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Update Messages
BGP systems send update messages to exchange network
reachability information. BGP systems use this information to construct
a graph that describes the relationships among all known ASs.
Update messages consist of the BGP header plus
the following optional fields:
- Unfeasible routes length—Length of the field that
lists the routes being withdrawn from service because they are no
longer deemed reachable
- Withdrawn routes—IP address prefixes for the routes
being withdrawn from service
- Total path attribute length—Length of the field
that lists the path attributes for a feasible route to a destination
- Path attributes—Properties of the routes, including
the path origin, the multiple exit discriminator (MED), the originating
system’s preference for the route, and information about aggregation,
communities, confederations, and route reflection
- Network layer reachability information (NLRI)—IP
address prefixes of feasible routes being advertised in the update
message
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