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Re: BGP Confederation config problem...

  • From: Andy McConnell
  • Date: Fri Mar 06 17:52:55 1998

More info:
Per Avi's suggestion, I implemented this route map for all outbound

	route-map addone permit 10
	  set metric +1

(assumes match all)

I'm running IOS 11.2 or 11.1(11) on the routers.  The Command reference
for 11.1 and 11.2 does not mention the ability to use +1 to increment, but
the online command "help" shows:

Enter configuration commands, one per line.  End with CNTL/Z.
router(config)#route-map addone
router(config-rou)#set metric ?
  +/-<metric>     Add or subtract metric
  <0-4294967295>  Metric value or IGRP bandwidth in Kbits per second

Does this really mean "Add one", or "Reset to Positive 1"?


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

Slogan of 105.9, the classic rock radio station in Chicago: "Of
all the radio stations in Chicago ... we're one of them."

On Fri, 6 Mar 1998, Andy McConnell wrote:

andym> I have a problem I cannot solve.  If anyone is running confederations like
andym> mine, I would appreciate some help :-)
andym> We have a 4-memberAS confederation, each with two IBGP peers.  The
andym> arrangement looks sort of like an octagon.  the "r" is AS3 is an internal 
andym> hop, not a BGP peer.  
andym>    _______     ______
andym>   |      R-----R---r |
andym>   |     / |   |    | |
andym>   |AS2 R  |   |AS3 R |
andym>    ----|--     ----|-
andym>        |           |
andym>    ____|__     ____|_
andym>   |AS4 R  |   |AS1 R |
andym>   |     \ |   |   /  |
andym>   |      R-------R   |
andym>    -------     ------
andym> The problem is this:  How do you get BGP to choose the shortest "AS PATH",
andym> since internal AS paths are ignored in selecting BGP routes?  Right now,
andym> to top router in AS4 will always choose a route through (2 3 1) instead of
andym> (1), because it prefers "external" routes (even external confederation
andym> routes) over internal routes.
andym> So, when given a choice, the router on the distant side of the AS will
andym> ALWAYS prefer the three-AS-hop path, because it is external.  Is there a
andym> way around this?!?
andym> Avi Freedman suggested using a +1 metric when leaving each member-AS. 
andym> (Thanks Avi!) But it doesn't seem to help - perhaps I didn't do it right. 
andym> In fact, it doesn't look like the metrics are adjusted more than 1.  for
andym> example, from AS2, the lower router sees every route outside of AS2 as
andym> having only metric 1!  The top router in AS3 does not add 1 to the metrics
andym> it readvertises to AS2.
andym> I've been stewing over this problem for some time... I believe there is
andym> some clue that I've missed.  Any help would be greatly appreciated!
andym> -Andy
andym> --
andym> Andy McConnell       アンディ マッコネル
andym> Network Architect, NTT Multimedia Communications Laboratories
andym> Writing about music is like dancing about architecture.