Junos Asymmetric Routing

Date: Sun, 30 May 2010 18:39:39 +0200
From: Randy Bush <randy@psg.com>

> your perfectly fine multihop BGP session could break when rerouting
> occurs.

one of the many reasons that there are no perfectly fine multi-hop bgp

I remember a posting to this list back in the late 90s from Tony Li, who
knows a bit about BGP. He urged that multi-hop BGP never be used and
pointed out that it had not been intended for use except as a test tool,
not a production one and should have been stripped from IOS before it
was shipped.

While there are a few good cases for using it, it is generally a bad,
bad idea. And this thread demonstrates that he had reason for the warning

I think you guys may be getting a tad carried away with the crusade
against multihop BGP. The only thing you're really giving up when you
use it is liveness detection, which as we all know BGP is actually
pretty terrible at implementing anyways (hows that 180 sec IOS default
working out for you?). There are much better mechanisms out there, like
BFD, which could be used to provide better liveness detection to BGP
through nexthop invalidation.

I'm not saying everyone should run out and do all their peering over
multihop EBGP without carefully considering a replacement for the
liveness detection component, I just hate it when people get religious
about such a simple concept for no good reason (well, other than Randy
Bush getting to do his best Andy Rooney impersonation :P). Multihop BGP
is no more evil than anything else we do with the Internet, and the fact
that we've all managed to use it successfully for IBGP proves that it
can work out just fine. There are some pretty interesting things you can
accomplish as far as large scale traffic engineering if you can free
yourself from the requirement of speaking EBGP with a directly connected
neighbor, processed by whatever slow overpriced router CPU could be
stuffed into that box. Again, I just hate to see the concepts dismissed
out of hand because of some old BGP ideology about a problem that can be
addressed any number of other ways.