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

[Gert Doering]

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.

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.

> 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).

> 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 :-)

gert

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Gert Doering - Munich, Germany                             gert at greenie.muc.de
fax: +49-89-35655025                        gert at net.informatik.tu-muenchen.de