BGP: update source

Just a note to remind myself that to establish eBGP connection if loopback is used as the source then neighbor <peer’s ip address> update-source loopback<id>is necessary.

By default BGP will use the interface ip as the source address to establish TCP connection. If update-source is not used then the BGP adjacency will never be formed, and will always stuck in Active state.

Active state is the state whereby a router is attempting to establish a TCP connection to the destination.

Think of the remote BGP router as a server which is constantly listening to port 179, my local BGP router needs to establish a tcp connection to destination port 179 and start the 3-way handshake, at least one BGP router must have an update-source statement in order to establish adjacency.

Here’s the commands:

1841-2#sh run | s router 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 2
neighbor 1.1.1.1 update-source Loopback200
no auto-summary

1841-1#sh run | i ip route
ip route 2.2.2.2 255.255.255.255 172.2.10.2

1841-1#sh run | s router bgp
router bgp 100
no synchronization
bgp log-neighbor-changes
neighbor 2.2.2.2 remote-as 200
neighbor 2.2.2.2 ebgp-multihop 2
neighbor 2.2.2.2 update-source Loopback100
no auto-summary

1841-2#sh run | i ip route
ip route 1.1.1.1 255.255.255.255 172.2.10.1

My mistake here is I have forgotten to use update-source keyword! Deadly mistake!