BGP Router-id BGP identifier wrong

[Alex Pfeil]

We have two routers (A and B) doing BGP with an ISP.  We had a scheduled outage to bring down side A. We did not have a static BGP router-id setup or a loopback interface configured on A or B.  We bring down side A, go through the scheduled outage, and then bring up side A. Somehow, B changed it's router-id to the IP address on A and they both had the same BGP router-id. Can somebody please explain this phenomenon?  Is this a bug?   

[Mike.Cifelli]

I assume once A came back up that it was unable to establish a peering session with your ISP if B is established and using the same router-ID, correct?

I believe BGP will use the IP configured with the command bgp router-id.  If not that, then by the 'update-source' as described below.  Or, by the highest IP on a loopback.  

In your BGP config you can specify your update source interface. For example:

neighbor x.x.x.x update-source (loopback0; intx/x; etc.)

I recommend manually configuring your router-id to avoid BGP flaps & any other potential issues in the future.  Hope this helps.

[Georg Pauwen]

Hello,

how are A and B connected ?

There are numerous bugs relating to router IDs...what platforms do you have (sh ver) ?

[Alex Pfeil]

Cisco 4451. The are connected over a VLAN sub-interface which has a few switches in between.

[Alex Pfeil]

Cisco IOS XE Software, Version 03.16.05.S - Extended Support Release
Cisco IOS Software, ISR Software (X86_64_LINUX_IOSD-UNIVERSALK9-M), Version 15.5(3)S5, RELEASE SOFTWARE (fc2)