Update messages are used to advertise and withdraw routes. An Update message provides the following information:
Withdrawn routes length
The length of the withdrawn routes field that is described next (may be 0).
Withdrawn routes
The IPv4 prefixes that are no longer considered reachable by the advertising router.
Total path attribute length
The length of the path attributes field that is discussed next (may be 0).
Path attributes
The path attributes presented in variable length TLV format. The path attributes apply to all the NLRI in the UPDATE message.
Network layer reachability information (NLRI)
The IPv4 prefixes that are considered reachable by the advertising router.
For fast routing convergence, as many NLRI as possible are packed into a single Update message as possible. This requires identifying all the routes that share the same path attribute values.