BGP problem 'lost middleman'

Mon Oct 4 02:54:22 UTC 1993

Dunno if this has happened to others before, but in case not:

I have a setup where 3 routers (ciscos, running 9.1.x if that's important)
are running IBGP.

	A---------B-----------C
	|
      ext-net

Router-A peers with 'B' & 'C', Router-B peers with B & C and C with B&A
(a full IBGP mesh).  Router A is an exit/border router. In addition to BGP,
the routers run IGRP. The BGP routes are kept separate from
the IGRP routes.

For whatever reason (not a configuration error), the BGP session between
A-B died but not between A-C, B-C.

The result was that C 'knew' via the BGP session with A, that to
get to external network 'ext-net', it should send traffic to A. It does
the IGRP lookup to get to A, and sends the traffic to 'B'.  However,
since B did not have a BGP session with A, it did not know how to get
to the ext-net  (in the real case, B was sending traffic back to C since
the default route was via C).

A potential disaster!! I realize that the problem was the unexpected
'lost' BGP session between A-B,  but are we sure that in a more 
complicated topology, something similar cannot happen ?

What really make me worried is that there doesn't seem to be way
to detect this kind of a problem easily.. how do others monitor
their BGP sessions ? 

	-vikas
	vikas at jvnc.net