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Unable to explain routing behavior?


I have come across an issue with a few branch routers on our WAN network. We have an router in the home office that connects to an MPLS network. Then we have multiple branch routers that connect to the same MPLS network via T1 connections that home back to the home office.

We an issue arise after a MLPPP conversion on a branch router (we will call this router A). After the conversion on Router A Router B when on our broadband back up connectivity, Keep in mind that both Routers have completely different CER and PER as well as BGP AS numbers and Router B has been installed without issues for a long while.

Now after the conversion of Router A I had forgotten to remove the old BGP neighbor statements from the configuration. Router A was routing traffic across the T1 circuit, but Router B was going across the back up connectivity.

A Trace route from my computer in my home office to Router B showed the hop path to be Main Router > MPLS network > old PER of Router A > old CER of Router A.

Connecting to Router B and pinging the IP of the interface assigned to the T1 would was not pingable even though the interface was up/up. This caused us to believe we had a routing issue with our MPLS network provider.

After some time of fighting with them I remembered to remove the old BGP neighbors from Router A, during this change I accidently removed the new BGP neighbor statement. Once this was done Router B started to pass traffic across the T1 to the MPLS network and the trace route was correct to the destination.

I am at a lost for words on explaining what happened. I hope someone out there might be able to explain this to me in better detail.

Please let me know.