[15539] in North American Network Operators' Group
Re: BGP Confederation config problem...
daemon@ATHENA.MIT.EDU (dorian@blackrose.org)
Sat Mar 7 14:39:01 1998
Date: Sat, 7 Mar 1998 14:35:32 -0500
From: dorian@blackrose.org
To: Andy McConnell <andym@nttlabs.com>, nanog@merit.edu
Mail-Followup-To: Andy McConnell <andym@nttlabs.com>, nanog@merit.edu
In-Reply-To: <Pine.GSO.3.95LJ1.1b4.980306095952.26668D-100000@alicia>; from Andy McConnell on Fri, Mar 06, 1998 at 10:01:41AM -0800
On Fri, Mar 06, 1998 at 10:01:41AM -0800, Andy McConnell wrote:
> The problem is this: How do you get BGP to choose the shortest "AS PATH",
> since internal AS paths are ignored in selecting BGP routes? Right now,
> to top router in AS4 will always choose a route through (2 3 1) instead of
> (1), because it prefers "external" routes (even external confederation
> routes) over internal routes.
>
> So, when given a choice, the router on the distant side of the AS will
> ALWAYS prefer the three-AS-hop path, because it is external. Is there a
> way around this?!?
>
> Avi Freedman suggested using a +1 metric when leaving each member-AS.
> (Thanks Avi!) But it doesn't seem to help - perhaps I didn't do it right.
> In fact, it doesn't look like the metrics are adjusted more than 1. for
> example, from AS2, the lower router sees every route outside of AS2 as
> having only metric 1! The top router in AS3 does not add 1 to the metrics
> it readvertises to AS2.
>
> I've been stewing over this problem for some time... I believe there is
> some clue that I've missed. Any help would be greatly appreciated!
This won't work because Cisco's bgp implementation skips over MED
from neighboring sub ASes inside a confederation in its decision
process.
What you probably need is to get cisco to implement a knob that will
evaluate MEDs across sub-ASes inside a confederation.
Cisco is working on this, but I'm not sure what the ETA is.
-dorian
