Bgp Peering

how bgp peering works :

Server Client Relashionship :

If Router A & Router B have Bgp relashionship than who will initiate connection :

By default,Highes BGP Router-id will initiate connection :

For example :

Rourter A(lo4:4.4.4.4/24)<===>Router B(lo5 5.5.5.5/24)
Server Client

To verify which side is Server or Client Currenlty :

sh tcp brief :
65B92ED8 5.5.5.5.21337 4.4.4.4.179 ESTAB
=============================================================
if you issue clear ip bgp * one of the side ( for example Router A) ,it will Initiate Connection :
if you issue clear ip bgp * at both side than you will see above output ( Higher router-id will become client ).

If you want to prevent one side to initiate the connection than there is couple of option :
Assumption : using loopback on both side :

R4 <=======>R5

R4 :
config t
router bgp 100
neighbor 5.5.5.5 remote-as 100
neighbor 5.5.5.5 ebgp-multihope
neighbor 5.5.5.5 transport connection-mode passive( will not initiate connection)

Obviosuly on R5 you can use neighbor 4.4.4.4 tranport connection-mode active ( to actively initiate connection)


R5

config t
router bgp 100
neighbor 4.4.4.4 remote-as 100
neighbor 4.4.4.4 ebgp-multihope
neighbor 4.4.4.4 transport connection-mode active