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 -- 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.
More info: Per Avi's suggestion, I implemented this route map for all outbound advertisements: 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 <cr> Does this really mean "Add one", or "Reset to Positive 1"? -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, 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> 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> _______ ______ andym> | R-----R---r | andym> | / | | | | andym> |AS2 R | |AS3 R | andym> ----|-- ----|- andym> | | andym> ____|__ ____|_ andym> |AS4 R | |AS1 R | andym> | \ | | / | andym> | R-------R | andym> ------- ------ 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> 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> 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> 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> andym> -Andy andym> andym> andym> -- andym> Andy McConnell アンディ マッコネル andym> Network Architect, NTT Multimedia Communications Laboratories andym> andym> Writing about music is like dancing about architecture. andym>
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
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Andy McConnell -
dorian＠blackrose.org