Cisco IOS Cookbook, 2nd Edition

Adjusting the Next-Hop Attribute

Problem

You want to change the next-hop attribute on routes while distributing them via iBGP so that the routes always point to a next-hop address that is inside your AS.

Solution

By default, the value of the next-hop attribute for an external route is the IP address of the external BGP router that announced this route to the AS. You can change this behavior so that the next-hop router is an internal router instead by using the next-hop-self command:

Router1#configure terminal
Enter configuration commands, one per line.  End with CNTL/Z.
Router1(config)#router bgp 65500
Router1(config-router)#neighbor 192.168.1.6 remote-as 65500
Router1(config-router)#neighbor 192.168.1.6 next-hop-self
Router1(config-router)#exit
Router1(config)#end
Router1#

Discussion

The next-hop attribute for a route depends on which router announces it. When a router passes route information to a peer in a different AS (using eBGP), it will generally update the next-hop attribute with its own IP address. However, by default iBGP peers will not change this attribute. For internal routes, the next-hop attribute will be the IP address of the router that sourced the internal route into BGP.

The result is that all of the routers inside of an AS will see the same external device as the next-hop BGP router, even if that router is actually several physical hops away. The following output shows the BGP table of one of the routers in our AS before we specified the next-hop-self command. All of ...