BGP Path Selection weirdness regarding next hops

I posted this question to the Cisco NSP list and I've also talked to a
couple of guys from Cisco Advanced Services and I'm still stumped about
something. I'll try my best to phrase it in a way that makes sense.

Router A is learning about a prefix from two route reflector clients. In
both cases, the next hop for the prefix is the loopback address of the
advertising routers. Their loopback addresses are being advertised into
OSPF.

So, from the perspective of Router A, it's BGP table for this prefix has
two paths:

1: 4.4.4.4 (loopback address of Router B, learned via OSPF) * winner due
to lower IGP metric
2. 5.5.5.5 (loopback address of Router C, learned via OSPF)

Now for the weirdness to begin. A network event occurs that causes the
loopback address of Router C to go away. This shouldn't affect Router A
because it is already selecting the shortest path to the network via Router
B (4.4.4.4).

However, Router A is also learning a default via BGP. That means that even
though 5.5.5.5 (loopback of Router C) disappeared and is unreachable, the
router is doing a recursive lookup and keeps the path in the BGP table;
5.5.5.5 is still reachable, it thinks, by using the default route.

The weird thing is that this causes Router A to start using the wrong path!
It seems to be preferring a path with a next hop learned via BGP to a path
with a next hop learned via OSPF. Why would it do this? I see no
documentation that would explain why a BGP-learned next hop is preferred
over an IGP-learned next hop.

Is the router still comparing IGP metrics even though the "wrong" path now
has no IGP metric?

It's not changing due to router ID, cluster length, or neighbor IP address.
I checked. So, why is it switching?

As soon as the BGP session from Router A to Router C times out, the
extraneous path gets removed from the BGP table and the router goes back to
using the correct path it should have been using all along.

So, is a BGP-learned next hop preferred over an IGP-learned next hop? If
so, why? If not, any idea why my router switches paths? I've turned on BGP
debugging and IP routing debugging and haven't found a suitable explanation
for the switch.

John

Blogs and organic groups at http://www.ccie.net
Received on Fri Nov 30 2012 - 19:21:25 ART