BGP problem 'lost middleman'

Daniel W. McRobb dwm at
Mon Oct 4 02:59:59 UTC 1993

	We monitor our peer sessions via the BGP3 MIB where we can.  But
we can't on our Ciscos because they haven't implemented the BGP3 MIB (or
any BGP MIB).

> 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

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