eBGP (external BGP) by default requires two Cisco IOS routers to be directly connected to each other in order to establish a neighbor adjacency. This is because eBGP routers use a TTL of one for their BGP packets. When the BGP neighbor is more than one hop away, the TTL will decrement to 0 and it will be discarded. When these two routers are not directly connected then we can still make it work but we’ll have to use multihop. This requirement does not apply to internal BGP
R2(config-router)#do
show run | s bgp
router bgp
200
no synchronization
bgp log-neighbor-changes
neighbor 1.1.1.1 remote-as 100
neighbor 1.1.1.1 ebgp-multihop 3
neighbor 1.1.1.1 update-source Loopback0
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