a bgp question

Unanswered Question
Mar 2nd, 2007

We have 2 routers(router1 & router2) that are connected to each other in NY city. Also, Router1 is connected to Router3 in Philly and Router2 is connected to Router4 in Boston. We r running BGP on the routers...The router1-router3 path is the primary path and had to be taken down to repair the circuit. So, the traffic is currently going through the router2-router4 path. When the router1-router 3 circuit is back up again, how do we ascertain that the BGP updates are going through it as opposed to the backup path?


I have this problem too.
0 votes
  • 1
  • 2
  • 3
  • 4
  • 5
Overall Rating: 0 (1 ratings)
Richard Burts Fri, 03/02/2007 - 19:24


Some additional information about your environment would help us to give you better answers. You tell us that there are 4 routers running BGP but you do not tell us whether they are all in the same AS (IBGP) or some in different AS (EBGP). It would also be useful to know whether router3 and router4 are advertising the same routes or are advertising unique routes.

But not knowing that helpful information there are some suggestions that I can make:

- on router1 perform show ip bgp neighbor. If router3 shows up as a neighbor then you know that BGP is established again over that circuit. If BGP is established then whatever was making it the primary route before should make it the primary route again.

- on router2 perform show ip bgp. Look for routes that are advertised from router3. If there are routes advertised uniquely from router3 this is easy. If not look for routes in the table with a next hop associated with router3.



axfalk1 Fri, 03/02/2007 - 19:51

Rick, Thanks for your response..

The router1 & router2 are in the same AS as are routers3 & 4. Also, the routers3 & 4 advertizing the same routes...

thanks again

Danilo Dy Sat, 03/03/2007 - 03:10


Since 4 routers are in the same AS, it would help if you can provide a diagram to show their connection. You know iBGP should be fully mesh, else assign RR (Route Reflector) for those that are not fully mesh. The RR should be the one with connections to all router in the AS and has connection to outside AS (eBGP)

axfalk Sat, 03/03/2007 - 18:05

I am sorry, the 4 router are not in the same AS - routers1 & 2 are in the same AS, however routers3 & 4 are in another...Sorry again...

Danilo Dy Sun, 03/04/2007 - 00:01

No problem Greg, thanks for clarifying. Rick suggestions still valid with this topology.

Richard Burts Sat, 03/03/2007 - 10:02


Thanks for clarifying the topology and the environment. I believe that the suggestions that I made in my first response are still good suggestions. If show ip bgp neighbor on router1 shows router3 as a neighbor that is a good sign that the circuit is back up and that bgp should be back to what you want. The more critical indication would be on router2. If show ip bgp shows routes advertised from router3 and especially if show ip route shows that traffic toward the internet is going from router2 to router1 then it confirms that BGP is back where you want it and the primary data path is router1 to router3.




This Discussion