[c-nsp] Very Weird Problem with BGP Peer Groups and iBGP

[Oliver Boehmer (oboehmer)]

John Neiberger <> wrote on Saturday, March 19, 2005 12:15 AM:

> Nevermind. It's a bug...er...a feature:  CSCec14415 BGP next-hop not
> being set properly amongst peer-groups.
> 
> I guess I'll be doing a software upgrade on these routers!

Either upgrade, or configure your iBGP sessions between loopbacks
(update-source lo0), then the next-hop will be the same on all updates
and you effectively avoid the problem fixed in CSCec14415.

	oli

>>>> "John Neiberger" <John.Neiberger at efirstbank.com> 3/18/05 4:04:01 PM
>>>> 
> I had something really weird just happen that I think might be a bug,
> but it happened at two different sites configured exactly the same
> way.
> 
> Imagine Router A that has an eBGP session to an ISP, and it is
> accepting a default route. Router A has four iBGP peers (connected via
> frame relay point-to-point subinterfaces) configured in a peer group.
> The peer group settings include their remote-as and "next-hop-self".
> If
> you look at the BGP table on one of the iBGP peers you would see that
> the next hop for all routes is Router A, specifically the serial
> subinterface that connects the router to Router A.
> 
> At one point today, the default route was withdrawn temporarily by the
> ISP and then readvertised in a couple of minutes. The weird thing is
> that I lost IP connectivity from yet another remote site to a couple
> of
> the iBGP peers of Router A. When I checked their BGP tables I saw that
> several of the next hops were wrong. Instead of the IP address of the
> subinterface that connects the peer to Router A, I was seeing the IP
> address of some of the *other* subinterfaces!! Freaking weird. That
> should never happen, should it? If next-hop-self is configured, the
> next
> hop should always be the directly connected interface, right?
> 
> This is very very odd. I hope I've explained it in a way that makes
> sense. The story has been modified and abbreviated because I'm in a
> hurry.
> 
> Any thoughts?
> 
> Thanks!
> John
> --
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