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

[Gert Doering]

Hi,

On Tue, Oct 25, 2005 at 09:35:04AM +0000, Kristofer Sigurdsson wrote:
> > 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).

Doing this might cause interesting routing loops...:

  Router A chooses "best path" -> B, but announces path via -> C to B
  Router B chooses "best path" -> A(->C)
    --> loop

[..]
> > 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.

Yes, if the "former best path" router crashes, it might take up to a minute
or so for BGP to learn that on the other 7500, and then some more time
to do new best-path calculations and announce the new paths out to the
other routers.

[..]
> > 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?  

Yes, but it might take some time.

> This seems not to have been the case.

In which case, this would be a bug - you'd need to check what its "new best"
route is, and see why it's not announcing it (filters, for example).

gert
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Gert Doering - Munich, Germany                             gert at greenie.muc.de
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