BGP Confederation config problem...
Andy McConnell
andym at nttlabs.com
Fri Mar 6 22:38:54 UTC 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
--
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>
blakew> What IGP are you running on this network?
blakew>
blakew> -Blake Willis
blakew> CAIS Engineering
blakew>
blakew> ---------------------------------------------------------------------------
blakew> Blake Willis 703-448-4470x483
blakew> Network Engineer, New Customers blakew at cais.net
blakew> CAIS Internet, a CGX Communications Company
blakew> ---------------------------------------------------------------------------
blakew>
blakew> On Fri, 6 Mar 1998, Andy McConnell wrote:
blakew>
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> >
blakew>
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