This topic explains how the BGP AS-path attribute is processed within an AS that contains a BGP confederation.
When BGP routing updates are sent by BGP-speaking routers over a BGP session, the BGP attributes are encoded in a binary structure. The AS-path attribute, which is printed out and displayed as a text string, is actually a type, length, value (TLV) binary field, which is composed of several segments. The intra-confederation part of the AS path is encoded by the intra-confederation router as a separate segment of the AS path with a new type code. This segment of the AS path contains a sequence of AS numbers that encode the member autonomous systems that the BGP update has traversed.
Because this segment is an extension to the original interpretation of the mandatory well-known BGP attribute AS path, a BGP implementation that does not support BGP confederations will not understand the intra-confederation part of the AS path. If a router receives a BGP update with a mandatory well-known attribute that the router cannot interpret, it will send a notification to the neighbor that sent the offending update and terminate the session. A router that does not support BGP confederations, therefore, cannot operate inside a BGP confederation.
6-60 Configuring BGP on Cisco Routers (BGP) v3.2 © 2005, Cisco Systems, Inc.
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