- Cisco BGP Overview
- BGP 4
- Configuring a Basic BGP Network
- BGP 4 Soft Configuration
- BGP Support for 4-byte ASN
- IPv6 Routing: Multiprotocol BGP Extensions for IPv6
- IPv6 Routing: Multiprotocol BGP Link-Local Address Peering
- IPv6 Multicast Address Family Support for Multiprotocol BGP
- Configuring Multiprotocol BGP (MP-BGP) Support for CLNS
- Connecting to a Service Provider Using External BGP
- BGP Route-Map Continue
- BGP Route-Map Continue Support for Outbound Policy
- Removing Private AS Numbers from the AS Path in BGP
- Configuring BGP Neighbor Session Options
- BGP Neighbor Policy
- BGP Dynamic Neighbors
- BGP Support for Next-Hop Address Tracking
- BGP Restart Neighbor Session After Max-Prefix Limit Reached
- BGP Support for Dual AS Configuration for Network AS Migrations
- Configuring Internal BGP Features
- BGP VPLS Auto Discovery Support on Route Reflector
- BGP FlowSpec Route-reflector Support
- BGP Flow Specification Client
- BGP NSF Awareness
- BGP Graceful Restart per Neighbor
- BGP Support for BFD
- IPv6 NSF and Graceful Restart for MP-BGP IPv6 Address Family
- BGP Link Bandwidth
- iBGP Multipath Load Sharing
- BGP Multipath Load Sharing for Both eBGP and iBGP in an MPLS-VPN
- Loadsharing IP Packets over More Than Six Parallel Paths
- BGP Policy Accounting
- BGP Policy Accounting Output Interface Accounting
- BGP Cost Community
- BGP Support for IP Prefix Import from Global Table into a VRF Table
- BGP Support for IP Prefix Export from a VRF Table into the Global Table
- BGP per Neighbor SoO Configuration
- Per-VRF Assignment of BGP Router ID
- BGP Next Hop Unchanged
- BGP Support for the L2VPN Address Family
- BGP Event-Based VPN Import
- BGP Best External
- BGP PIC Edge for IP and MPLS-VPN
- Detecting and Mitigating a BGP Slow Peer
- Configuring BGP: RT Constrained Route Distribution
- Configuring a BGP Route Server
- BGP Diverse Path Using a Diverse-Path Route Reflector
- BGP Enhanced Route Refresh
- Configuring BGP Consistency Checker
- BGP—Origin AS Validation
- BGP MIB Support
- BGP 4 MIB Support for Per-Peer Received Routes
- BGP Support for Nonstop Routing (NSR) with Stateful Switchover (SSO)
- BGP NSR Auto Sense
- BGP NSR Support for iBGP Peers
- BGP Graceful Shutdown
- BGP — mVPN BGP sAFI 129 - IPv4
- BGP-MVPN SAFI 129 IPv6
- BFD—BGP Multihop Client Support, cBit (IPv4 and IPv6), and Strict Mode
- BGP Attribute Filter and Enhanced Attribute Error Handling
- BGP Additional Paths
- BGP-Multiple Cluster IDs
- BGP-VPN Distinguisher Attribute
- BGP-RT and VPN Distinguisher Attribute Rewrite Wildcard
- VPLS BGP Signaling
- Multicast VPN BGP Dampening
- BGP—IPv6 NSR
- BGP-VRF-Aware Conditional Advertisement
- BGP—Selective Route Download
- BGP—Support for iBGP Local-AS
- eiBGP Multipath for Non-VRF Interfaces (IPv4/IPv6)
- L3VPN iBGP PE-CE
- BGP NSR Support for MPLS VPNv4 and VPNv6 Inter-AS Option B
- BGP-RTC for Legacy PE
- BGP PBB EVPN Route Reflector Support
- BGP Monitoring Protocol
- VRF Aware BGP Translate-Update
- BGP Support for MTR
- BGP Accumulated IGP
- BGP MVPN Source-AS Extended Community Filtering
- BGP AS-Override Split-Horizon
- BGP Support for Multiple Sourced Paths Per Redistributed Route
- Finding Feature Information
- Restrictions for Support for iBGP Local-AS
- Information About Support for iBGP Local-AS
- Support for iBGP Local-AS
- Benefits of iBGP Local-AS
- How to Configure iBGP Local-AS
- Configuring iBGP Local-AS
- Configuration Examples for iBGP Local-AS
- Example: Configuring iBGP Local-AS
- Additional References for Support for iBGP Local-AS
- Feature Information for BGP—Support for iBGP Local-AS
BGP—Support for iBGP Local-AS
Prior to the BGP—Support for iBGP Local-AS feature, the neighbor local-as command was used on a route reflector to customize AS_PATH attributes for routes received from an eBGP neighbor. The neighbor local-as command can now be used to enable the sending of the iBGP attributes (LOCAL_PREF, ORIGINATOR_ID, CLUSTER_ID, and CLUSTER_LIST) over an iBGP local-AS session. This functionality is useful when merging two autonomous systems, when it is advantageous to keep the iBGP attributes in routes.
Prior to the BGP—Support for iBGP Local-AS feature, the RR should not have been configured to change iBGP attributes. With the introduction of this feature, the RR can be configured to change iBGP attributes, providing more flexibility.
- Finding Feature Information
- Restrictions for Support for iBGP Local-AS
- Information About Support for iBGP Local-AS
- Support for iBGP Local-AS
- Benefits of iBGP Local-AS
- How to Configure iBGP Local-AS
- Configuring iBGP Local-AS
- Configuration Examples for iBGP Local-AS
- Example: Configuring iBGP Local-AS
- Additional References for Support for iBGP Local-AS
- Feature Information for BGP—Support for iBGP Local-AS
Finding Feature Information
Your software release may not support all the features documented in this module. For the latest caveats and feature information, see Bug Search Tool and 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 table.
Use Cisco Feature Navigator to find information about platform support and Cisco software image support. To access Cisco Feature Navigator, go to www.cisco.com/go/cfn. An account on Cisco.com is not required.
Restrictions for Support for iBGP Local-AS
- This feature is not supported for a peer that belongs to a confederation.
- iBGP neighbors that are in a single AS are put into a separate update group from iBGP neighbors that are configured with the iBGP Local-AS feature.
- Two iBGP neighbors that are in two different autonomous systems and that are configured with the same iBGP Local-AS number are put into separate update groups.
Information About Support for iBGP Local-AS
Support for iBGP Local-AS
Prior to the Support for iBGP Local-AS feature, the neighbor local-as command was used to customize AS_PATH attributes for routes received from an eBGP neighbor. When a peer (or peer group) was configured with the neighbor local-as command and the neighbor remote-as command that specified the same AS number, that peer acted as an iBGP local-AS client, causing the local-AS number to be prepended to routes from that neighbor. However, the connection with the neighbor was not an iBGP connection, which meant that some attributes were not preserved in outgoing updates. Updates were propagated as if the peer were an eBGP peer, and therefore the LOCAL_PREF, ORIGINATOR_ID, CLUSTER_ID, and CLUSTER_LIST attributes were not added to the route during re-advertisement because those attributes apply to iBGP.
There can be scenarios where the network administrator wants those attributes to be passed unmodified, typically to facilitate the merging of two autonomous systems. The Support for iBGP Local-AS feature causes BGP to treat an iBGP local-AS connection as an iBGP connection and pass those four attributes unmodified.
In such a scenario, an RR is in one AS; the routers in the other AS become RR clients and keep their existing AS number. The RR is configured with the neighbor remote-as command and the neighbor local-as command that specify that neighbor (client) or peer group and its AS number. The session with that neighbor is treated as iBGP when the RR advertises routes received in the session to iBGP peers that have no neighbor local-as command configured. The RR transparently passes BGP updates to and from the client routers in the other AS, passing the LOCAL_PREF, ORIGINATOR_ID, CLUSTER_ID, and CLUSTER_LIST attributes unmodified instead of dropping them.
The figure below illustrates a scenario where this feature is being used to facilitate the merging of two autonomous systems. The route reflector (R3) and R4 belong to AS 1000; R1 and R6 belong to AS 3000. The RR is configured with the neighbor local-as command and the neighbor remote-as command, both specifying AS 3000. Even though the routers belong to two different autonomous systems, the local preference value is preserved in the updates from the client (R6) to the RR, and out to the other clients.
Prior to the Support for iBGP Local-AS feature, the RR should not have been configured to change iBGP attributes. With the introduction of this feature, the RR can be configured to change iBGP attributes. This flexibility is achieved by configuring the neighbor allow-policy command on the RR, and configuring an outbound route map to send attributes downstream to peers.
Benefits of iBGP Local-AS
One use case for this feature is when merging two ISPs that have different autonomous system numbers. It is desirable to preserve attributes that are considered internal (LOCAL_PREF, ORIGINATOR_ID, CLUSTER_ID, and CLUSTER_LIST) in the routes that are being passed to a different autonomous system. Configuring this feature causes these iBGP attributes in the advertised routes to be passed on to the second AS rather than dropped.
Prior to the BGP—Support for iBGP Local-AS feature, the RR should not have been configured to change iBGP attributes. With the introduction of this feature, the RR can be configured to change iBGP attributes. This flexibility is achieved by configuring the neighbor allow-policy command on the RR, and configuring an outbound route map to send iBGP attributes downstream to peers.
How to Configure iBGP Local-AS
Configuring iBGP Local-AS
Configure the iBGP Local-AS feature on a route reflector when you want iBGP attributes to be preserved in updates to a neighbor or peer group in another AS. Optionally, you can enable the route reflector to change iBGP attributes sent to a neighbor.
Note | This task includes the optional neighbor allow-policy command that enables you to configure the route reflector to change iBGP attributes, but it does not include the route map that actually changes iBGP attributes. |
1.
enable
2.
configure
terminal
3.
ipv6
unicast-routing
4.
router
bgp
autonomous-system-number
5.
neighbor peer-group-name peer-group
6.
neighbor {ip-address | ipv6-address} peer-group peer-group-name
7.
neighbor {ip-address
| ipv6-address | peer-group} remote-as as-number
8.
neighbor {ip-address
| ipv6-address | peer-group}
local-as as-number
9.
neighbor {ip-address
| ipv6-address | peer-group}
route-reflector-client
10.
address-family
vpnv4
11.
neighbor {ip-address
| ipv6-address | peer-group}
allow-policy
12.
exit
13.
address-family
vpnv6
14.
neighbor {ip-address
| ipv6-address | peer-group}
allow-policy
15.
end
16.
show ip bgp vpnv4 all neighbors
{ip-address
| ipv6-address}
policy
17.
show ip bgp vpnv4 all update-group update-group
18.
show ip bgp vpnv4 all neighbors
{ip-address
| ipv6-address}
DETAILED STEPS
Configuration Examples for iBGP Local-AS
Example: Configuring iBGP Local-AS
The example configures a route reflector (RR) in AS 4000 to treat BGP sessions with the peer group rr-client-2 in AS 2500 as iBGP sessions. That is, iBGP attributes (LOCAL_PREF, ORIGINATOR_ID, CLUSTER_ID, and CLUSTER_LIST) will not be dropped from routes in advertisements to and from the peer group; the attributes will be passed unmodified. AS 2500 will be prepended to the AS_PATH attribute in routes to and from the peer group.
Additionally, the neighbor allow-policy command configures that the network administrator can configure iBGP policies on the RR. That is, an outbound route map can be configured to change attributes that are sent to the downstream peers. In this example, the feature is applied to VPNv4 and VPNv6 address families.
router bgp 4000 neighbor rr-client-2 peer-group neighbor 192.168.1.1 peer-group rr-client-2 neighbor 192.168.4.1 peer-group rr-client-2 neighbor rr-client-2 remote-as 2500 neighbor rr-client-2 local-as 2500 neighbor rr-client-2 route-reflector-client address-family vpnv4 neighbor rr-client-2 allow-policy ! address-family vpnv6 neighbor rr-client-2 allow-policy
Additional References for Support for iBGP Local-AS
Related Documents
Related Topic |
Document Title |
---|---|
Cisco IOS commands |
|
BGP commands |
|
Migration of autonomous systems |
“BGP Support for Dual AS Configuration for Network AS Migrations” module in the IP Routing: BGP Configuration Guide, Cisco IOS XE Release 3S |
Technical Assistance
Description | Link |
---|---|
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Feature Information for BGP—Support for iBGP Local-AS
The following table provides release information about the feature or features described in this module. This table 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.
Use Cisco Feature Navigator to find information about platform support and Cisco software image support. To access Cisco Feature Navigator, go to www.cisco.com/go/cfn. An account on Cisco.com is not required.