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BGP Diverse Path Using a Diverse-Path Route Reflector
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BGP Diverse Path Using a Diverse-Path Route Reflector

Table Of Contents

BGP Diverse Path Using a Diverse-Path Route Reflector

Finding Feature Information

Contents

Prerequisites for BGP Diverse Path Using a Diverse-Path Route Reflector

Restrictions for BGP Diverse Path Using a Diverse-Path Route Reflector

Information About BGP Diverse Path Using a Diverse-Path Route Reflector

Limitation that a BGP Diverse Path Overcomes

BGP Diverse Path Using a Diverse-Path Route Reflector

Triggers to Compute a BGP Diverse Path

IGP Metric Check

Route Reflector Determination

How to Configure a BGP Diverse-Path Route Reflector

Determine Whether You Need to Disable the IGP Metric Check

Configure the Route Reflector for BGP Diverse Path

Configuration Examples for BGP Diverse Path Using a Diverse-Path Route Reflector

Example: BGP Diverse Path Where Additional Path Is the Backup Path

Example: BGP Diverse Path Where Additional Path Is the Multipath

Example: BGP Diverse Path Where Both Multipath and Backup Path Calculations Are Triggered

Example: Triggering Computation and Installation of a Backup Path

Additional References

Related Documents

Standards

MIBs

RFCs

Technical Assistance

Feature Information for BGP Diverse Path Using a Diverse-Path Route Reflector


BGP Diverse Path Using a Diverse-Path Route Reflector


First Published: July 25, 2011
Last Updated: July 25, 2011

The BGP Diverse Path Using a Diverse-Path Route Reflector feature allows BGP to distribute an alternative path other than the best path between BGP speakers when route reflectors are deployed. This feature is meant to provide path diversity within an AS, within a single cluster only. That is, an RR is allowed to advertise the diverse path to its client peers only.

Finding Feature Information

Your software release may not support all the features documented in this module. For the latest feature information and caveats, see the release notes for your platform and software release. To find information about the features documented in this module, and to see a list of the releases in which each feature is supported, see the "Feature Information for BGP Diverse Path Using a Diverse-Path Route Reflector" section.

Use Cisco Feature Navigator to find information about platform support and Cisco software image support. To access Cisco Feature Navigator, go to http://www.cisco.com/go/cfn. An account on Cisco.com is not required.

Contents

Prerequisites for BGP Diverse Path Using a Diverse-Path Route Reflector

Restrictions for BGP Diverse Path Using a Diverse-Path Route Reflector

Information About BGP Diverse Path Using a Diverse-Path Route Reflector

How to Configure a BGP Diverse-Path Route Reflector

Configuration Examples for BGP Diverse Path Using a Diverse-Path Route Reflector

Additional References

Feature Information for BGP Diverse Path Using a Diverse-Path Route Reflector

Prerequisites for BGP Diverse Path Using a Diverse-Path Route Reflector

You should understand the BGP Best External feature.

Restrictions for BGP Diverse Path Using a Diverse-Path Route Reflector

A diverse path can be configured on an RR only.

Only one shadow RR is allowed per existing RR, which will calculate one additional best path (the second best path). In other words, only one additional plane (topology) is configured.

Path diversity is configured within an AS, within a single RR cluster. That is, the RR will advertise the diverse path to its RR client peers only.

Diverse path functionality is not supported on a route server.

Information About BGP Diverse Path Using a Diverse-Path Route Reflector

Limitation that a BGP Diverse Path Overcomes

BGP Diverse Path Using a Diverse-Path Route Reflector

Triggers to Compute a BGP Diverse Path

IGP Metric Check

Route Reflector Determination

Limitation that a BGP Diverse Path Overcomes

As a path vector routing protocol, BGP-4 requires a router to advertise to its neighbors only the best path for a destination. However, multiple paths to the same destination would allow mechanisms that can improve resilience, quickly recover from failures, and load balance, for example.

The use of route reflectors (RR) is one of the main reasons for poor path diversity within an autonomous system (AS). In a network with RRs, even if a prefix is learned from multiple egress routers, the RR reflects only the best path to its clients. Figure 1 shows how deploying RRs might reduce path diversity in an AS, even when the BGP Best External feature is deployed.

Figure 1 Route Reflectors in an AS Reduce Path Diversity

In Figure 1, P1 and P2 are diverse paths for prefix p. Assume Path2 (P2) has a lower MED and higher local preference than P1. The BGP Best External feature on PE1 will make sure that P1 is propagated to the RRs, regardless of P2 having a lower MED and higher local preference. The RRs will have path diversity; they will learn both P1 and P2 with different exit points PE1 and PE2 (assuming that PE1 and PE2 have the set ip next-hop self command configured). However, both RRs select the best path as P2 due to its lower MED/higher local preference and advertise it to PE3. PE3 will not learn P1 (that is, PE3 will not learn about existing path diversity).

The BGP Diverse Path Using a Diverse-Path Route Reflector feature is a way to resolve that limitation and achieve path diversity.

BGP Diverse Path Using a Diverse-Path Route Reflector

The BGP Diverse Path Using a Diverse-Path Route Reflector feature overcomes the lack of path diversity in an AS containing RRs. This feature is meant to provide path diversity within an AS, within a single cluster only. That is, an RR is allowed to advertise the diverse path to its client peers only.

For each RR in the AS, a shadow RR is added to distribute the second best path, also known as the diverse path. Figure 2 shows the shadow RR for RR2. The shadow RR improves path diversity because PE3 can now learn both P1 (from RR1/RR2) and learn P2 from the shadow RR.


Note The primary RR and shadow RR must have the exact same connections (physical/control plane) to the rest of the routers in the network.


Shadow RRs can be both control plane RRs and data plane RRs.

Figure 2 Shadow RR Provides Second Best Path for Path Diversity

Figure 3 shows a diverse path in greater detail, indicating the next hops:

BR2 announces to RR1 and shadow RR2 that R2 (BR2) is the Next Hop for those who want to reach Prefix Z. Likewise, BR3 announces to RR1 and shadow RR2 that R3 (BR3) is the Next Hop for those who want to reach Prefix Z

RR1 sends a packet to BR1 announcing that the Next Hop is R2 if BR1 wants to reach Prefix Z. The second best path (or diverse path) comes from shadow RR2, which sends a packet to BR1 announcing that the Next Hop is R3 if BR1 want to reach Prefix Z.

At BR1 (far right), we see there are two (diverse) paths to Prefix Z.

Figure 3 RR and Shadow RR Provide Two Different Next Hops for Path Diversity

Triggers to Compute a BGP Diverse Path

Computation of a diverse path per address family is triggered by any of the following commands:

bgp additional-paths install

bgp additional-paths select

maximum-paths ebgp

maximum-paths ibgp

The bgp additional-paths install command will install the type of path that is specified in the bgp additional-paths select command. If the bgp additional-paths select command specifies both keyword options (best-external and backup), the system will install a backup path.

The maximum-paths ebgp and maximum-paths ibgp commands trigger a multipath computation, and multipaths are automatically installed as primary paths.

On the other hand, the bgp additional-paths install command triggers computation of a backup path or best-external path.

If the bgp additional-paths select command is not configured, the bgp additional-paths install command will trigger both computation and installation of a backup path (as is done with the BGP PIC feature).

IGP Metric Check

Disabling the IGP metric check and configuring BGP Diverse Path are independent of each other. One does not imply the other. That is, configuring bgp bestpath igp-metric ignore does not imply that the BGP Diverse Path feature is enabled. Conversely, enabling the BGP Diverse Path feature might not require that bgp bestpath igp-metric ignore be configured (because, for example, the RR and shadow RR are co-located).

The bgp bestpath igp-metric ignore command can be configurd at RRs and PEs.


Note Per-VRF functionality for the bgp bestpath igp-metric ignore command is not supported. If you use it anyway, it is at your own risk.


Route Reflector Determination

If a router's configuration includes either one of the following commands, the router is a route reflector:

bgp cluster-id

neighbor route-reflector-client

How to Configure a BGP Diverse-Path Route Reflector

Perform the tasks in this section to configure a BGP Diverse-Path Route Reflector.

Determine Whether You Need to Disable the IGP Metric Check (required)

Configure the Route Reflector for BGP Diverse Path (required)

Determine Whether You Need to Disable the IGP Metric Check

Before you configure a shadow RR in order to get a BGP diverse path, determine whether you need to disable the IGP metric check. The IGP metric is a configurable value indicating physical distance, and is used by an Interior Gateway Protocol, such as OSPF, EIGRP, or RIP. A smaller IGP metric is preferred over a larger IGP metric.

The locations of the RR and shadow RR determine whether or not you need to disable the IGP metric check, as follows:

When the RR and shadow RR are co-located—They have the same IP subnetwork address and are connected to the Ethernet switch with different links. Failure of such a link is equivalent to the RR going down. When RRs are co-located, their IGP metrics cannot be different from each other, and therefore there is no need to disable the IGP metric check during the best path calculation at any RR. Since there is no need to disable the IGP metric check, the first plane RRs do not need to be upgraded to Cisco IOS XE Release 3.4S.

When the shadow RR is in a different IGP place from the RR (it is not co-located with its best path RR)—In this case, the IGP metric check is ignored on both the best path RR and shadow RR when the best path and second best path are being calculated. The IGP metric check must be disabled on the primary RR by configuring the bgp bestpath igp-metric ignore command. This command is available beginning with Cisco IOS XE Release 3.4S, which means you need to upgrade to that release.

Configure the Route Reflector for BGP Diverse Path

Perform this task to configure a route reflector for the BGP Diverse Path feature. This task specifies the IPv4 address family, but other address families are also supported.

SUMMARY STEPS

1. enable

2. configure terminal

3. router bgp autonomous-system-number

4. neighbor ip-address remote-as autonomous-system-number

5. address-family ipv4 unicast

6. neighbor ip-address activate

7. maximum-paths ibgp number-of-paths

8. bgp bestpath igp-metric ignore

9. bgp additional-paths select [backup]

10. bgp additional-paths install

11. neighbor ip-address route-reflector-client

12. neighbor ip-address advertise diverse-path backup [mpath]

13. end

14. show ip bgp neighbor ip-address advertised-routes

DETAILED STEPS

 
Command or Action
Purpose

Step 1 

enable

Example:

Router> enable

Enables privileged EXEC mode.

Enter your password if prompted.

Step 2 

configure terminal

Example:

Router# configure terminal

Enters global configuration mode.

Step 3 

router bgp autonomous-system-number

Example:

Router(config)# router bgp 1

Enters router configuration mode for the BGP routing process.

Step 4 

neighbor ip-address remote-as autonomous-system-number

Example:

Router(config-router)# neighbor 10.1.1.1 remote-as 1

Adds an entry to the BGP or multiprotocol BGP neighbor table.

Step 5 

address-family ipv4 unicast

Example:

Router(config-router)# address-family ipv4 unicast

Specifies the address family and enters address family configuration mode.

Supported address families are ipv4 unicast, vpnv4 unicast, ipv6 unicast, vpnv6 unicast, ipv4+label, and ipv6+label.

Step 6 

neighbor ip-address activate

Example:

Router(config-router-af)# neighbor 10.1.1.1 activate

Enables the exchange of information with a BGP neighbor.

Step 7 

maximum-paths ibgp number-of-paths

Example:

Router(config-router-af)# maximum-paths ibgp 4

Controls the maximum number of parallel internal BGP (iBGP) routes that can be installed in a routing table.

Step 8 

bgp bestpath igp-metric ignore

Example:

Router(config-router-af)# bgp bestpath igp-metric ignore

Configures the system to ignore the Interior Gateway Protocol (IGP) metric during BGP best path selection.

Step 9 

bgp additional-paths select [backup]

Example:

Router(config-router-af)# bgp additional-paths select backup

Configures the system to calculate a second BGP bestpath.

Step 10 

bgp additional-paths install

Example:

Router(config-router-af)# bgp additional-paths install

Enables BGP to calculate a backup path for a given address family and to install it into the RIB and CEF.

Step 11 

neighbor ip-address route-reflector-client

Example:

Router(config-router-af)# neighbor 10.1.1.1 route-reflector-client

Configures the router as a BGP route reflector and configures the specified neighbor as its client.

Step 12 

neighbor ip-address advertise diverse-path [backup] [mpath]

Example:

Router(config-router-af)# neighbor 10.1.1.1 advertise diverse-path backup

(Optional) Configures a neighbor to receive the diverse path in an advertisement.

Step 13 

end

Example:

Router(config-router-af)# end

(Optional) Exits address family configuration mode and returns to privileged EXEC mode.

Step 14 

show ip bgp neighbor ip-address advertised-routes

Example:

Router# show ip bgp neighbor 10.1.1.1 advertised-routes

(Optional) Displays the routes advertised to the specified neighbor.

Configuration Examples for BGP Diverse Path Using a Diverse-Path Route Reflector

Example: BGP Diverse Path Where Additional Path Is the Backup Path

Example: BGP Diverse Path Where Additional Path Is the Multipath

Example: BGP Diverse Path Where Both Multipath and Backup Path Calculations Are Triggered

Example: Triggering Computation and Installation of a Backup Path

Example: BGP Diverse Path Where Additional Path Is the Backup Path

Diverse path functionality is contained to within a single cluster; that is, only the clients of an RR can be configured to advertise the diverse path. A diverse path is advertised to the clients of an RR only if the client is configured to get the additional path.

A shadow RR can be added to calculate and advertise the additional path, or an existing RR can be configured to calculate and advertise the additional path. In Figure 4, instead of adding a shadow RR, RR2 (the existing backup RR) is configured to calculate the additional path and advertise it to a particular neighbor.

In Figure 4, assume that from the RRs, the path to CE1 via PE1 is preferred over the path via PE2. Without the diverse path feature, both RRs will advertise to PE3 that the path to CE1 is via PE1. If the connection between RR1 and PE1 fails (or the path between PE1 and CE1 fails), there is no other path.

Figure 4 BGP Diverse Path Scenario

In the following configuration example based on Figure 4, RR2 is configured with an additional path, which is a backup path.

If RR1 and RR2 are not co-located, you must configure the bgp bestpath igp-metric ignore command before the additional path is calculated. (If RR1 and RR2 are co-located, do not configure that command.)

The bgp additional-paths select backup command triggers calculation of the backup path at RR2, which is the path via PE2.

The bgp additional-paths install command installs the backup path if RR2 is in the forwarding plane. (Do not configure this command if RR2 is in the control plane.)

The address of PE3 is 10.1.1.1, and that address is used in the neighbor advertise diverse-path backup command on RR2. This command triggers advertisement of the backup path to PE3. PE3 will learn the best path, (which is the path via PE1) from RR1, and it will learn the backup path from RR2.

RR2

router bgp 1
 neighbor 10.1.1.1 remote-as 1
 address-family ipv4 unicast
 neighbor 10.1.1.1 activate
 maximum-paths ibgp 4
 bgp bestpath igp-metric ignore
 bgp additional-paths select backup
 bgp additional-paths install
 neighbor 10.1.1.1 route-reflector-client
 neighbor 10.1.1.1 advertise diverse-path backup

Example: BGP Diverse Path Where Additional Path Is the Multipath

In the following example based on Figure 4, assume that paths toward CE1 via PE1 and PE2 are multipaths. The maximum-paths ibgp command will trigger calculation of multipaths.

The address of PE3 is 10.1.1.1, and that address is used in the neighbor advertise diverse-path mpath command on RR2. This command will trigger advertisement of the multipath, that is, the second best path, to PE3. PE3 will learn the best path, path via PE1 from RR1, and will learn second best path from RR2.

RR2

router bgp 1
 neighbor 10.1.1.1 remote-as 1
 address-family ipv4 unicast
 neighbor 10.1.1.1 activate
 maximum-paths ibgp 4
 neighbor 10.1.1.1 remote-as 1
 neighbor 10.1.1.1 route-reflector-client
 neighbor 10.1.1.1 advertise diverse-path mpath

Example: BGP Diverse Path Where Both Multipath and Backup Path Calculations Are Triggered

The following example is based on Figure 4. The maximum-paths ibgp command will trigger calculation of multipaths. When both multipath and backup path calculations are triggered, the backup path and the second multipath (which is the second best path) are the same paths and it will be installed as the active path, regardless of whether the RR is in the control plane or forwarding plane.

The address of PE3 is 10.1.1.1, and that address is used in the neighbor advertise diverse-path backup mpath command on RR2. This command causes RR2 to advertise the second best path, which is the second multipath, to PE3.

RR2

router bgp 1
 neighbor 10.1.1.1 remote-as 1
 address-family ipv4 unicast
 neighbor 10.1.1.1 activate
 maximum-paths ibgp 4
 bgp additional-paths select backup
 neighbor 10.1.1.1 remote-as 1
 neighbor 10.1.1.1 route-reflector-client
 neighbor 10.1.1.1 advertise diverse-path backup mpath

Example: Triggering Computation and Installation of a Backup Path

When the bgp additional-paths install command is configured without configuring bgp additional-paths select backup, the former command will trigger both computation and installation of the backup path (as it is with the existing BGP PIC feature).

The address of PE3 is 10.1.1.1, and that address is used in the neighbor advertise diverse-path backup command on RR2. This command will trigger advertisement of a backup path to PE3. PE3 will learn the best path, a path via PE1 from RR1, and it will learn a backup path from RR2.

RR2

router bgp 1
 neighbor 10.1.1.1 remote-as 1
 address-family ipv4 unicast
 neighbor 10.1.1.1 activate
 maximum-paths ibgp 4
 bgp additional-paths install
 neighbor 10.1.1.1 remote-as 1
 neighbor 10.1.1.1 route-reflector-client
 neighbor 10.1.1.1 advertise diverse-path backup

Additional References

Related Documents

Related Topic
Document Title

Cisco IOS commands

Cisco IOS Master Commands List, All Releases

BGP commands

Cisco IOS IP Routing: BGP Command Reference

Configuring BGP Best External Path on an RR for Intercluster

"BGP Best External" module

BGP configuration tasks

Cisco IOS XE IP Routing: BGP Configuration Guide


Standards

Standard
Title

draft-ietf-grow-diverse-bgp-path-dist-02.txt

Distribution of Diverse BGP Paths


MIBs

MIB
MIBs Link

No new or modified MIBs are supported by this feature, and support for existing MIBs has not been modified by this feature.

To locate and download MIBs for selected platforms, Cisco software releases, and feature sets, use Cisco MIB Locator found at the following URL:

http://www.cisco.com/go/mibs


RFCs


Technical Assistance

Description
Link

The Cisco Support and Documentation website provides online resources to download documentation, software, and tools. Use these resources to install and configure the software and to troubleshoot and resolve technical issues with Cisco products and technologies. Access to most tools on the Cisco Support and Documentation website requires a Cisco.com user ID and password.

http://www.cisco.com/cisco/web/support/index.html


Feature Information for BGP Diverse Path Using a Diverse-Path Route Reflector

Table 1 lists the release history for this feature.

Use Cisco Feature Navigator to find information about platform support and software image support. Cisco Feature Navigator enables you to determine which software images support a specific software release, feature set, or platform. To access Cisco Feature Navigator, go to http://www.cisco.com/go/cfn. An account on Cisco.com is not required.


Note Table 1 lists only the software release that introduced support for a given feature in a given software release train. Unless noted otherwise, subsequent releases of that software release train also support that feature.


Table 1 Feature Information for BGP Diverse Path Using a Diverse-Path Route Reflector

Feature Name
Releases
Feature Information

BGP Diverse Path Using a Diverse-Path Route Reflector

Cisco IOS XE Release 3.4S

This feature allows BGP to distribute an alternative path other than the best path between BGP speakers when route reflectors are deployed.

The following commands were introduced:

bgp additional-paths select

bgp bestpath igp-metric ignore

debug ip bgp igp-metric ignore

neighbor advertise best-external

neighbor advertise diverse-path