eBGP Multihop Concepts
The fifth configuration step for using loopback IP addresses with eBGP peers refers to a feature called eBGP multihop. By default, when building packets to send to an eBGP peer, Cisco IOS sets the IP Time-To-Live (TTL) field in the IP header to a value of 1. With this default action, the eBGP neighborship fails to complete when using loopback interface IP addresses. The reason is that when the packet with TTL=1 arrives at the neighbor, the neighbor decrements the TTL value to 0 and discards the packet.
The logic of discarding the BGP packets can be a bit surprising, so an example can help. For this example, assume that the default action of TTL=1 is used and that eBGP multihop is not configured yet. Router E1 from Figure ...
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