BGP Confederation config problem...

Sat Mar 7 19:35:32 UTC 1998

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