On Wed, Mar 18, 2009 at 11:00:32AM +0100, Arnoud Vermeer wrote:
> I have a problem with filtering on the current route server
> implementation. I currently have the following setup:
>
> >* 10.0.1.0/24 10.0.1.0/24
>
> +-----------+ +-----------+
> | AS1 | | AS2 |
> | 10.0.0.50 | | 10.0.0.51 |
> +-----------+ +-----------+
> | |
> | |
> +---+-----------+---+
> | RS |
> | 10.0.0.49 |
> +-----+-----+
> |
> | deny to { 10.0.0.52 } AS 1
> |
> +-----+-----+
> | AS3 |
> | 10.0.0.52 |
> +-----------+
>
> (or http://www.freshway.biz/files/20090318-problem-filter.txt for the
> correct ASCII)
>
> Both AS1 and AS2 announce the same prefix, but the route server selects
> the AS1 path because of the lower nexthop value. Now I add a filter to
> AS3. I deny to send any prefixes to AS3 that match AS1. Now AS3 doesn't
> receive the 10.0.1.0/24 prefix at all. It should however receive it from
> AS2.
>
> Quagga overcomes this problem by making a per-filtered-peer RIB and then
> do best path selection
> (http://www.quagga.net/docs/docs-multi/Description-of-the-Route-Server-model.html).
>
> I think this is just an ugly and complicated work-around as it doesn't
> solve the core of the problem.
>
> In my eyes the best solution will be to disable the best-path-selection
> on the route server altogether, and send all routes (except the
> filtered) to the peer.
>
> Arguments to do this:
> - As shown above, the best path selection breaks on the route server
> when applying filters.
> - A route server should not make any best-path selection, because the
> peers criteria could be completely different than the route server.
> - The function of the route server is to 'collect' all the routes and
> send them to all of the peers, not to 'collect a subset' of the routes
> and send that to its peers.
>
> I would love to hear your thoughts on this subject. Would it be hard to
> implement this feature?
>
BGP only supports one path per prefix and peer. If you send multiple ones
as you propose the later ones will overwrite the first one no matter what.
To support your idea we would need a per-filtered-peer local-RIB because
the route-server needs to do the best path selection for the peer.
--
:wq Claudio
