3.1. Routes: Advertisement and Storage

3.1. Routes: Advertisement and Storage

For the purpose of this protocol, a route is defined as a unit of information that pairs a set of destinations with the attributes of a path to those destinations. The set of destinations are systems whose IP addresses are contained in one IP address prefix that is carried in the Network Layer Reachability Information (NLRI) field of an UPDATE message, and the path is the information reported in the path attributes field of the same UPDATE message.

Routes are advertised between BGP speakers in UPDATE messages. Multiple routes that have the same path attributes can be advertised in a single UPDATE message by including multiple prefixes in the NLRI field of the UPDATE message.

Routes are stored in the Routing Information Bases (RIBs): namely, the Adj-RIBs-In, the Loc-RIB, and the Adj-RIBs-Out, as described in Section 3.2.

If a BGP speaker chooses to advertise a previously received 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 peers 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 There are three methods by which a given BGP speaker can indicate that a route has been withdrawn from service:

a) the IP prefix that expresses the destination for a previously advertised route can be advertised in the WITHDRAWN ROUTES field in the UPDATE message, thus marking the associated route as being no longer available for use,

b) a replacement route with the same NLRI can be advertised, or

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

Changing the attribute(s) of a route is accomplished by advertising a replacement route. The replacement route carries new (changed) attributes and has the same address prefix as the original route.