3.1 Routes: Advertisement and Storage

For purposes of this protocol a route is defined as a unit of information that pairs a destination with the attributes of a path to that destination:

• Routes are advertised between a pair of BGP speakers in UPDATE messages: the destination is the systems whose IP addresses are reported in the Network Layer Reachability Information (NLRI) field, and the the path is the information reported in the path attributes fields of the same UPDATE message.

• Routes are stored in the Routing Information Bases (RIBs): namely, the Adj-RIBs-In, the Loc-RIB, and the Adj-RIBs-Out. Routes that will be advertised to other BGP speakers must be present in the Adj-RIB-Out; routes that will be used by the local BGP speaker must be present in the Loc-RIB, and the next hop for each of these routes must be present in the local BGP speaker's forwarding information base; and routes that are received from other BGP speakers are present in the Adj-RIBs-In.

If a BGP speaker chooses to advertise the route, it may add to or modify the path attributes of the route before advertising it to a peer.

BGP provides mechanisms by which a BGP speaker can inform its peer that a previously advertised route is no longer available for use. There are three methods by which a given BGP speaker can indicate that a route has been withdrawn from service:

1. the IP prefix that expresses destinations for a previously advertised route can be advertised in the WITHDRAWN ROUTES field in the UPDATE message, thus marking the associated route as being no longer available for use

2. a replacement route with the same Network Layer Reachability Information can be advertised, or

3. the BGP speaker - BGP speaker connection can be closed, which implicitly removes from service all routes which the pair of speakers had advertised to each other.