2. How to Identify a Path

As defined in [RFC4271], a path refers to the information reported in the Path Attribute field of an UPDATE message. As the procedures specified in [RFC4271] allow only the advertisement of one path for a particular address prefix, a path for an address prefix from a BGP peer can be keyed on the address prefix.

In order for a BGP speaker to advertise multiple paths for the same address prefix, a new identifier (termed "Path Identifier" hereafter) needs to be introduced so that a particular path for an address prefix can be identified by the combination of the address prefix and the Path Identifier.

The assignment of the Path Identifier for a path by a BGP speaker is purely a local matter. However, the Path Identifier MUST be assigned in such a way that the BGP speaker is able to use the (Prefix, Path Identifier) to uniquely identify a path advertised to a neighbor. A BGP speaker that re-advertises a route MUST generate its own Path Identifier to be associated with the re-advertised route. A BGP speaker that receives a route should not assume that the identifier carries any particular semantics.