[c-nsp] BGP routes co-existing with different local-preference

[Kristofer Sigurdsson]

On Tue, 2005-10-25 at 11:09 +0200, Gert Doering wrote:
> Hi,
> 
> On Tue, Oct 25, 2005 at 08:25:18AM +0000, Kristofer Sigurdsson wrote:
> > Now, when the 7500 has 2 routes to the same destination, from it's 
> > upstream and from the other edge router and the route received from
> > the other edge router has a higher local preference, the 7500 does
> > not advertise his route (the one with the "normal" local preference)
> > to other routers within our AS.
> 
> That's the way local-pref and BGP works.  local-pref will force this
> route to be "best" - and BGP will never announce non-"best" routes to
> peers.

Ack'd.  However, I thought (this seems to be where I'm mistaken) that
the prefix itself would still be advertised, even though it was not in
the routing table (to give the other routers the option of choosing 
which route to use based on their own engineering or something else, at 
least in iBGP).


> 
> Routes learned from iBGP peers (edge-1 to edge-2) will also never be
> re-announced to other iBGP peers.
> 
> So your router is doing what the BGP specs say it should do.
> 
> > If I set the weight of those routes (in the 7500) to 250, it advertises
> > them.
> 
> Sure - in that case, you force them to be "best" on this router, and thus
> they are advertised.

Yes, this was really meant as a hack to get the route into the other 
router's BGP tables, so they could quickly change which route went into
the routing table in case of failure (based on last time, where the
edge that survived does not seem to have started advertising the less
preferred routes when the other edge router got disconnected).  This may
be due to some timers, though, it was a rather short outage.

> 
> > Now, shouldn't the router advertise it's routes, even if there's another
> > route that is preferred?  The outcome from this would be if
> > the preferred outgoing edge router lost connectivity, the less preferred
> > route would not be in the other routers BGP tables, so no connectivity,
> > which is a bad thing (TM).
> 
> As soon as the "best route" is gone, all routers will select a new "best 
> route" *and* will advertise it (if it's coming from eBGP).

In this case, the 7500 would choose a best route (the eBGP neighbor
instead of the other edge router) and start advertising it, right?  This
seems not to have been the case.

> 
> > Cisco 7200 running 12.2S seems to do this the way I want (advertise all
> > routes to the other routers within the AS and let them choose).
> 
> I'd be very surprised if the 7200/12.2S violates the BGP spec here :-)

Yeah, me too.  This observation was done about 7 AM, where it was 
debatable whether I was awake or not and definately before the first
caffeine of the day...:-)

Thanks,
Kristofer