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Update: oversized AS paths

Update @ 2009-02-20: Root cause analysis, Detailed Cisco IOS bug description

I was wrong about the details of yesterday's Internet brownout: older IOS releases don't recognize AS-paths having more than 128 AS numbers due to improper handling of extended length flag in the BGP UPDATE message (CSCdr54230).

However, quick stress tests indicate that classic IOS releases (including 12.2SRC) can't handle AS-paths having more than 255 AS numbers. IOS is able to accept (and properly process) inbound updates with two AS_SEQUENCE segments, but does not generate valid AS-path attribute in outbound update when there are more than 255 AS numbers in the AS-path, resulting in a NOTIFICATION message and continuously flapping BGP session. The only global protection you have against this behavior is the bgp maxas-limit router configuration command.

I've also updated the Wiki article.

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