[c-nsp] Weird BGP Routing Problem

[Gert Doering]

Hi,

On Tue, Nov 04, 2008 at 03:49:33PM -0500, Paul Stewart wrote:
> Sometime this morning, we noted a sudden increase in outbound traffic to one
> of our transit providers.  Have now realized that some routes that should
> prefer peering are now going via transit.
> 
> What makes this very strange is that in our routing table, the best route
> chosen is not being honoured - very confused about this...
> 
> Below is an example:
> 
> core1-rtr-to#sh ip bgp xxx.xxx.xxx.105
> BGP routing table entry for xxx.xxx.xxx.0/18, version 36369947
> Paths: (5 available, best #1, table default)
>   Not advertised to any peer
>   xxxx
>     xxx.32.245.67 from xx.75.100.39 (xx.75.100.39)
>       Origin IGP, metric 0, localpref 200, valid, internal, best
>       Community: 5645:5000 11666:2000 11666:2001
> 
> Highest localpref, low metric and all kinds of other good reasons state this
> is the best route.  But this isn't the route being chosen and I don't know
> why...??

We don't know either, because you're not showing the relevant data to
answer, and my crystal ball seems to be cloudy today.

One possible guess would be that there is a more-specific route for the
specific destination that you're observing.

Start with "show ip route xxx.xx.xx.105", then check where that route
is coming from, and why it's not the BGP route.

gert
-- 
USENET is *not* the non-clickable part of WWW!
                                                           //www.muc.de/~gert/
Gert Doering - Munich, Germany                             gert at greenie.muc.de
fax: +49-89-35655025                        gert at net.informatik.tu-muenchen.de
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