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Specifying Peers That Are Not Directly Connected

Normally, EBGP speakers are directly connected. When you cannot connect EBGP speakers directly, you can use the neighbor ebgp-multihop command to specify that the neighbor is more than one hop away. You generally need static routes to configure multihop connections. By default, the one-hop limitation per EBGP peers is enforced by the time-to-live attribute. You can override this default limit by using the ttl variable to specify the maximum number of hops to the peer.

In Figure 12, router Boston and router LA are connected together through router NY, rather than by a direct connection. Routers Boston and LA are configured as external peers with the neighbor ebgp-multihop command because no direct connection exists between them. Because router NY is not a BGP speaker, static routes are configured on routers Boston and LA. The configuration for router NY is not shown, because it does not involve BGP.

Figure 12: Using EBGP-Multihop

Image g013170.gif

The following commands achieve the BGP configuration.

To configure router Boston:



host1(config)#ip route 10.7.4.0 255.255.255.0
10.1.10.2 

host1(config)#router bgp 100 

host1(config-router)#neighbor 10.7.4.3 remote-as
300 

host1(config-router)#neighbor 10.7.4.3 ebgp-multihop

To configure router LA:



host2(config)#ip route 10.1.10.0 255.255.255.0
10.7.4.4 

host2(config)#router bgp 300 

host2(config-router)#neighbor 10.1.10.1 remote-as
100 

host2(config-router)#neighbor 10.1.10.1 ebgp-multihop

neighbor ebgp-multihop

Use to configure BGP to accept route updates from external peers in networks that are not directly connected to the local peer.
If you specify a BGP peer group by using the peerGroupName argument, all the members of the peer group inherit the characteristic configured with this command unless it is overridden for a specific peer.
Any external BGP peering that is not resolved by a connected route is treated as a multihop. Configurations with loopback-to-loopback external BGP peering require the neighbor ebgp-multihop command to work properly. In these configurations, the neighbor remote-as command is issued with the address of a loopback interface.
This command takes effect immediately and automatically bounces the BGP session.
Use the no version to return BGP to halt acceptance of such routers. Use the default version to remove the explicit configuration from the peer or peer group and reestablish inheritance of the feature configuration.
See neighbor ebgp-multihop

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