> I have the following problem. I'm seeking advice on how to handle it.
> Surely someone else has been presented with this situation!
>
>
> AS X <----------> AS Y
> | |
> | |
> | |
> AS Z AS W
>
> AS Z and AS W are transit providers and are backbones or have connections
> to backbones (not shown). AS X and AS Y are interconnected to each other
> to that AS X has the benefit of using AS W for transit and vice versa.
>
> The idea is that AS X and AS Y pool together to buy 2 links, and they
> use each other's link.
Presumably you are not trying to use the link for backup only?
>
> Problem:
>
> AS X is likely to have a general tendancy to use AS Z too often because
> every AS path from AS W has an extra Y prepended to it, so it's probably
> longer.
>
> Conversely, AS Y is more likely to select AS W for the same reason.
>
> AS X could prepend once to paths from AS Z. That was the paths are
> longer by 1 whether they come from Z or W, so it will probably make a
> better selection. But this simply makes the problem worse for AS Y
> which is now going to receive paths from Z which are longer by 2.
>
> Is there a way in IOS to prepend an AS to an AS path, or otherwise
> influence the route selection process, for the local router or AS
> only, but send the original path received to external peers?
The real solution here is to use localpref as an AS exit tiebreak
rather than AS path length. This will achieve the result you are looking
for. Use AS Path stuffing (if you must) when sending routes from X to
Z and from Y to W. You can match the path in a routemap before stuffing
so you can manipulate how much paths from each downstream AS ae stuffed.
This will help you manipulate inbound traffic which is far more hard
than sorting outbound traffic (where localpref will do exactly what
you require).
-- Alex Bligh GX Networks (formerly Xara Networks)
This archive was generated by hypermail 2b29 : Sun Aug 04 2002 - 04:13:15 EDT