Using a loopback interface to define neighbors is common with iBGP, but not with eBGP. Normally the loopback interface is used to make sure the IP address of the neighbor stays up and is independent of hardware functioning properly. In the case of eBGP, peer routers are frequently directly connected and loopback doesn't apply.
If you use the IP address of a loopback interface in the neighbor command, you need some extra
configuration on the neighbor router. The neighbor router needs to tell BGP it's using a loopback interface rather than a physical interface to initiate the BGP neighbor TCP connection. The command used to indicate a loopback interface is:
neighbor ip−address update−source interface
The following example illustrates the use of this command.
RTA#
router bgp 100RTB#
neighbor 190.225.11.1 remote−as 100
neighbor 190.225.11.1 update−source loopback 1
router bgp 100
neighbor 150.212.1.1 remote−as 100
