of the backup path in the FIB.
Router# show cef 1.1.1.1/32 detail
Fri Oct 10 10:24:33.079 UTC
1.1.1.1/32, version 1, internal 0x40000001 (0xa94c0574) [1], 0x0 (0x0), 0x0
(0x0)
Updated Oct 9 16:49:06.795
Prefix Len 32, traffic index 0, precedence routine (0)
gateway array (0xa8d9b130) reference count 4, flags 0x80200, source rib
(3),
[1 type 3 flags 0x901101 (0xa8ec6b90) ext 0x0 (0x0)]
LW-LDI[type=0, refc=0, ptr=0x0, sh-ldi=0x0]
Level 1 - Load distribution: 0
[0] via 12.24.0.1, recursive
via 12.24.0.1, 3 dependencies, recursive
next hop 12.24.0.1 via 12.24.0.1/32
via 12.24.0.2, 3 dependencies, recursive, backup
next hop 12.24.0.2 via 12.24.0.2/32Load distribution: 0 (refcount 1)
Hash OK Interface Address
0 Y MgmtEth0/RP0/CPU0/0 12.24.0.1
Verify the presence of the backup (best external) path for BGP.
Router# show bgp vrf foo 206.1.1.1/32
BGP routing table entry for 206.1.1.1/32
Versions:
Process bRIB/RIB SendTblVer
Speaker 6 6
Local Label: 3
Paths: (1 available, best #1)
Advertised to peers (in unique update groups):
100.100.100.1
Path #1: Received by speaker 0
1.1.1.1 from 1.1.1.1 (200.200.200.1)
Origin incomplete, metric 0, localpref 100, weight 32768, valid,
internal, best
2.2.2.2 from 2.2.2.2 (100.100.100.1)
Origin incomplete, metric 0, localpref 100, weight 32768, valid,
external, backup, best-external
Configure BGP PIC between Autonomous Systems
This section describes the procedure to configure BGP PIC between autonomous systems. .
Note
BGP PIC is supported only for Option A and Option B scenarios. The following section describes a sample configuration for
Option B.
Topology
For example, consider the topology shown in the following illustration.
Figure 10. Prefix-Independent Convergence between Autonomous Systems
For traffic from Router PE1 to Router PE2, ASBR1 is the primary router and ASBR2 is the backup router. The ASBR1-ASBR3 eBGP
path is the primary path. The ASBR2-ASBR4 eBGP path is the backup path. For traffic from Router PE2 to Router PE1, ASBR3 is
the primary router and ASBR4 is the backup router. The ASBR3-ASBR1 eBGP path is the primary path and the ASBR4-ASBR2 eBGP
path is the backup path.
Before you Begin
Before you can configure the BGP PIC feature, ensure that you have configured the loopback and network interfaces as per the
illustrated topology.
Configuration
Use the configuration in this section to configure BGP PIC feature for the illustrated topology.
Router ASBR1
Configure Router ASBR1 to install the backup (best external) path advertised by Router ASBR2, and the period for which the
local label must be retained on convergence, as shown.
The provided configuration is for traffic from Router PE1 to Router PE2. Similarly, configure Router ASBR4 for traffic from
Router PE2 to Router PE1.
Verify BGP PIC
Run the following commands on Router PE2 (for traffic from Router PE1 to Router PE2) or on Router PE1 (for traffic from Router
PE2 to Router PE1) to verify the BGP PIC feature in operation.
Verify the presence of the backup path in the FIB.
Router# show cef 1.1.1.1/32 detail
Fri Oct 10 10:24:33.079 UTC
1.1.1.1/32, version 1, internal 0x40000001 (0xa94c0574) [1], 0x0 (0x0), 0x0
(0x0)
Updated Oct 9 16:49:06.795
Prefix Len 32, traffic index 0, precedence routine (0)
gateway array (0xa8d9b130) reference count 4, flags 0x80200, source rib
(3),
[1 type 3 flags 0x901101 (0xa8ec6b90) ext 0x0 (0x0)]
LW-LDI[type=0, refc=0, ptr=0x0, sh-ldi=0x0]
Level 1 - Load distribution: 0
[0] via 12.24.0.1, recursive
via 12.24.0.1, 3 dependencies, recursive
next hop 12.24.0.1 via 12.24.0.1/32
via 12.24.0.2, 3 dependencies, recursive, backup
next hop 12.24.0.2 via 12.24.0.2/32Load distribution: 0 (refcount 1)
Hash OK Interface Address
0 Y MgmtEth0/RP0/CPU0/0 12.24.0.1
Verify the presence of the backup (best external) path for BGP.
Router# show bgp vrf foo 206.1.1.1/32
BGP routing table entry for 206.1.1.1/32
Versions:
Process bRIB/RIB SendTblVer
Speaker 6 6
Local Label: 3
Paths: (1 available, best #1)
Advertised to peers (in unique update groups):
100.100.100.1
Path #1: Received by speaker 0
1.1.1.1 from 1.1.1.1 (200.200.200.1)
Origin incomplete, metric 0, localpref 100, weight 32768, valid,
internal, best
2.2.2.2 from 2.2.2.2 (100.100.100.1)
Origin incomplete, metric 0, localpref 100, weight 32768, valid,
external, backup, best-external
Command Line Interface (CLI) Consistency for BGP Commands
The Border Gateway Protocol (BGP) commands use disable keyword to disable a feature. The keyword inheritance-disable disables the inheritance of the feature properties from the parent level.
BGP Additional Paths
Table 9. Feature History Table
Feature Name
Release Information
Feature Description
Additonal path control per neighbor
Release 7.3.15
This features allows flexibility and granular control of the
advertisement of additional paths based on the neighbor outbound
policy configuration.
This is done by allowing configuration of combinations diff erent
path selection procedures unlike singular path selection, and
extending neighbor outpound policy to have finer control of the path
types to be advertised.
This feature enables operational efficiency to manage additional
paths and reduce scale of the paths in a typical clustered network
architecture.
Without this feature, the path scale limitation of the memory is
impacted, and control plane convergence issues develop because of
the excessive number of paths.
The Border Gateway Protocol (BGP) Additional Paths feature modifies the BGP protocol machinery for a BGP speaker to be able
to send multiple paths for a prefix. This gives 'path diversity' in the network. The add path enables BGP prefix independent
convergence (PIC) at the edge routers.
BGP add path enables add path advertisement in an iBGP network and advertises the following types of paths for a prefix:
Backup paths—to enable fast convergence and connectivity restoration.
Group-best paths—to resolve route oscillation.
All paths—to emulate an iBGP full-mesh.
iBGP Multipath Load Sharing
When a Border Gateway Protocol (BGP) speaking router that has no local policy configured, receives multiple network layer
reachability information (NLRI) from the internal BGP (iBGP) for the same destination, the router will choose one iBGP path
as the best path. The best path is then installed in the IP routing table of the router. The iBGP Multipath Load Sharing feature
enables the BGP speaking router to select multiple iBGP paths as the best paths to a destination. The best paths or multipaths
are then installed in the IP routing table of the router.
Configure iBGP Multipath Load Sharing
Perform this task to configure the iBGP Multipath Load Sharing:
SUMMARY STEPS
configure
routerbgpas-number
address-family {ipv4|ipv6} {unicast|multicast}
maximum-pathsibgpnumber
Use the
commit or
end command.
DETAILED STEPS
Step 1
configure
Example:
RP/0/RP0/CPU0:router# configure
Enters
mode.
Step 2
routerbgpas-number
Example:
Router(config)# router bgp 100
Specifies the autonomous system number and enters the BGP configuration mode, allowing you to configure the BGP routing process.
Step 3
address-family {ipv4|ipv6} {unicast|multicast}
Example:
Router(config-bgp)# address-family ipv4 multicast
Specifies either the IPv4 or IPv6 address family and enters address family configuration submode.
Step 4
maximum-pathsibgpnumber
Example:
Router(config-bgp-af)# maximum-paths ibgp 30
Configures the maximum number of iBGP paths for load sharing.
Step 5
Use the
commit or
end command.
commit—Saves the configuration changes and remains
within the configuration session.
end—Prompts user to take one of these actions:
Yes— Saves configuration changes and exits the
configuration session.
No—Exits the configuration session without
committing the configuration changes.
Cancel—Remains in the configuration session,
without committing the configuration changes.
iBGP Multipath Loadsharing Configuration: Example
The following is a sample configuration where 30 paths are used for loadsharing:
This feature enables you to implement multiple contiguous BGP
Autonomous Systems under a single administration.
You can allow BGP to make its routing decisions based on the IGP
metric just as an IGP would do.
Overview of BGP AIGP
The Accumulated IGP (AIGP) Attribute for BGP is an optional non-transitive BGP path
Attribute. IANA assigned the attribute type code for the AIGP attribute. The value
field of the AIGP attribute is defined as a set of Type/Length/Value elements
(TLVs). The AIGP TLV contains the Accumulated IGP metric.
The AIGP feature is required in the network to simulate the current OSPF behavior of
computing the distance associated with a path. OSPF or LDP carries the prefix or
label information only in the local area. Then, BGP carries the prefix label to all
the remote areas by redistributing the routes into BGP at area boundaries. The
routes or labels are then advertised using LSPs. The next hop for the route is
changed at each ABR to local router which removes the need to leak OSPF routes
across area boundaries. The bandwidth available on each of the core links is mapped
to OSPF cost, hence it is imperative that BGP carries this cost correctly between
each of the PEs. This functionality is achieved by using the AIGP.
Originate Prefixes with AIGP
Origination of routes with the accumulated interior gateway protocol (AIGP) metric is
controlled by configuration. AIGP attributes are attached to redistributed routes
that satisfy following conditions.
The protocol redistributing the route is enabled for AIGP.
The route is an interior gateway protocol (IGP) route redistributed into
border gateway protocol (BGP). The value assigned to the AIGP attribute is
the value of iGP next hop to the route or as set by a route-policy.
The route is a static route redistributed into BGP. The value assigned is the
value of next hop to the route or as set by a route-policy.
The route is imported into BGP through network statement. The value assigned
is the value of next hop to the route or as set by a route-policy.
Router# show bgp 10.0.0.1
Thu Sep 30 21:21:15.279 EDT
BGP routing table entry for 10.0.0.1/32
Versions:
Process bRIB/RIB SendTblVer
Speaker 4694 4694
Last Modified: Sep 30 21:20:09.000 for 00:01:06
Paths: (2 available, best #1)
Not advertised to any peer
Path #1: Received by speaker 0
Not advertised to any peer
Local
192.168.0.1 (metric 2) from 192.168.0.1 (192.168.0.6)
Received Label 24000
Origin IGP, localpref 80, aigp metric 900, valid, internal, best, group-best, labeled-unicast
Received Path ID 1, Local Path ID 1, version 4694
Originator: 192.168.0.6, Cluster list: 192.168.0.1
Total AIGP metric 902 <-- AIGP attribute received.
Accumulated Interior Gateway Protocol Attribute
The Accumulated Interior Gateway Protocol (AiGP)Attribute is an optional non-transitive BGP Path Attribute. The attribute
type code for the AiGP Attribute is to be assigned by IANA. The value field of the AiGP Attribute is defined as a set of Type/Length/Value
elements (TLVs). The AiGP TLV contains the Accumulated IGP Metric.
The AiGP feature is required in the 3107 network to simulate the current OSPF behavior of computing the distance associated
with a path. OSPF/LDP carries the prefix/label information only in the local area. Then, BGP carries the prefix/lable to all
the remote areas by redistributing the routes into BGP at area boundaries. The routes/labels are then advertised using LSPs.
The next hop for the route is changed at each ABR to local router which removes the need to leak OSPF routes across area boundaries.
The bandwidth available on each of the core links is mapped to OSPF cost, hence it is imperative that BGP carries this cost
correctly between each of the PEs. This functionality is achieved by using the AiGP.
BGP Accept Own
The BGP Accept Own feature enables handling of self-originated VPN routes, which a BGP speaker receives from a route-reflector
(RR). A "self-originated" route is one which was originally advertized by the speaker itself. As per BGP protocol [RFC4271],
a BGP speaker rejects advertisements that were originated by the speaker itself. However, the BGP Accept Own mechanism enables
a router to accept the prefixes it has advertised, when reflected from a route-reflector that modifies certain attributes
of the prefix. A special community called ACCEPT-OWN is attached to the prefix by the route-reflector, which is a signal to
the receiving router to bypass the ORIGINATOR_ID and NEXTHOP/MP_REACH_NLRI check. Generally, the BGP speaker detects prefixes
that are self-originated through the self-origination check (ORIGINATOR_ID, NEXTHOP/MP_REACH_NLRI) and drops the received
updates. However, with the Accept Own community present in the update, the BGP speaker handles the route.
One of the applications of BGP Accept Own is auto-configuration of extranets within MPLS VPN networks. In an extranet configuration,
routes present in one VRF is imported into another VRF on the same PE. Normally, the extranet mechanism requires that either
the import-rt or the import policy of the extranet VRFs be modified to control import of the prefixes from another VRF. However,
with Accept Own feature, the route-reflector can assert that control without the need for any configuration change on the
PE. This way, the Accept Own feature provides a centralized mechanism for administering control of route imports between different
VRFs.
BGP Accept Own is supported only for VPNv4 and VPNv6 address families in neighbor configuration mode.
Route-Reflector Handling Accept Own Community and RTs
The ACCEPT_OWN community is originated by the InterAS route-reflector (InterAS-RR) using an outbound route-policy. To minimize
the propagation of prefixes with the ACCEPT_OWN community attribute, the attribute will be attached on the InterAS-RR using
an outbound route-policy towards the originating PE. The InterAs-RR adds the ACCEPT-OWN community and modifies the set of
RTs before sending the new Accept Own route to the attached PEs, including the originator, through intervening RRs. The route
is modified via route-policy.
Accept Own Configuration Example
In this configuration example:
PE11 is configured with Customer VRF and Service VRF.
OSPF is used as the IGP.
VPNv4 unicast and VPNv6 unicast address families are enabled between the PE and RR neighbors and IPv4 and IPv6 are enabled
between PE and CE neighbors.
The Accept Own configuration works as follows:
CE1 originates prefix X.
Prefix X is installed in customer VRF as (RD1:X).
Prefix X is advertised to IntraAS-RR11 as (RD1:X, RT1).
IntraAS-RR11 advertises X to InterAS-RR1 as (RD1:X, RT1).
InterAS-RR1 attaches RT2 to prefix X on the inbound and ACCEPT_OWN community on the outbound and advertises prefix X to IntraAS-RR31.
IntraAS-RR31 advertises X to PE11.
PE11 installs X in Service VRF as (RD2:X,RT1, RT2, ACCEPT_OWN).
Remote PE: Handling of Accept Own Routes
Remote PEs (PEs other than the originator PE), performs bestpath calculation among all the comparable routes. The bestpath
algorithm has been modified to prefer an Accept Own path over non-Accept Own path. The bestpath comparison occurs immediately
before the IGP metric comparison. If the remote PE receives an Accept Own path from route-reflector 1 and a non-Accept Own
path from route-reflector 2, and if the paths are otherwise identical, the Accept Own path is preferred. The import operates
on the Accept Own path.
Configuring BGP Accept Own
Perform this task to configure BGP Accept Own:
SUMMARY STEPS
configure
router bgpas-number
neighborip-address
remote-asas-number
update-sourcetypeinterface-path-id
address-family {vpnv4 unicast | vpnv6 unicast}
accept-own [inheritance-disable]
DETAILED STEPS
Command or Action
Purpose
Step 1
configure
Example:
RP/0/RP0/CPU0:router# configure
Enters
mode.
Step 2
router bgpas-number
Example:
Router(config)#router bgp 100
Specifies the autonomous system number and enters the BGP configuration mode, allowing you to configure the BGP routing process.
Step 3
neighborip-address
Example:
Router(config-bgp)#neighbor 10.1.2.3
Places the router in neighbor configuration mode for BGP routing and configures the neighbor IP address as a BGP peer.
Step 4
remote-asas-number
Example:
Router(config-bgp-nbr)#remote-as 100
Assigns a remote autonomous system number to the neighbor.
Step 5
update-sourcetypeinterface-path-id
Example:
Router(config-bgp-nbr)#update-source Loopback0
Allows sessions to use the primary IP address from a specific interface as the local address when forming a session with a
neighbor.
Specifies the address family as VPNv4 or VPNv6 and enters neighbor address family configuration mode.
Step 7
accept-own [inheritance-disable]
Example:
Router(config-bgp-nbr-af)#accept-own
Enables handling of self-originated VPN routes containing Accept_Own community.
Use the inheritance-disable keyword to disable the "accept own" configuration and to prevent inheritance of "acceptown" from a parent configuration.
BGP Link-State
BGP Link-State (LS) is an Address Family Identifier (AFI) and Sub-address Family Identifier (SAFI) originally defined to carry
interior gateway protocol (IGP) link-state information through BGP. The BGP Network Layer Reachability Information (NLRI)
encoding format for BGP-LS and a new BGP Path Attribute called the BGP-LS attribute are defined in RFC7752. The identifying key of each Link-State object, namely a node, link, or prefix, is encoded in the NLRI and the properties
of the object are encoded in the BGP-LS attribute.
Note
IGPs do not use BGP LS data from remote peers. BGP does not download the received BGP LS data to any other component on the
router.
An example of a BGP-LS application is the Segment Routing Path Computation Element (SR-PCE). The SR-PCE can learn the SR capabilities
of the nodes in the topology and the mapping of SR segments to those nodes. This can enable the SR-PCE to perform path computations
based on SR-TE and to steer traffic on paths different from the underlying IGP-based distributed best-path computation.
The following figure shows a typical deployment scenario. In each IGP area, one or more nodes (BGP speakers) are configured
with BGP-LS. These BGP speakers form an iBGP mesh by connecting to one or more route-reflectors. This way, all BGP speakers
(specifically the route-reflectors) obtain Link-State information from all IGP areas (and from other ASes from eBGP peers).
Exchange Link State Information with BGP Neighbor
The following example shows how to exchange link-state information with a BGP neighbor:
A given BGP node may have connections to multiple, independent routing domains. IGP link-state database distribution into
BGP-LS is supported for both OSPF and IS-IS protocols in order to distribute this information on to controllers or applications
that desire to build paths spanning or including these multiple domains.
To distribute OSPFv2 link-state data using BGP-LS, use the distribute link-state command in router configuration mode.
The identifier field of BGP-LS (referred to as the Instance-ID) identifies the IGP routing domain where the NLRI belongs.
The NLRIs representing link-state objects (nodes, links, or prefixes) from the same IGP routing instance must use the same
Instance-ID value.
When there is only a single protocol instance in the network where BGP-LS is operational, we recommend configuring the Instance-ID
value to 0.
Assign consistent BGP-LS Instance-ID values on all BGP-LS Producers within a given IGP domain.
NLRIs with different Instance-ID values are considered to be from different IGP routing instances.
Unique Instance-ID values must be assigned to routing protocol instances operating in different IGP domains. This allows the
BGP-LS Consumer (for example, SR-PCE) to build an accurate segregated multi-domain topology based on the Instance-ID values,
even when the topology is advertised via BGP-LS by multiple BGP-LS Producers in the network.
If the BGP-LS Instance-ID configuration guidelines are not followed, a BGP-LS Consumer may see duplicate link-state objects
for the same node, link, or prefix when there are multiple BGP-LS Producers deployed. This may also result in the BGP-LS Consumers
getting an inaccurate network-wide topology.
Configuring BGP Link-state
To exchange BGP link-state (LS) information with a BGP neighbor, perform these steps:
Procedure
Step 1
configure
Example:
RP/0/RP0/CPU0:router# configure
Enters
mode.
Step 2
router bgp as-number
Example:
Router(config)# router bgp 100
Specifies the BGP AS number and enters the BGP configuration mode, allowing you to configure the BGP routing process.
Step 3
neighborip-address
Example:
Router(config-bgp)# neighbor 10.0.0.2
Configures a CE neighbor. The ip-address argument must be a private address.
Configures unique identifier four-octet ASN. Range is from 1 to 4294967295.
Step 5
Use the
commit or
end command.
commit—Saves the configuration changes and remains
within the configuration session.
end—Prompts user to take one of these actions:
Yes— Saves configuration changes and exits the
configuration session.
No—Exits the configuration session without
committing the configuration changes.
Cancel—Remains in the configuration session,
without committing the configuration changes.
BGP Permanent Network
BGP permanent network feature supports static routing through BGP. BGP routes to IPv4 or IPv6 destinations (identified by
a route-policy) can be administratively created and selectively advertised to BGP peers. These routes remain in the routing
table until they are administratively removed. A permanent network is used to define a set of prefixes as permanent, that
is, there is only one BGP advertisement or withdrawal in upstream for a set of prefixes. For each network in the prefix-set,
a BGP permanent path is created and treated as less preferred than the other BGP paths received from its peer. The BGP permanent
path is downloaded into RIB when it is the best-path.
The permanent-network command in global address family configuration mode uses a route-policy to identify the set of prefixes (networks) for which
permanent paths is to be configured. The advertise permanent-network command in neighbor address-family configuration mode is used to identify the peers to whom the permanent paths must be advertised.
The permanent paths is always advertised to peers having the advertise permanent-network configuration, even if a different
best-path is available. The permanent path is not advertised to peers that are not configured to receive permanent path.
The permanent network feature supports only prefixes in IPv4 unicast and IPv6 unicast address-families under the default
Virtual Routing and Forwarding (VRF).
Restrictions
These restrictions apply while configuring the permanent network:
Permanent network prefixes must be specified by the route-policy on the global address family.
You must configure the permanent network with route-policy in global address family configuration mode and then configure
it on the neighbor address family configuration mode.
When removing the permanent network configuration, remove the configuration in the neighbor address family configuration
mode and then remove it from the global address family configuration mode.
Configuring BGP Permanent Network
Perform this task to configure BGP permanent network. You must configure at least one route-policy to identify the set of
prefixes (networks) for which the permanent network (path) is to be configured.
Enters prefix set configuration mode and defines a prefix set for contiguous and non-contiguous set of bits.
Step 3
exit
Example:
Router(config-pfx)# exit
Exits prefix set configuration mode and enters global configuration mode.
Step 4
route-policyroute-policy-name
Example:
Router(config)# route-policy POLICY-PERMANENT-NETWORK-IPv4
Router(config-rpl)# if destination in PERMANENT-NETWORK-IPv4 then
Router(config-rpl)# pass
Router(config-rpl)# endif
Creates a route policy and enters route policy configuration mode, where you can define the route policy.
Step 5
end-policy
Example:
Router(config-rpl)# end-policy
Ends the definition of a route policy and exits route policy configuration mode.
Step 6
router bgpas-number
Example:
Router(config)# router bgp 100
Specifies the autonomous system number and enters the BGP configuration mode.
Step 7
address-family { ipv4 | ipv6 } unicast
Example:
Router(config-bgp)# address-family ipv4 unicast
Specifies either an IPv4 or IPv6 address family unicast and enters address family configuration submode.
Specifies the peers to whom the permanent network (path) is advertised.
Step 7
Use the
commit or
end command.
commit—Saves the configuration changes and remains
within the configuration session.
end—Prompts user to take one of these actions:
Yes— Saves configuration changes and exits the
configuration session.
No—Exits the configuration session without
committing the configuration changes.
Cancel—Remains in the configuration session,
without committing the configuration changes.
Step 8
show bgp {ipv4 | ipv6} unicast neighborip-address
Example:
Router# show bgp ipv4 unicast neighbor 10.255.255.254
(Optional) Displays whether the neighbor is capable of receiving BGP permanent networks.
BGP-RIB Feedback Mechanism for Update Generation
The Border Gateway Protocol-Routing Information Base (BGP-RIB) feedback mechanism for update generation feature avoids premature
route advertisements and subsequent packet loss in a network. This mechanism ensures that routes are installed locally, before
they are advertised to a neighbor.
BGP waits for feedback from RIB indicating that the routes that BGP installed in RIB are installed in forwarding information
base (FIB) before BGP sends out updates to the neighbors. RIB uses the the BCDL feedback mechanism to determine which version
of the routes have been consumed by FIB, and updates the BGP with that version. BGP will send out updates of only those routes
that have versions up to the version that FIB has installed. This selective update ensures that BGP does not send out premature
updates resulting in attracting traffic even before the data plane is programmed after router reload, LC OIR, or flap of a
link where an alternate path is made available.
To configure BGP to wait for feedback from RIB indicating that the routes that BGP installed in RIB are installed in FIB,
before BGP sends out updates to neighbors, use the update wait-install command in router address-family IPv4 or router address-family VPNv4 configuration mode. The show bgp, show bgp neighbors, and show bgp process performance-statistics commands display the information from update wait-install configuration.
Default-originate Under VRF
BGP advertises default routes to provider-edge neighbors, based on per-VRF configuration.
User-Defined Martian Address Check
When you configure BGP on a Cisco 8000 Series Router, you can prevent routers from accessing certain sites with certain IP
address prefixes. These routers drop packets from such IP addresses, and such IP addresses are known as Martian addresses.
However, you can enable routers with BGP IPv4 address-family or BGP IPv6 address-family configuration to access these sites
by configuring the command default-martian-check disable. These sites are sites with certain IPv4 and IPv6 prefixes as follows:
IPv4 address prefixes
0.0.0.0/8
127.0.0.0/8
224.0.0.0/4
IPv6 address prefixes
::
::0002 - ::ffff
::ffff:a.b.c.d
fe80:xxxx
ffxx:xxxx
Restrictions
Routers with OSPF or IS-IS Protocols cannot access these sites even by having the default-martian-check disable command configured.
Configuration Example
To allow routes from Martian addresses, use the following steps:
Enter BGP IPv4 or BGP IPv6 address-family configuration mode.
Configure the address-family modifier as a unicast address.
Disable the Martian address check.
Configuration
/* Enter BGP IPv4 or BGP IPv6 address-family configuration mode. */
Router# configure
Router(config)# router bgp 100
/* Configure the address-family modifier as unicast. */
Router(config-bgp)# address-family ipv4 unicast
/* Disable the martian address check. */
Router(config-bgp-af)# default-martian-check disable
Router(config-bgp-af)# commit
Verification
To verify if you have enabled or disabled a Martian address check, you can use the show bgp ipv4 unicast command or show bgp ipv6 unicast command:
Router# show bgp ipv6 unicast
BGP router identifier 2.2.2.1, local AS number 1
BGP generic scan interval 60 secs
Non-stop routing is enabled
BGP table state: Active
Table ID: 0xe0800000 RD version: 29
BGP main routing table version 29
BGP NSR Initial initsync version 4 (Reached)
BGP NSR/ISSU Sync-Group versions 0/0
Dampening enabled
BGP scan interval 60 secs
Status codes: s suppressed, d damped, h history, * valid, > best
i - internal, r RIB-failure, S stale, N Nexthop-discard
Origin codes: i - IGP, e - EGP, ? - incomplete
Network Next Hop Metric LocPrf Weight Path
*>i::/0 1:1:1:1:1:1:1:1 100 0 i
* i192:1::/112 1.1.1.1 0 100 0 ?
*>i 1:1:1:1:1:1:1:1 0 100 0 ?
* iff11:1123::/64 1.1.1.1 2 100 0 ?
*>i 1:1:1:1:1:1:1:1 2 100 0 ?
BGP Multipath Enhancements
Overwriting of next-hop calculation for multipath prefixes is not allowed. The next-hop-unchanged multipath command disables overwriting of next-hop calculation for multipath prefixes.
The ability to ignore as-path onwards while computing multipath is added. The bgp multipath as-path ignore onwards command ignores as-path onwards while computing multipath.
When multiple connected routers start ignoring as-path onwards while computing multipath, it causes routing loops. Therefore,
you should not configure the bgp multipath as-path ignore onwards command on routers that can form a loop.
Figure 11. Topology to illustrate formation of loops
Consider three routers R1, R2 and R3 in different autonomous systems (AS-1, AS-2, and AS-3). The routers are connected with
each other. R1 announces a prefix to R2 and R3. Both R2 and R3 are configured with multipath and also with bgp multipath as-path
ignore onwards command. Since R3 is configured as multipath, R2 will send part of its traffic to R3. Similarly, R3 will send
part of its traffic to R2. This creates a forwarding loop between R3 and R2. Therefore, to avoid such forwarding loops you
should not configure the bgp multipath as-path ignore onwards command on connected routers.
Overview of BGP Monitoring Protocol
The BGP Monitoring Protocol (BMP) feature enables monitoring of BGP speakers (called BMP clients). You can configure a device
to function as a BMP server, which monitors either one or several BMP clients, which in turn, has several active peer sessions
configured. You can also configure a BMP client to connect to one or more BMP servers. The BMP feature enables configuration
of multiple BMP servers (configured as primary servers) to function actively and independent of each other, simultaneously
to monitor BMP clients.
The BMP Protocol provides access to the Adjacent Routing Information Base, Incoming (Adj-RIB-In) table of a peer on an ongoing
basis and a periodic dump of certain statistics that the monitoring station can use for further analysis. The BMP provides
pre-policy view of the Adj-RIB-In table of a peer.
There can be several BMP servers configured globally across all the BGP instances. The BMP severs configured are common across
multiple speaker instances and each BGP peer in an instance can be configured for monitoring by all or a subset of the BMP
servers, giving a 'any-to-any' map between BGP peers and BMP servers from the point of view of a BGP speaker. If a BMP server
is configured before any of the BGP peers come up, then the monitoring will start as soon as the BGP peers come up. A BMP
server configuration can be removed only when there are no BGP peers configured to be monitored by that particular BMP server.
Sessions between BMP clients and BMP servers operate over plain TCP (no encryption/encapsulation). If a TCP session with the
BMP server is not established, the client retries to connect every 7 seconds.
The BMP server does not send any messages to its clients (BGP speakers). The message flow is in one direction only—from BGP
speakers to the BMP servers
A maximum of eight BMP servers can be configured on the router. Each BMP server is specified by a server ID and certain parameters
such as IP address, port number, etc are configurable. Upon successful configuration of a BMP server with host and port details,
the BGP speaker attempts to connect to BMP Server. Once the TCP connection is setup, an Initiation message is sent as first
message.
The bmp server command enables the user to configure multiple—independent and asynchronous—BMP server connections.
All neighbors for a BGP speaker need not necessarily be BMP clients. BMP clients are the ones that have direct TCP connection
with a BMP server. Each of these BGP speakers can have many BGP neighbors or peers. Under a BGP speaker, if any of its neighbors
are configured for BMP monitoring, only that particular peer router's messages are sent to BMP servers.
The session connection to BMP server is attempted after an initial-delay at the BMP client. This initial-delay can be configured.
If the initial-delay is not configured, then the default connection delay of 7 seconds is used. Configuring the initial delay
becomes significant under certain circumstances where, if multiple BMP servers' states toggle closely and refresh delay is
so small, then this might result in redundant route-refreshes being generated. This causes considerable network traffic and
load on the device. Having different initial delays can reduce the load spike on the network and router.
After the initial delay, TCP connection to BMP servers are attempted. Once the server connections are up, it is checked if
there are any peers enabled for monitoring. Once a BGP peer that is already being monitored is in the “ESTAB” state, speaker
sends a “peer-up” message for that peer to the BMP server. After the BGP peer receives a route-refresh request, neighbor sends
the updates. This route refresh is initiated based on a delay configured for each BMP server. This is called route refresh
delay. When there are multiple neighbors to be monitored, each neighbor is set a refresh delay based upon the BMP server they
are enabled for. Once all the BGP neighbors have sent the updates in response to the refresh requests, the tables will be
up to date in the BMP Server. If a neighbor establishes connection after BMP monitoring has begun, it does not require a route-refresh
request. All received routes from that neighbor is sent to BMP servers.
Note
In the case of BMP Pre Inbound Policy Route monitoring, when a new BMP server comes up, route refresh requests are sent to
the peer router by the BGP speaker. However, in the case of BMP Post Inbound Policy Route Monitoring route refresh request
are not sent to the peer routers when the new BMP server comes up because the BMP table is used for update generation.
It is advantageous to batch up refresh requests to BGP peers, if several BMP servers are activated in quick succession. Use
the bmp server initial-refresh-delay command to configure a delay in triggering the refresh mechanism when the first BMP server comes up. If other BMP servers
come online within this time-frame, only one set of refresh requests is sent to the BGP peers. You can also configure the
bmp server initial-refresh-delay skip command to skip all refresh requests from BGP speakers and just monitor all incoming messages from the peers.
In a client-server configuration, it is recommended that the resource load of the devices be kept minimal and adding excessive
network traffic must be avoided. In the BMP configuration, you can configure various delay timers on the BMP server to avoid
flapping during connection between the server and client.
BGP—Multiple Cluster IDs
The BGP—Multiple Cluster IDs feature allows an iBGP neighbor (usually a route reflector) to have multiple cluster IDs: a global
cluster ID and additional cluster IDs that are assigned to clients (neighbors). Prior to the introduction of this feature,
a device could have a single, global cluster ID.
When a network administrator configures per-neighbor cluster IDs:
The loop prevention mechanism based on a CLUSTER_LIST is automatically modified to take into account multiple cluster IDs.
A network administrator can disable client-to-client route reflection based on cluster ID.
Restriction
The BGP Multiple Cluster-IDs feature only works in default VRF.
BGP Flowspec Overview
Table 11. Feature History Table
Feature Name
Release Information
Feature Description
Scaling BGP Flowspec to 6000 Rules
Release 7.5.2
You can now assign 6000 BGP Flowspec rules for Cisco 8800 series routers and 3000 BGP Flowspec rules for Cisco 8100 and 8200
series routers. This feature thus provide enhanced mitigation against Distributed Denial-of-Service (DDoS) attacks.
In earlier releases, you could assign 2000 BGP Flowspec rules. These are one dimensional scale numbers; the numbers vary based
on other intersecting features like AccessList (ACL), Quality of Service (QoS), and Local Path Transport Switching (LPTS).
The BGP flow specification (flowspec) feature allows you to rapidly deploy and propagate filtering and policing functionality
among many BGP peer routers to mitigate the effects of a distributed denial-of-service (DDoS) attack over your network.
BGP Flowspec feature allows you to construct instructions to match a particular flow with IPv4 and IPv6 source, IPv4 and IPv6
destination, L4 parameters and packet specifics such as length, fragment, destination port and source port, actions that must
be taken, such as dropping the traffic, or policing it at a definite rate, or redirect the traffic, through a BGP update.
In the BGP update, the flowspec matching criteria is represented by Network Layer Reachability Information (BGP NLRI) and
the actions are represented by BGP extended communities.
You can use the BGP Flowspec feature for mitigation of DDoS attack. When a DDoS attack occurs on a particular host inside
a network, you can send a flowspec update to the border routers so that the attack traffic can be policed or dropped, or even
redirected elsewhere. For example, to an appliance that cleans the traffic by filtering out the bad traffic and forward only
the good traffic toward the affected host.
Once flowspecs have been received by a router and programmed in applicable line cards, any active L3 ports on those line cards
start processing ingress traffic according to flowspec rules.
The BGP Flowspec feature cannot coexist with MAP-E and PBR on a given interface. If you
configure BGP Flowspec with PBR, the router does not display any error or system
message. The router ignores the BGP Flowspec configuration and the feature will not
function.
Flow Specifications
A flow specification is an n-tuple consisting of several matching criteria that can be applied to IP traffic. A given IP packet
matches the defined flow if it matches all the specified criteria.
Every flow-spec route is effectively a rule, consisting of a matching part (encoded in the NLRI field) and an action part
(encoded as a BGP extended community). The BGP flowspec rules are converted internally to equivalent C3PL policy representing
match and action parameters. The match and action support can vary based on underlying platform hardware capabilities. Sections
Supported Matching Criteria and Actions and Traffic Filtering Actions provide information on the supported match (tuple definitions) and action parameters.
Note
Cisco 8800 series routers support up to 6,000 flowspec rules.
Cisco 8200 and 8100 series routers support up to 3,000 flowspec rules.
Supported Matching Criteria and Actions
Table 12. Feature History Table
Feature Name
Release Name
Description
Additional BGP FlowSpec Actions for Enhanced Security
Release 7.3.3
This release introduces additional BGP FlowSpec actions for enhanced security against distributed denial-of-service (DDoS)
attacks.
Redirect Nexthop VRF only: Redirects the traffic to a different Autonomous System Number (ASN).
Rate Limit and Redirect IPv4 or IPv6 Nexthop: Redirects the traffic to the indicated nexthop IPv4 or IPv6 address. Policer
rate regulates the traffic.
Rate Limit and Redirect Nexthop VRF: Redirects the traffic to the next hop IPv4 address through a VRF. Policer rate regulates
the traffic. This action is supported only on Q200 Silicon One ASIC.
Table 13. Feature History Table
Feature Name
Release Name
Description
BGP FlowSpec NLRI types
Release 7.3.15
A BGP flow specification consists of several matching criteria encoded in the NLRI that is applied to IP traffic. A given
IP packet must match all the specified criteria. Network layer reachability information (NLRI) exchanges routing information
and matching criteria between BGP peers, indicating how to reach the destination.
The following NLRI types are supported:
Type 7: IPv4 or IPv6 ICMP type
Type 8: IPv4 or IPv6 ICMP code
Type 9: IPv4 TCP flags (2 bytes include reserved bits)
Type 10: IPv4 Packet length
Type 11: IPv4 or IPv6 DSCP
Type 12: IPv4 fragmentation bits
BGP FlowSpec Actions
Release 7.3.15
This feature provides information on the actions that can be associated with a BGP flow. The traffic filtering flow specification
is applied based on the specified rule. The following extended community values that can be used to specify particular action:
Set DSCP
Redirect IPv4 or IPv6 next hop
Overview
A flow specification NLRI type may include several components such as destination prefix, source prefix, protocol, ports,
and so on. This NLRI is treated as an opaque bit string prefix by BGP. Each bit string identifies a key to a database entry
with which a set of attributes can be associated. This NLRI information is encoded using MP_REACH_NLRI and MP_UNREACH_NLRI
attributes. Whenever the corresponding application does not require Next-Hop information, this is encoded as a 0-octet length
Next Hop in the MP_REACH_NLRI attribute, and ignored. The NLRI field of the MP_REACH_NLRI and MP_UNREACH_NLRI is encoded as
a 1- or 2-octet NLRI length field followed by a variable-length NLRI value. The NLRI length is expressed in octets.
The flow specification NLRI type consists of several optional sub-components. A specific packet is considered to match the
flow specification when it matches the intersection and of all the components present in the specification. The following
are the supported component types or tuples that you can define:
BGP Flowspec NLRI type
QoS Match Fields
Description and Syntax Construction
Value Input Method
Type 1
IPv4 or IPv6 destination address
Defines the destination prefix to match. Prefixes are encoded in the BGP UPDATE messages as a length in bits followed by
enough octets to contain the prefix information.
Defines a list of {operation, value} pairs that matches source or destination TCP or UDP ports. Values are encoded as 1- or
2-byte quantities. Port, source port, and destination port components evaluate to FALSE if the IP protocol field of the packet
has a value other than TCP or UDP. If the packet is fragmented and this is not the first fragment, or if the system in unable
to locate the transport header.
Defines a list of {operation, value} pairs used to match the type field of an ICMP packet. Values are encoded using a single
byte. The ICMP type and code specifiers evaluate to FALSE whenever the protocol value is not ICMP.
IPv4 or IPv6 TCP flags (2 bytes include reserved bits)
Note
Reserved and NS bit not supported
Bitmask values can be encoded as a 1- or 2-byte bitmask. When a single byte is specified, it matches byte 13 of the TCP header,
which contains bits 8 through 15 of the 4th 32-bit word. When a 2-byte encoding is used, it matches bytes 12 and 13 of the
TCP header with the data offset field having a "don't care" value. As with port specifier, this component evaluates to FALSE
for packets that are not TCP packets. This type uses the bitmask operand format, which differs from the numeric operator format
in the lower nibble.
Encoding: <type (1 octet), [op, bitmask]+>
Syntax:
matchtcp-flag value bit-mask mask_value
Bit mask
Type 10
IPv4 or IPv6 Packet length
Starting from Release 7.10.1, the IPv6 packet length is supported.
Note
Reserved and NS bit not supported
IPv4 or IPv6 support is available for the packets that are not the first fragment packets.
Match on the total IP packet length (excluding Layer 2, but including IP header). Values are encoded using 1- or 2-byte quantities.
Defines a list of (operation, value) pairs used to match the 6-bit DSCP field. Values are encoded using a single byte, whereas
the two most significant bits are zero and the six least significant bits contain the DSCP value.
Note
The DSCP does not contain Flowspec statistics.
Encoding: <type (1 octet), [op, value]+>
Syntax:
matchdscp{dscp-value | min-value - max-value}
Multi-value range
Type 12
IPv4 Fragmentation bits
Note
IPv4 support is available for the packets that are not the first fragment packets.
IPv6 BGP flowspec does not supports Type 12 NRLI.
Identifies a fragment-type as the match criterion for a class map.
Encoding: <type (1 octet), [op, bitmask]+>
Syntax:
matchfragmenttype[is-fragment]
Bit mask
In a given flowspec rule, 2-tuple action combinations can be specified without restrictions. However, mixing address family between matching criterion and actions are not allowed. For example, IPv4 matches cannot be
combined with IPv6 actions and vice versa.
Limitations for BGP FlowSpec
These limitations apply to the BGP FlowSpec feature.
BGP Flowspec statistics are supported when there is a policer rate limit.
The policer action scale is limited to a maximum of 128 per slice.
BGP Flowspec statistics are supported in Redirect action only when a policer is attached. BGP Flowspec statistics is not supported
for Redirect action alone.
VRF to default VRF redirect is not supported.
Traffic Filtering Actions
The default action for a traffic filtering flow specification is to accept IP traffic that matches that particular rule.
The following extended community values can be used to specify particular actions:
Note
The BGP flowspec actions rate limit and redirect are not supported
together.
The BGP flowspec action redirect is supported only for nexthop IPv4 and IPv6
not with nexthop VRF IPv4 and IPv6.
Type
Extended Community
PBR Action
Description
0x8006
traffic-rate 0
traffic-rate <rate>
Drop
Police
The traffic-rate extended community is a non-transitive extended
community across the autonomous-system boundary and uses following
extended community encoding:
The first two octets carry the 2-octet id, which can be assigned from
a 2-byte AS number. When a 4-byte AS number is locally present, the
2 least significant bytes of such an AS number can be used. This
value is informational. The remaining 4 octets carry the rate
information in IEEE floating point [IEEE.754.1985] format, bytes per
second. A traffic-rate of 0 should result on all traffic for the
particular flow to be discarded.
Command syntax
police rate < > | drop
0x8009
traffic-marking
Set DSCP
The traffic marking extended community instructs a system to modify
the differentiated service code point (DSCP) bits of a transiting IP
packet to the corresponding value. This extended community is
encoded as a sequence of 5 zero bytes followed by the DSCP value
encoded in the 6 least significant bits of 6th byte.
Command syntax
set dscp <6 bit value>
0x0800
Redirect IP NH
Redirect IPv4 or IPv6
Nexthop
Announces the reachability of one or more flowspec NLRI. When a BGP
speaker receives an UPDATE message with the redirect-to- IP extended
community it is expected to create a traffic filtering rule for
every flow-spec NLRI in the message that has this path as its best
path. The filter entry matches the IP packets described in the NLRI
field and redirects them or copies them towards the IPv4 or IPv6
address specified in the Network Address of Next-Hop field of the
associated MP_REACH_NLRI.
Note
The redirect-to-IP extended community is valid with any other set
of flow-spec extended communities except if that set includes a
redirect-to-VRF extended community (type 0x8008) and in that
case the redirect-to-IP extended community should be ignored.
The BGP Flowspec model comprises of a client and a server Controller. The Controller is responsible for sending or injecting
the flowspec NRLI entry. The client (acting as a BGP speaker) receives that NRLI and programs the hardware forwarding to act
on the instruction from the Controller. An illustration of this model is provided below.
BGP Flowspec Client
Here, the Controller on the left-hand side injects the flowspec NRLI, and the client on the right-hand side receives the information,
sends it to the flowspec manager, configures the ePBR (Enhanced Policy-based Routing) infrastructure, which in turn programs
the hardware from the underlaying platform in use.
BGP Flowspec Controller
The Controller is configured using CLI to provide an entry for NRLI injection.
Configure BGP Flowspec
The following sections show how to configure BGP Flowspec feature.
Figure 12. BGP Flowspec
The controller or the server with IP address 10.2.3.4 sends the Flowspec NLRI to the client with IP address 10.2.3.3. The
NLRI consists of matching criteria, the client processes based on this criteria. Traffic is dropped or accepted based on the
configured criteria.
The following section describes how you can configure BGP Flowspec on the client:
/*Configure BGP Flowspec */
Router(config)# flowspec
Router(config-flowspec)# address-family ipv4
Router(config-flowspec-af)# local-install interface-all
Router(config-flowspec-af)# exit
Router(config-flowspec)# address-family ipv6
Router(config-flowspec-af)# local-install interface-all
Router(config-flowspec-af)# exit
/* Configure the policy to accept all presented routes without modifying the routes */
Router(config)# route-policy pass-all
Router(config)# pass
Router(config)# end-policy
/* Configure the policy to reject all presented routes without modifying the routes */
Router(config)# route-policy drop-all
Router(config)# drop
Router(config)# end-policy
/* Configure BGP towards flowspec server */
Router(config)# router bgp 1
Router(config-bgp)# nsr
Router(config-bgp)# bgp router-id 10.2.3.3
Router(config-bgp)# address-family ipv4 flowspec
Router(config-bgp-af)# exit
Router(config-bgp)# address-family ipv6 flowspec
Router(config-bgp-af)# exit
Router(config-bgp)# neighbor 10.2.3.4
Router(config-bgp-nbr)# remote-as 1
Router(config-bgp-nbr)# address-family ipv4 flowspec
Router(config-bgp-nbr-af)# route-policy pass-all in
Router(config-bgp-nbr-af)# route-policy drop-all out
Router(config-bgp-af)# exit
Router(config-bgp-nbr)# address-family ipv6 flowspec
Router(config-bgp-nbr-af)# route-policy pass-all in
Router(config-bgp-nbr-af)# route-policy drop-all out
Router(config-bgp-nbr-af)# exit
Router(config-bgp-nbr)# update-source Loopback0
/* Disable BGP Flowspec */
Router(config)# interface bundle-ether 3.1
Router(config-subif)# ipv4 flowspec disable
Router(config-subif)# ipv6 flowspec disable
The following section describes how you can configure BGP Flowspec on the server:
/* Configure the policy to accept all presented routes without modifying the routes */
Router(config)# route-policy pass-all
Router(config)# pass
Router(config)# end-policy
/* Configure the policy to reject all presented routes without modifying the routes */
Router(config)# route-policy drop-all
Router(config)# drop
Router(config)# end-policy
/* Configure BGP towards flowspec client */
Router(config)# router bgp 1
Router(config-bgp)# nsr
Router(config-bgp)# bgp router-id 10.2.3.4
Router(config-bgp)# address-family ipv4 flowspec
Router(config-bgp-af)# exit
Router(config-bgp)# address-family ipv6 flowspec
Router(config-bgp-af)# exit
Router(config-bgp)# neighbor 10.2.3.3
Router(config-bgp-nbr)# remote-as 1
Router(config-bgp-nbr)# address-family ipv4 flowspec
Router(config-bgp-nbr-af)# route-policy pass-all in
Router(config-bgp-nbr-af)# route-policy pass-all out
Router(config-bgp-nbr-af)# exit
Router(config-bgp-nbr)# update-source Loopback0
/* Configure IPv4 flowspec to be advertised to client. Define traffic classes. */
Router(config)# class-map type traffic match-all ipv4_fragment
Router(config-cmap)# match destination-address ipv4 10.2.1.1 255.255.255.255
Router(config-cmap)# match source-address ipv4 172.16.0.1 255.255.255.255
Router(config-cmap)# end-class-map
Router(config)# class-map type traffic match-all ipv4_icmp
Router(config-cmap)# match destination-address ipv4 10.2.1.1 255.255.255.255
Router(config-cmap)# match source-address ipv4 172.16.0.1 255.255.255.255
Router(config-cmap)# end-class-map
/* Define a policy map and associate it with traffic classes.
Router(config)# policy-map type pbr scale_ipv4
Router(config-pmap)# class type traffic ipv4_fragment
Router(config-pmap-c)# drop
Router(config-pmap-c)# exit
Router(config-pmap)# class type traffic ipv4_icmp
Router(config-pmap-c)# exit
Router(config-pmap)# class type traffic class-default
Router(config-pmap-c)# end-policy-map
Router(config)# flowspec
Router(config)# address-family ipv4
Router(config-af)# service-policy type pbr scale_ipv4
/* Configure IPv6 flowspec to be advertised to client. Define traffic classes. */
Router(config)# class-map type traffic match-all ipv6_tcp
Router(config-cmap)# match destination-address ipv6 70:1:1::5a/128
Router(config-cmap)# match source-address ipv4 ipv6 80:1:1::5a/128
Router(config-cmap)# match destination-port 22
Router(config-cmap)# match source-port 4000
Router(config-cmap)# end-class-map
Router(config)# class-map type traffic match-all ipv6_icmp
Router(config-cmap)# match destination-address ipv6 70:2:1::1/128
Router(config-cmap)# match source-address ipv4 ipv6 80:2:1::1/128
Router(config-cmap)# end-class-map
/* Define a policy map and associate it with traffic classes.
Router(config)# policy-map type pbr scale_ipv6
Router(config-pmap)# class type traffic ipv6_tcp
Router(config-pmap-c)# exit
Router(config-pmap)# class type traffic ipv6_icmp
Router(config-pmap-c)# exit
Router(config-pmap)# class type traffic class-default
Router(config-pmap-c)# end-policy-map
Router(config)# flowspec
Router(config)# address-family ipv6
Router(config-af)# service-policy type pbr scale_ipv6
/* Class map configuration with DSCP */
Router(config-map)# class-map type traffic match-all class_dscp_5
Router(config-cmap)# match destination-address ipv4 192.0.2.254 255.255.255.0
Router(config-cmap)# match dscp 10-12
/* Policy map configuration with IPv4 Redirect and Rate Limiter */
Router(config-pmap)#class type traffic class_dscp_5
Router(config-pmap-c)#redirect ipv4 nexthop 10.26.245.2
Router(config-pmap-c)#police rate 5 mbps
Router(config-pmap-c)# root
Running Configuration
/* Client-side configuration */
flowspec
address-family ipv4
local-install interface-all
!
address-family ipv6
local-install interface-all
!
!
route-policy pass-all
pass
end-policy
!
route-policy drop-all
drop
end-policy
!
router bgp 1
nsr
bgp router-id 10.2.3.3
address-family ipv4 flowspec
!
address-family ipv6 flowspec
!
neighbor 10.2.3.4
remote-as 1
address-family ipv4 flowspec
route-policy pass-all in
route-policy drop-all out
!
address-family ipv6 flowspec
route-policy pass-all in
route-policy drop-all out
!
update-source Loopback0
!
!
vrf vrf1
address-family ipv4 unicast
import route-target
4787:13
!
export route-target
4787:13
!
!
address-family ipv6 unicast
import route-target
4787:13
!
export route-target
4787:13
!
!
!
router static
vrf vrf1
address-family ipv4 unicast
10.0.0.0/8 200.255.55.2
!
!
!
/* Disable the flowspec. This is optional configuration */
interface Bundle-Ether3.1
ipv4 flowspec disable
ipv6 flowspec disable
!
/* Server-side Configuration */
route-policy pass-all
pass
end-policy
!
route-policy drop-all
drop
end-policy
!
router bgp 1
nsr
bgp router-id 10.2.3.4
address-family ipv4 flowspec
!
address-family ipv6 flowspec
!
neighbor 10.2.3.3
remote-as 1
address-family ipv4 flowspec
route-policy drop-all in
route-policy pass-all out
exit
update-source Loopback0
!
!
class-map type traffic match-all ipv4_fragment
match destination-address ipv4 10.2.1.1 255.255.255.255
end-class-map
!
class-map type traffic match-all ipv4_icmp
match destination-address ipv4 10.2.1.1 255.255.255.255
match source-address ipv4 172.16.0.1 255.255.255.255
end-class-map
!
policy-map type pbr scale_ipv4
class type traffic ipv4_fragment
drop
!
class type traffic ipv4_icmp
!
!
class type traffic class-default
!
end-policy-map
!
flowspec
address-family ipv4
service-policy type pbr scale_ipv4
!
!
class-map type traffic match-all ipv6_tcp
match destination-address ipv6 70:1:1::5a/128
match source-address ipv6 80:1:1::5a/128
match protocol tcp
match destination-port 22
match source-port 4000
end-class-map
!
class-map type traffic match-all ipv6_icmp
match destination-address ipv6 70:2:1::1/128
match source-address ipv6 80:2:1::1/128
end-class-map
!
policy-map type pbr scale_ipv6
class type traffic ipv6_tcp
!
!
class type traffic ipv6_icmp
!
!
class type traffic class-default
!
!
flowspec
address-family ipv6
service-policy type pbr scale_ipv6
!
!
Verification
The following show output displays the status of the flowspec from the client side.
Router# show bgp ipv4 flowspec
GP router identifier 202.158.0.1, local AS number 4787
BGP generic scan interval 60 secs
Non-stop routing is enabled
BGP table state: Active
Table ID: 0x0 RD version: 7506
BGP main routing table version 7506
BGP NSR Initial initsync version 130 (Reached)
BGP NSR/ISSU Sync-Group versions 7506/0
BGP scan interval 60 secs
Status codes: s suppressed, d damped, h history, * valid, > best
i - internal, r RIB-failure, S stale, N Nexthop-discard
Origin codes: i - IGP, e - EGP, ? - incomplete
Network Next Hop Metric LocPrf Weight Path
*>iDest:10.1.1.1/32,Proto:=6,DPort:=80,SPort:=3000,Length:=200,DSCP:=10/176
0.0.0.0 10 0 ?
*>iDest:10.1.1.2/32,Proto:=6,DPort:=80,SPort:=3000,Length:=200,DSCP:=10/176
0.0.0.0 10 0 ?
*>iDest:10.1.1.3/32,Proto:=6,DPort:=80,SPort:=3000,Length:=200,DSCP:=10/176
0.0.0.0 10 0 ?
*>iDest:10.1.1.4/32,Proto:=6,DPort:=80,SPort:=3000,Length:=200,DSCP:=10/176
0.0.0.0 10 0 ?
*>iDest:10.1.1.5/32,Proto:=6,DPort:=80,SPort:=3000,Length:=200,DSCP:=10/176
0.0.0.0 10 0 ?
Router# show bgp ipv6 flowspec
BGP router identifier 202.158.0.1, local AS number 4787
BGP generic scan interval 60 secs
Non-stop routing is enabled
BGP table state: Active
Table ID: 0x0 RD version: 1503
BGP main routing table version 1504
BGP NSR Initial initsync version 2 (Reached)
BGP NSR/ISSU Sync-Group versions 1504/0
BGP scan interval 60 secs
Status codes: s suppressed, d damped, h history, * valid, > best
i - internal, r RIB-failure, S stale, N Nexthop-discard
Origin codes: i - IGP, e - EGP, ? - incomplete
Network Next Hop Metric LocPrf Weight Path
*>iDest:70:1:1::1/0-128,Source:80:1:1::1/0-128,NH:=6,DPort:=22,SPort:=4000,TCPFlags:=0x10,Length:=300,DSCP:=12/464
202:158:2::1 100 0 i
*>iDest:70:1:1::2/0-128,Source:80:1:1::2/0-128,NH:=6,DPort:=22,SPort:=4000,TCPFlags:=0x10,Length:=300,DSCP:=12/464
202:158:2::1 100 0 i
*>iDest:70:1:1::3/0-128,Source:80:1:1::3/0-128,NH:=6,DPort:=22,SPort:=4000,TCPFlags:=0x10,Length:=300,DSCP:=12/464
202:158:2::1 100 0 i
*>iDest:70:1:1::4/0-128,Source:80:1:1::4/0-128,NH:=6,DPort:=22,SPort:=4000,TCPFlags:=0x10,Length:=300,DSCP:=12/464
202:158:2::1 100 0 i
*>iDest:70:1:1::5/0-128,Source:80:1:1::5/0-128,NH:=6,DPort:=22,SPort:=4000,TCPFlags:=0x10,Length:=300,DSCP:=12/464
202:158:2::1 100 0 i
Router# show bgp vpnv4 flowspec
BGP router identifier 202.158.0.1, local AS number 4787
BGP generic scan interval 60 secs
Non-stop routing is enabled
BGP table state: Active
Table ID: 0x0 RD version: 0
BGP main routing table version 5
BGP NSR Initial initsync version 3 (Reached)
BGP NSR/ISSU Sync-Group versions 5/0
BGP scan interval 60 secs
Status codes: s suppressed, d damped, h history, * valid, > best
i - internal, r RIB-failure, S stale, N Nexthop-discard
Origin codes: i - IGP, e - EGP, ? - incomplete
Network Next Hop Metric LocPrf Weight Path
Route Distinguisher: 202.158.0.1:0 (default for vrf customer_1)
*>iDest:202.158.3.2/32,Source:202.158.1.2/32/96
0.0.0.0 100 0 i
Route Distinguisher: 202.158.0.2:1
*>iDest:202.158.3.2/32,Source:202.158.1.2/32/96
0.0.0.0 100 0 i
Processed 2 prefixes, 2 paths
Router# show bgp vpnv6 flowspec
BGP router identifier 202.158.0.1, local AS number 4787
BGP generic scan interval 60 secs
Non-stop routing is enabled
BGP table state: Active
Table ID: 0x0 RD version: 0
BGP main routing table version 5
BGP NSR Initial initsync version 4 (Reached)
BGP NSR/ISSU Sync-Group versions 5/0
BGP scan interval 60 secs
Status codes: s suppressed, d damped, h history, * valid, > best
i - internal, r RIB-failure, S stale, N Nexthop-discard
Origin codes: i - IGP, e - EGP, ? - incomplete
Network Next Hop Metric LocPrf Weight Path
Route Distinguisher: 202.158.0.1:0 (default for vrf customer_1)
*>iDest:200:158:3::2/0-128,Source:200:158:1::2/0-128,NH:=6,DPort:=22,SPort:=4000,Length:=300,DSCP:=12/440
0.0.0.0 100 0 i
Route Distinguisher: 202.158.0.2:1
*>iDest:200:158:3::2/0-128,Source:200:158:1::2/0-128,NH:=6,DPort:=22,SPort:=4000,Length:=300,DSCP:=12/440
0.0.0.0 100 0 i
Processed 2 prefixes, 2 paths
Router# show bgp ipv6 flowspec summary
BGP router identifier 202.158.0.1, local AS number 4787
BGP generic scan interval 60 secs
Non-stop routing is enabled
BGP table state: Active
Table ID: 0x0 RD version: 1503
BGP main routing table version 1504
BGP NSR Initial initsync version 2 (Reached)
BGP NSR/ISSU Sync-Group versions 1504/0
BGP scan interval 60 secs
BGP is operating in STANDALONE mode.
Process RcvTblVer bRIB/RIB LabelVer ImportVer SendTblVer StandbyVer
Speaker 1504 1504 1504 1504 1504 1504
Neighbor Spk AS MsgRcvd MsgSent TblVer InQ OutQ Up/Down St/PfxRcd
200.255.1.5 0 4787 6957 2957 1504 0 0 04:48:02 0
200.255.1.6 0 50011 3015 3010 0 0 0 05:27:50 (NoNeg)
202.158.2.1 0 4787 1548 1648 1504 0 0 1d01h 750 <-- this
many flowspecs were received from server
202.158.3.1 0 4787 1683 1644 1504 0 0 1d01h 751
202.158.4.1 0 4787 1543 1649 1504 0 0 1d01h 0
Router# show bgp vpnv4 flowspec summary
BGP router identifier 202.158.0.1, local AS number 4787
BGP generic scan interval 60 secs
Non-stop routing is enabled
BGP table state: Active
Table ID: 0x0 RD version: 0
BGP main routing table version 5
BGP NSR Initial initsync version 3 (Reached)
BGP NSR/ISSU Sync-Group versions 5/0
BGP scan interval 60 secs
BGP is operating in STANDALONE mode.
Process RcvTblVer bRIB/RIB LabelVer ImportVer SendTblVer StandbyVer
Speaker 5 5 5 5 5 5
Neighbor Spk AS MsgRcvd MsgSent TblVer InQ OutQ Up/Down St/PfxRcd
202.158.2.1 0 4787 1549 1648 5 0 0 1d01h 1 <-- this
many flowspecs were received from server
202.158.3.1 0 4787 1684 1644 5 0 0 1d01h 0
202.158.4.1 0 4787 1543 1649 5 0 0 1d01h 0
Router# show bgp vpnv6 flowspec summary
BGP router identifier 202.158.0.1, local AS number 4787
BGP generic scan interval 60 secs
Non-stop routing is enabled
BGP table state: Active
Table ID: 0x0 RD version: 0
BGP main routing table version 5
BGP NSR Initial initsync version 4 (Reached)
BGP NSR/ISSU Sync-Group versions 5/0
BGP scan interval 60 secs
BGP is operating in STANDALONE mode.
Process RcvTblVer bRIB/RIB LabelVer ImportVer SendTblVer StandbyVer
Speaker 5 5 5 5 5 5
Neighbor Spk AS MsgRcvd MsgSent TblVer InQ OutQ Up/Down St/PfxRcd
202.158.2.1 0 4787 1549 1649 5 0 0 1d01h 1 <-- this
many flowspecs were received from server
202.158.3.1 0 4787 1684 1645 5 0 0 1d01h 0
202.158.4.1 0 4787 1543 1650 5 0 0 1d01h 0
Router# show flowspec ipv4 detail
AFI: IPv4
Flow :Dest:10.1.1.1/32,Proto:=6,DPort:=80,SPort:=3000,Length:=200,DSCP:=10
Actions :Traffic-rate: 0 bps (bgp.1)
Statistics (packets/bytes)
Matched : 18174999/3707699796
Transmitted : 0/0
Dropped : 18174999/3707699796
Router# show flowspec ipv6 detail
AFI: IPv6
Flow
:Dest:70:1:1::1/0-128,Source:80:1:1::1/0-128,NH:=6,DPort:=22,SPort:=4000,TCPFlags:=0x10,Length:=300,DSCP:=12
Actions :Traffic-rate: 1000000 bps DSCP: cs1 Nexthop: 202:158:2::1 (bgp.1)
Statistics (packets/bytes)
Matched : 64091597/19483845488
Transmitted : 33973978/10328089312
Dropped : 30117619/9155756176
Router# show flowspec vrf customer_1 ipv4 detail
VRF: customer_1 AFI: IPv4
Flow :Dest:202.158.3.2/32,Source:202.158.1.2/32
Actions :Traffic-rate: 250000000 bps DSCP: cs6 Redirect: VRF dirty_dancing
Route-target: ASN2-4787:666 (bgp.1)
Statistics (packets/bytes)
Matched : 37260786850/4098686553500
Transmitted : 21304093027/2343450232970
Dropped : 15956693823/1755236320530
Router# show flowspec vrf customer_1 ipv6 detail
VRF: customer_1 AFI: IPv6
Flow
:Dest:200:158:3::2/0-128,Source:200:158:1::2/0-128,NH:=6,DPort:=22,SPort:=4000,Length:=300,DSCP:=12
Actions :Traffic-rate: 250000000 bps DSCP: cs6 Redirect: VRF dirty_dancing
Route-target: ASN2-4787:666 (bgp.1)
Statistics (packets/bytes)
Matched : 16130480136/4903665961344
Transmitted : 8490755776/2581189755904
Dropped : 7639724360/2322476205440
Router# show flowspec ipv4 nlri
AFI: IPv4
NLRI (hex) :0x01204601010103810605815006910bb80a81c80b810a
Actions :Traffic-rate: 0 bps (bgp.1)
Router# show flowspec ipv6 nlri
AFI: IPv6
NLRI (hex)
:0x018000007000010001000000000000000000010280000080000100010000000000000000000103810605811606910fa00981100a91012c0b810c
Actions :Traffic-rate: 1000000 bps DSCP: cs1 Nexthop: 202:158:2::1 (bgp.1)
Router# show flowspec vrf customer_1 ipv4 nlri
VRF: customer_1 AFI: IPv4
NLRI (hex) :0x0120ca9e03020220ca9e0102
Actions :Traffic-rate: 250000000 bps DSCP: cs6 Redirect: VRF dirty_dancing
Route-target: ASN2-4787:666 (bgp.1)
Router# show flowspec vrf customer_1 ipv6 nlri
VRF: customer_1 AFI: IPv6
NLRI (hex)
:0x018000020001580003000000000000000000020280000200015800010000000000000000000203810605811606910fa00a91012c0b810c
Actions :Traffic-rate: 250000000 bps DSCP: cs6 Redirect: VRF dirty_dancing
Route-target: ASN2-4787:666 (bgp.1)
Router# show policy-map transient type pbr
policy-map type pbr __bgpfs_default_IPv4
handle:0x36000004
table description: L3 IPv4 and IPv6
class handle:0x760013eb sequence 1024
match destination-address ipv4 10.1.1.1 255.255.255.255
match protocol tcp
match destination-port 80
match source-port 3000
Router# show flowspec vrf all afi-all summary
Flowspec VRF+AFI table summary:
VRF: default
AFI: IPv4
Total Flows: 1
Total Service Policies: 1
VRF: default
AFI: IPv6
Total Flows: 0
Total Service Policies: 0
Router# show flowspec ipv4 detail
Flow :Dest:192.0.2.254/24,DSCP:>=10&<=12
Actions :Traffic-rate: 5000000 bps Nexthop: 10.26.245.2 (bgp.1)
Statistics (packets/bytes)
Matched : 1169087/233817400
Transmitted : 369952/73990400
Dropped : 799135/159827000
Enabling BGP Flowspec for IPv6 Packet Length
Table 14. Feature History Table
Feature Name
Release Information
Feature Description
Enabling BGP Flowspec for IPv6 Packet Length
Release 7.10.1
Services such as end-to-end security, quality of service (QoS), and globally unique addresses are now supported for IPv6 packet
lengths, which allows your networks to scale and provides them with global reachability. Support for IPv6 packet lengths also
means that, in terms of the matching criteria, support for BGP Network Layer Reachability Information (BGP NLRI) type-10 flowspec
for IPv6 is added.
This feature introduces the following to enable BGP flowspec for IPv6 packet length:
An IPv6 address has 128 bits, or 16 bytes. The address is divided into eight 16-bit hexadecimal blocks separated by colons
(:) in the format: x:x:x:x:x:x:x:x. BGP Flowspec match conditions for IPv6 packet length support the standard length of 16
bits (2 bytes) or /128 IPv6 source IP address matches. By default, this IPv6 packet length is disabled.
This feature introduces the hw-module profile flowspec ipv6-packet-len-enable command that enables BGP Flowspec for IPv6 packet length. Support for IPv6 packet lengths also means that, in terms of the
matching criteria, support for BGP Network Layer Reachability Information (BGP NLRI) type-10 flowspec for IPv6 is added.
After configuring the command, you must reload the router for the feature to take effect.
Restriction
This packet length feature is supported only in the ingress direction for non-compression ACLs.
This feature is supported on:
8201-32FH
88-LC0-36FH-M
88-LC0-36FH-MO
8102-64H
8101-32H
8101-32H-O
8101-32FH
8202-32FH-M
88-LC0-34H14FH
88-LC1-36EH
Configuration
To enable BGP flowspec IPv6 packet length, perform the following actions:
Enter the IOS XR configuration mode.
Router#config
Enable the flowspec IPv6 packet length profile for an IPv6 interface.
Router(config)#hw-module profile flowspec ipv6-packet-len-enable
Thu Dec 15 09:15:49.226 UTC
In order to activate/deactivate this flowspec IPv6 packet-len profile, you must manually reload the chassis/all line cards
Commit the changes.
Router(config)#commit
After configuring the command, you must reload the router for the feature to take effect.
You can then configure IPv6 flowspec on the server router which acts as a BGP flowspec (bgpfs) server, and then define a policy
map and associate it with traffic classes.
Router(config)# class-map type traffic match-all class1
Router(config-cmap)# match protocol tcp
Router(config-cmap)# match destination-address ipv6 2:1:1::1/64
Router(config-cmap)# match packet length 0 65535
Router(config-cmap)# end-class-map
Router(config)# policy-map type pbr policy1
Router(config-pmap)# class type traffic class1
Router(config-pmap-c)# drop
Router(config-pmap-c)# end
Running Configuration
hw-module profile flowspec ipv6-packet-len-enable
!
class-map type traffic match-all class1
match protocol tcp
match destination-address ipv6 2:1:1::1/64
match packet length 0 65535
end-class-map
!
!
policy-map type pbr policy1
class type traffic class1
drop
end
!
!
!
Verification
This example shows sample output from show flowspec command when ipv6 keyword is used to display flowspec policy applied on IPv6 interfaces.
This example shows sample output from show flowspec command when afi-all keyword is used to display flowspec policy applied on IPv4 and IPv6 interfaces.
This feature allows you to maintain stale routing information from a failed BGP peer for longer periods of time than that
is configured in the Graceful Restart atribute. However, this feature ensures that the BGP neighbor considers the stale routes
as new routes.
When a BGP peer fails, the Extended Route Rention feature applies the route retention policy to the routes to modify the route
attributes. This feature modifies the route attributes in addition to the modification that occur due to neighbor's inbound
policy. This feature enables the use of route retention policy in place of LLGR, when the BGP hold timer expires or when the
BGP session fails to reestablish as a receiving speaker within the configured graceful retart timer.
When you apply LLGR, you cannot remove the LLGR_STALE community when the stale route is advertised, and the route will treat
it as the least preferred. Also, stale routes may be advertised to those neighbors that would not have advertised the LLGR
capability under the following confitions:
The neighbors must be internal (IBGP or confederation) neighbors.
The NO_EXPORT community must be attached to the stale routes.
The stale routes must have their LOCAL_PREF community set to zero.
This feature provides you the flexibility to advertise stale routes to eBGP neighbors and enable you to specify local preference
values for any stale route that is retained within the iBGP system.
Restrictions
The neighbor should be capable of graceful restart.
When the BGP neighbor fails, the graceful retart functionality is applied till the graceful restart timer is valid.
The Extend Route Retention feature starts, when the graceful restart timer expires,
Soft-reconfiguration inbound configuration is a mandatory configuration. If required, configure the inbound policy.
The Extended Route Retention feature starts only when BGP peer goes down, that is, on the expiry of the hold-down timer.
For any other trigger, such as the expiry of a timer, the routes will not be indicated as stale and the routes is purged.
The Extended Route Retention feature is applicable only to the following address-family modes:
IPv4 and IPv6 unicast address family mode
IPv4 and IPv4 labelled unicast address family mode
You cannot configure both LLGR and Extended Route Retention feature on the same neighbor.
When you configure the Extended Route Retention feature, the capablity attribute is not sent.
Configuration Example
How a CLUSTER_LIST Attribute is Used
The CLUSTER_LIST propagation rules differ among releases, depending on whether the device is running a Cisco software release
generated before or after the BGP—Multiple Cluster IDs feature was implemented. The same is true for loop prevention based
on the CLUSTER_LIST.
The CLUSTER_LIST behavior is described below. Classic refers to the behavior of software released before the multiple cluster
IDs feature was implemented; MCID refers to the behavior of software released after the feature was implemented.
CLUSTER_LIST Propagation Rules
Classic—Before reflecting a route, the RR appends the global cluster ID to the CLUSTER_LIST. If the received route had no
CLUSTER_LIST attribute, the RR creates a new CLUSTER_LIST attribute with that global cluster ID.
MCID—Before reflecting a route, the RR appends the cluster ID of the neighbor the route was received from to the CLUSTER_LIST.
If the received route had no CLUSTER_LIST attribute, the RR creates a new CLUSTER_LIST attribute with that cluster ID. This
behavior includes a neighbor that is not a client of the speaker. If the nonclient neighbor the route was received from does
not have an associated cluster ID, the RR uses the global cluster ID.
Loop Prevention Based on CLUSTER_LIST
Classic—When receiving a route, the RR discards the route if the RR's global cluster ID is contained in the CLUSTER_LIST of
the route.
MCID—When receiving a route, the RR discards the route if the RR's global cluster ID or any of the cluster IDs assigned to
any of the iBGP neighbors is contained in the CLUSTER_LIST of the route.
Configure a Cluster ID per Neighbor
Perform this task on an iBGP peer ,usually a route reflector, to configure a cluster ID per neighbor. Configuring a cluster
ID per neighbor causes the loop-prevention mechanism based on the CLUSTER_LIST to be automatically modified to take into account
multiple cluster IDs. Also, you gain the ability to disable client-to-client route reflection on the basis of cluster ID.
The software tags the neighbor so that you can disable route reflection with the use of another command.
Note
When you change a cluster ID for a neighbor, BGP automatically does an inbound soft refresh and an outbound soft refresh for
all iBGP peers.
The following example shows that if a cluster-id is configured on any level, either global or per-neighbour, it will be added
to the active cluster IDs regardless of the neighbour state. BGP does not track the neighbour state for this feature.
Router# show bgp process detail
BGP Process Information:
BGP is operating in STANDALONE mode
Autonomous System number format: ASPLAIN
Autonomous System: 65000
Router ID: 10.10.1.92 (manually configured)
Default Cluster ID: 10.10.1.92
Active Cluster IDs: 10.10.1.92, 10.10.3.93, 10.10.4.20
10.10.5.20, 198.51.100.254
...
Router# show configuration commit change last 1
Building configuration...
!! IOS XR Configuration 6.1.3
router bgp 65000
neighbor 198.51.100.254 <<< not operational, no AFs etc
remote-as 65000
cluster-id 198.51.100.254
!
!
end
Disable Client-to-Client Reflection for Specified Cluster IDs
Note
When the software changes reflection state for a given cluster ID, BGP sends an outbound soft refresh to all clients.
The following show command output shows that client-to-client reflection for the cluster IDs has been disabled.
Router# show bgp process
BGP Process Information:
BGP is operating in STANDALONE mode
Autonomous System number format: ASPLAIN
Autonomous System: 65000
Router ID: 0.0.0.0
Active Cluster IDs: 0.0.0.1
Fast external fallover enabled
Platform RLIMIT max: 2147483648 bytes
Maximum limit for BMP buffer size: 409 MB
Default value for BMP buffer size: 307 MB
Current limit for BMP buffer size: 307 MB
Current utilization of BMP buffer limit: 0 B
Neighbor logging is enabled
Enforce first AS enabled
Default local preference: 100
Default keepalive: 60
Non-stop routing is enabled
Update delay: 120
Generic scan interval: 60
Address family: IPv4 Unicast
Dampening is not enabled
Client reflection is not enabled in global config
Dynamic MED is Disabled
Dynamic MED interval : 10 minutes
Dynamic MED Timer : Not Running
Dynamic MED Periodic Timer : Not Running
Scan interval: 60
Total prefixes scanned: 0
Prefixes scanned per segment: 100000
Number of scan segments: 1
Nexthop resolution minimum prefix-length: 0 (not configured)
Main Table Version: 2
Table version synced to RIB: 2
Table version acked by RIB: 2
IGP notification: IGPs notified
RIB has converged: version 0
RIB table prefix-limit reached ? [No], version 0
Permanent Network Unconfigured
Node Process Nbrs Estb Rst Upd-Rcvd Upd-Sent Nfn-Rcv Nfn-Snt
node0_0_CPU0 Speaker 1 0 2 0 0 0 3
How to Implement BGP
Information About Implementing BGP
To implement BGP, you need to understand the following concepts:
Adjust BGP Timers
BGP uses certain timers to control periodic activities, such as the sending of keepalive messages and the interval after which
a neighbor is assumed to be down if no messages are received from the neighbor during the interval. The values set using the
timers bgp command in router configuration mode can be overridden on particular neighbors using the timers command in the neighbor configuration mode.
Perform this task to set the timers for BGP neighbors.
Procedure
Step 1
configure
Example:
RP/0/RP0/CPU0:router# configure
Enters
mode.
Step 2
router bgp as-number
Example:
Router(config)# router bgp 123
Specifies the autonomous system number and enters the BGP configuration mode, allowing you to configure the BGP routing process.
Step 3
timers bgpkeepalive hold-time
Example:
Router(config-bgp)# timers bgp 30 90
Sets a default keepalive time and a default hold time for all neighbors.
Step 4
neighborip-address
Example:
Router(config-bgp)# neighbor 172.168.40.24
Places the router in neighbor configuration mode for BGP routing and configures the neighbor IP address as a BGP peer.
Step 5
timerskeepalive hold-time
Example:
Router(config-bgp-nbr)# timers 60 220
(Optional) Sets the keepalive timer and the hold-time timer for the BGP neighbor.
Step 6
Use the
commit or
end command.
commit—Saves the configuration changes and remains
within the configuration session.
end—Prompts user to take one of these actions:
Yes— Saves configuration changes and exits the
configuration session.
No—Exits the configuration session without
committing the configuration changes.
Cancel—Remains in the configuration session,
without committing the configuration changes.
Enabling BGP Routing
Perform this task to enable BGP routing and establish a BGP routing process. Configuring BGP neighbors is included as part
of enabling BGP routing.
Note
At least one neighbor and at least one address family must be configured to enable BGP routing. At least one neighbor with
both a remote AS and an address family must be configured globally using the address family and remote as commands.
Before you begin
BGP must be able to obtain a router identifier (for example, a configured loopback address). At least, one address family
must be configured in the BGP router configuration and the same address family must also be configured under the neighbor.
Note
If the neighbor is configured as an external BGP (eBGP) peer, you must configure an inbound and outbound route policy on the
neighbor using the route-policy command.
Note
Instead of configuring an inbound and outbound route policy, you can configure the unsafe eBGP policy to allow all eBGP neighbors
to pass routes using the bgp unsafe-ebgp-policy command.
Note
While establishing eBGP neighborship between two peers, BGP checks if the two peers are directly connected. If the peers are
not directly connected, BGP does not try to establish a relationship by default. If two BGP peers are not directly connected
and peering is required between the loop backs of the routers, you can use the ignore-connected-check command. This command overrides the default check that BGP performs which is to verify if source IP in BGP control packets
is in same network as that of destination. In this scenario, a TTL value of 1 is sufficient if ignore-connected-check is used.
Configuring egp-multihopttl is needed when the peers are not directly connected and there are more routers in between. If the egp-multihopttl command is not configured, eBGP sets the TTL of packets carrying BGP messages to 1 by default. When eBGP needs to be setup
between routers which are more than one hop away, you need to configure a TTL value which is at least equal to the number
of hops between them. For example, if there are 2 hops (R2, R3) between two BGP peering routers R1 and R4, you need to set
a TTL value of 3.
Procedure
Step 1
configure
Example:
RP/0/RP0/CPU0:router# configure
Enters
mode.
Step 2
route-policyroute-policy-name
Example:
Routing(config)# route-policy drop-as-1234
Routing(config-rpl)# if as-path passes-through '1234' then
Routing(config-rpl)# apply check-communities
Routing(config-rpl)# else
Routing(config-rpl)# pass
Routing(config-rpl)# endif
(Optional) Creates a route policy and enters route policy configuration mode, where you can define the route policy.
Step 3
end-policy
Example:
Routing(config-rpl)# end-policy
(Optional) Ends the definition of a route policy and exits route policy configuration mode.
Step 4
Use the
commit or
end command.
commit—Saves the configuration changes and remains
within the configuration session.
end—Prompts user to take one of these actions:
Yes— Saves configuration changes and exits the
configuration session.
No—Exits the configuration session without
committing the configuration changes.
Cancel—Remains in the configuration session,
without committing the configuration changes.
Step 5
configure
Example:
RP/0/RP0/CPU0:router# configure
Enters
mode.
Step 6
router bgp as-number
Example:
Routing(config)# router bgp 120
Specifies the BGP AS number and enters the BGP configuration mode, allowing you to configure the BGP routing process.
Step 7
bgp router-idip-address
Example:
Routing(config-bgp)# bgp router-id 192.168.70.24
Configures the local router with a specified router ID.
Step 8
address-family {ipv4 | ipv6} unicast
Example:
Routing(config-bgp)# address-family ipv4 unicast
Specifies either the IPv4 or IPv6 address family and enters address family configuration submode.
To see a list of all the possible keywords and arguments for this command, use the CLI help (?).
Step 9
exit
Example:
Routing(config-bgp-af)# exit
Exits the current configuration mode.
Step 10
neighborip-address
Example:
Routing(config-bgp)# neighbor 172.168.40.24
Places the router in neighbor configuration mode for BGP routing and configures the neighbor IP address as a BGP peer.
Step 11
remote-asas-number
Example:
Routing(config-bgp-nbr)# remote-as 2002
Creates a neighbor and assigns a remote autonomous system number to it.
Specifies either the IPv4 or IPv6 address family and enters address family configuration submode.
To see a list of all the possible keywords and arguments for this command, use the CLI help (?).
Step 13
route-policyroute-policy-name {in | out}
Example:
Routing(config-bgp-nbr-af)# route-policy drop-as-1234 in
(Optional) Applies the specified policy to inbound IPv4 unicast routes.
Step 14
Use the
commit or
end command.
commit—Saves the configuration changes and remains
within the configuration session.
end—Prompts user to take one of these actions:
Yes— Saves configuration changes and exits the
configuration session.
No—Exits the configuration session without
committing the configuration changes.
Cancel—Remains in the configuration session,
without committing the configuration changes.
Configure Multiple BGP Instances for a Specific Autonomous System
Perform this task to configure multiple BGP instances for a specific autonomous system. All configuration changes for a single
BGP instance can be committed together. However, configuration changes for multiple instances cannot be committed together.
Configures a fixed router ID for the BGP-speaking router (BGP instance).
Note
You must manually configure unique router ID for each BGP instance.
Step 4
Use the
commit or
end command.
commit—Saves the configuration changes and remains
within the configuration session.
end—Prompts user to take one of these actions:
Yes— Saves configuration changes and exits the
configuration session.
No—Exits the configuration session without
committing the configuration changes.
Cancel—Remains in the configuration session,
without committing the configuration changes.
Configure Routing Domain Confederation for BGP
Perform this task to configure the routing domain confederation for BGP. This includes specifying a confederation identifier
and autonomous systems that belong to the confederation.
Configuring a routing domain confederation reduces the internal BGP (iBGP) mesh by dividing an autonomous system into multiple
autonomous systems and grouping them into a single confederation. Each autonomous system is fully meshed within itself and
has a few connections to another autonomous system in the same confederation. The confederation maintains the next hop and
local preference information, and that allows you to retain a single Interior Gateway Protocol (IGP) for all autonomous systems.
To the outside world, the confederation looks like a single autonomous system.
Procedure
Step 1
configure
Example:
RP/0/RP0/CPU0:router# configure
Enters
mode.
Step 2
router bgp as-number
Example:
Router# router bgp 120
Specifies the autonomous system number and enters the BGP configuration mode, allowing you to configure the BGP routing process.
Specifies that the BGP autonomous systems belong to a specified BGP confederation identifier. You can associate multiple AS
numbers to the same confederation identifier, as shown in the example.
Step 5
Use the
commit or
end command.
commit—Saves the configuration changes and remains
within the configuration session.
end—Prompts user to take one of these actions:
Yes— Saves configuration changes and exits the
configuration session.
No—Exits the configuration session without
committing the configuration changes.
Cancel—Remains in the configuration session,
without committing the configuration changes.
BGP Confederation: Example
The following is a sample configuration that shows several peers in a confederation. The confederation consists of three internal
autonomous systems with autonomous system numbers 6001, 6002, and 6003. To the BGP speakers outside the confederation, the
confederation looks like a normal autonomous system with autonomous system number 666 (specified using the bgp confederation identifier command).
In a BGP speaker in autonomous system 6001, the bgp confederation peers command marks the peers from autonomous systems 6002 and 6003 as special eBGP peers. Hence, peers 171.16 .232.55 and 171.16 .232.56 get the local preference, next hop, and MED unmodified in the updates. The router at 171 .19 .69.1 is a normal eBGP speaker, and the updates received by it from this peer are just like a normal eBGP update from a peer
in autonomous system 666.
In a BGP speaker in autonomous system 6002, the peers from autonomous systems 6001 and 6003 are configured as special eBGP
peers. Peer 171 .17 .70.1 is a normal iBGP peer, and peer 199.99.99.2 is a normal eBGP peer from autonomous system 700.
In a BGP speaker in autonomous system 6003, the peers from autonomous systems 6001 and 6002 are configured as special eBGP
peers. Peer 192 .168 .200.200 is a normal eBGP peer from autonomous system 701.
The following is a part of the configuration from the BGP speaker 192 .168 .200.205 from autonomous system 701 in the same example. Neighbor 171.16 .232.56 is configured as a normal eBGP speaker from autonomous system 666. The internal division of the autonomous system
into multiple autonomous systems is not known to the peers external to the confederation.
Resetting an eBGP Session Immediately Upon Link Failure
By default, if a link goes down, all BGP sessions of any directly adjacent external peers are immediately reset. Use the bgp fast-external-fallover disable command to disable automatic resetting. Turn the automatic reset back on using the no bgp fast-external-fallover disable command.
eBGP sessions flap when the node reaches 3500 eBGP sessions with BGP timer values set as 10 and 30. To support more than 3500
eBGP sessions, increase the packet rate by using the lpts pifib hardware police locationlocation-id command. Following is a sample configuration to increase the eBGP sessions:
Logging neighbor changes is enabled by default. Use the log neighbor changes disable command to turn off logging. The no log neighbor changes disable command can also be used to turn logging back on if it has been disabled.
Change BGP Default Local Preference Value
Perform this task to set the default local preference value for BGP paths.
Procedure
Step 1
configure
Example:
RP/0/RP0/CPU0:router# configure
Enters
mode.
Step 2
router bgp as-number
Example:
Router(config)# router bgp 120
Specifies the autonomous system number and enters the BGP configuration mode, allowing you to configure the BGP routing process.
Sets the default local preference value from the default of 100, making it either a more preferable path (over 100) or less
preferable path (under 100).
Step 4
Use the
commit or
end command.
commit—Saves the configuration changes and remains
within the configuration session.
end—Prompts user to take one of these actions:
Yes— Saves configuration changes and exits the
configuration session.
No—Exits the configuration session without
committing the configuration changes.
Cancel—Remains in the configuration session,
without committing the configuration changes.
Configure MED Metric for BGP
Perform this task to set the multi exit discriminator (MED) to advertise to peers for routes that do not already have a metric
set (routes that were received with no MED attribute).
Procedure
Step 1
configure
Example:
RP/0/RP0/CPU0:router# configure
Enters
mode.
Step 2
router bgp as-number
Example:
Routing(config)# router bgp 120
Specifies the autonomous system number and enters the BGP configuration mode, allowing you to configure the BGP routing process.
Step 3
default-metricvalue
Example:
Routing(config-bgp)# default metric 10
Sets the default metric, which is used to set the MED to advertise to peers for routes that do not already have a metric set
(routes that were received with no MED attribute).
Step 4
Use the
commit or
end command.
commit—Saves the configuration changes and remains
within the configuration session.
end—Prompts user to take one of these actions:
Yes— Saves configuration changes and exits the
configuration session.
No—Exits the configuration session without
committing the configuration changes.
Cancel—Remains in the configuration session,
without committing the configuration changes.
Configure BGP Weights
A weight is a number that you can assign to a path so that you can control the best-path selection process. If you have particular
neighbors that you want to prefer for most of your traffic, you can use the weight command to assign a higher weight to all routes learned from that neighbor. Perform this task to assign a weight to routes
received from a neighbor.
Procedure
Step 1
configure
Example:
RP/0/RP0/CPU0:router# configure
Enters
mode.
Step 2
router bgp as-number
Example:
Routing(config)# router bgp 120
Specifies the autonomous system number and enters the BGP configuration mode, allowing you to configure the BGP routing process.
Step 3
neighborip-address
Example:
Routing(config-bgp)# neighbor 172.168.40.24
Places the router in neighbor configuration mode for BGP routing and configures the neighbor IP address as a BGP peer.
Step 4
remote-asas-number
Example:
Routing(config-bgp-nbr)# remote-as 2002
Creates a neighbor and assigns a remote autonomous system number to it.
Specifies either the IPv4 or IPv6 address family and enters address family configuration submode.
To see a list of all the possible keywords and arguments for this command, use the CLI help (?).
Step 6
weightweight-value
Example:
Routing(config-bgp-nbr-af)# weight 41150
Assigns a weight to all routes learned through the neighbor.
Step 7
Use the
commit or
end command.
commit—Saves the configuration changes and remains
within the configuration session.
end—Prompts user to take one of these actions:
Yes— Saves configuration changes and exits the
configuration session.
No—Exits the configuration session without
committing the configuration changes.
Cancel—Remains in the configuration session,
without committing the configuration changes.
What to do next
You the clear bgp command for the newly configured weight to take effect.
Tune BGP Best-Path Calculation
BGP routers typically receive multiple paths to the same destination. The BGP best-path algorithm determines the best path
to install in the IP routing table and to use for forwarding traffic. The BGP best-path comprises of three steps:
Step 1—Compare two paths to determine which is better.
Step 2—Iterate over all paths and determines which order to compare the paths to select the overall best path.
Step 3—Determine whether the old and new best paths differ enough so that the new best path should be used.
Note
The order of comparison determined by Step 2 is important because the comparison operation is not transitive; that is, if
three paths, A, B, and C exist, such that when A and B are compared, A is better, and when B and C are compared, B is better,
it is not necessarily the case that when A and C are compared, A is better. This nontransitivity arises because the multi
exit discriminator (MED) is compared only among paths from the same neighboring autonomous system (AS) and not among all paths.
Perform this task to change the default BGP best-path calculation behavior.
Procedure
Step 1
configure
Example:
RP/0/RP0/CPU0:router# configure
Enters
mode.
Step 2
router bgp as-number
Example:
Router(config)# router bgp 126
Specifies the autonomous system number and enters the BGP configuration mode, allowing you to configure the BGP routing process.
