1 BGP NEIGHBORS/PEERSTWO BGP ROUTERS BECOME NEIGHBORS OR PEERS ONCE...

Two BGP routers become neighbors or peers once they establish a TCP

connection between one another. The TCP connection is essential in order

for the two peer routers to start exchanging routing updates.

Two BGP speaking routers trying to become neighbors will first bring up

the TCP connection between one another and then send open messages in

order to exchange values such as the AS number, the BGP version they are

running (version 3 or 4), the BGP router ID and the keepalive hold time,

etc. After these values are confirmed and accepted the neighbor

connection will be established. Any state other than established is an

indication that the two routers did not become neighbors and hence the

BGP updates will not be exchanged.

The neighbor command used to establish a TCP connection is:

neighbor

ip-address remote-as number

The remote-as number is the AS number of the router we are trying to

connect to via BGP.

The ip-address is the next hop directly connected address for EBGP

¹

and

any IP address

²

on the other router for IBGP.

It is essential that the two IP addresses used in the neighbor command of

the peer routers be able to reach one another. One sure way to verify

reachability is an extended ping between the two IP addresses, the

extended ping forces the pinging router to use as source the IP address

specified in the neighbor command rather than the IP address of the

interface the packet is going out from.