If you have started to look at BGP you will very soon come across the BGP update-source command. This command tells BGP which interface to source the BGP packets out of. The short demo below will highlight the problem.
I have a very simple setup with 2 routers connected with a serial interface and an ethernet interface
I have setup BGP and set the the neighbor statements on each router to point to the ethernet address.
R1#sh run | beg router bgp router bgp 1 no synchronization bgp router-id 10.0.0.3 bgp log-neighbor-changes neighbor 10.0.0.2 remote-as 1
R2#sh run | beg router bgp router bgp 1 no synchronization bgp log-neighbor-changes neighbor 10.0.0.1 remote-as 1
BGP comes up fine R2#sh ip bgp summary BGP router identifier 10.0.0.2, local AS number 1 BGP table version is 1, main routing table version 1
Neighbor V AS MsgRcvd MsgSent TblVer InQ OutQ Up/Down State/PfxRcd 10.0.0.1 4 1 24 24 1 0 0 00:20:15 0
However if we now setup ip addresses on the Serial interfaces and then change the neighbor statement on R1 to try and peer to the Serial address while the neighbor statement on R2 is trying to peer with the ethernet interface the BGP neighborship will fail.
As the router is receiving a BGP request on an interface that is not configured to to receive a reply on.
*Mar 1 06:43:13.138: BGP: 10.0.0.2 open active, local address 10.0.0.1 *Mar 1 06:43:13.146: BGP: 10.0.0.2 open failed: Connection refused by remote host, open active delayed 200ms
To fix this we need to use the command neighbor update-source
This tells BGP to send the update out the other interface and thus R2 will receive the BGP request on the interface it is expecting to hear on and the BGP relationship will come up.
R1 router bgp 1 no synchronization bgp log-neighbor-changes neighbor 126.96.36.199 remote-as 1 neighbor 188.8.131.52 update-source Serial0/0
R1# *Mar 1 06:49:11.430: %BGP-5-ADJCHANGE: neighbor 10.0.0.2 Up R1#
Now because we are telling the router to source the BGP packets out of serial interface the BGP neighborship comes up.
More info here