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BGP Confederation config problem...

  • From: Andy McConnell
  • Date: Fri Mar 06 13:10:26 1998

I have a problem I cannot solve.  If anyone is running confederations like
mine, I would appreciate some help :-)

We have a 4-memberAS confederation, each with two IBGP peers.  The
arrangement looks sort of like an octagon.  the "r" is AS3 is an internal 
hop, not a BGP peer.  

   _______     ______
  |      R-----R---r |
  |     / |   |    | |
  |AS2 R  |   |AS3 R |
   ----|--     ----|-
       |           |
   ____|__     ____|_
  |AS4 R  |   |AS1 R |
  |     \ |   |   /  |
  |      R-------R   |
   -------     ------

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!


Andy McConnell       アンディ マッコネル
Network Architect, NTT Multimedia Communications Laboratories

Writing about music is like dancing about architecture.