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Re: BGP Confederation config problem...

  • From: Andy McConnell
  • Date: Fri Mar 06 17:45:22 1998

At each member AS, either IGRP or OSPF.  They do not interact.  By
design... two of the AS's are across the Pacific from the other two :-)


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, Blake Willis wrote:

blakew> Andy,
blakew> 	What IGP are you running on this network?
blakew> 	-Blake Willis
blakew> 	CAIS Engineering
blakew> ---------------------------------------------------------------------------
blakew> Blake Willis           					703-448-4470x483
blakew> Network Engineer, New Customers				[email protected]
blakew> CAIS Internet, a CGX Communications Company
blakew> ---------------------------------------------------------------------------
blakew> On Fri, 6 Mar 1998, Andy McConnell wrote:
blakew> > I have a problem I cannot solve.  If anyone is running confederations like
blakew> > mine, I would appreciate some help :-)
blakew> > 
blakew> > We have a 4-memberAS confederation, each with two IBGP peers.  The
blakew> > arrangement looks sort of like an octagon.  the "r" is AS3 is an internal 
blakew> > hop, not a BGP peer.  
blakew> > 
blakew> >    _______     ______
blakew> >   |      R-----R---r |
blakew> >   |     / |   |    | |
blakew> >   |AS2 R  |   |AS3 R |
blakew> >    ----|--     ----|-
blakew> >        |           |
blakew> >    ____|__     ____|_
blakew> >   |AS4 R  |   |AS1 R |
blakew> >   |     \ |   |   /  |
blakew> >   |      R-------R   |
blakew> >    -------     ------
blakew> > 
blakew> > The problem is this:  How do you get BGP to choose the shortest "AS PATH",
blakew> > since internal AS paths are ignored in selecting BGP routes?  Right now,
blakew> > to top router in AS4 will always choose a route through (2 3 1) instead of
blakew> > (1), because it prefers "external" routes (even external confederation
blakew> > routes) over internal routes.
blakew> > 
blakew> > So, when given a choice, the router on the distant side of the AS will
blakew> > ALWAYS prefer the three-AS-hop path, because it is external.  Is there a
blakew> > way around this?!?
blakew> > 
blakew> > Avi Freedman suggested using a +1 metric when leaving each member-AS. 
blakew> > (Thanks Avi!) But it doesn't seem to help - perhaps I didn't do it right. 
blakew> > In fact, it doesn't look like the metrics are adjusted more than 1.  for
blakew> > example, from AS2, the lower router sees every route outside of AS2 as
blakew> > having only metric 1!  The top router in AS3 does not add 1 to the metrics
blakew> > it readvertises to AS2.
blakew> > 
blakew> > I've been stewing over this problem for some time... I believe there is
blakew> > some clue that I've missed.  Any help would be greatly appreciated!
blakew> > 
blakew> > -Andy
blakew> > 
blakew> > 
blakew> > --
blakew> > Andy McConnell       アンディ マッコネル
blakew> > Network Architect, NTT Multimedia Communications Laboratories
blakew> > 
blakew> > Writing about music is like dancing about architecture.
blakew> >