5. Operation

The Path Identifier specified in Section 3 can be used to advertise multiple paths for the same address prefix without subsequent advertisements replacing the previous ones. Apart from the fact that this is now possible, the route advertisement rules of [RFC4271] are not changed. In particular, a new advertisement for a given address prefix and a given Path Identifier replaces a previous advertisement for the same address prefix and Path Identifier. If a BGP speaker receives a message to withdraw a prefix with a Path Identifier not seen before, it SHOULD silently ignore it.

For a BGP speaker to be able to send multiple paths to its peer, that BGP speaker MUST advertise the ADD-PATH Capability with the Send/Receive field set to either 2 or 3, and MUST receive from its peer the ADD-PATH Capability with the Send/Receive field set to either 1 or 3, for the corresponding <AFI, SAFI>.

A BGP speaker MUST follow the procedures defined in [RFC4271] when generating an UPDATE message for a particular <AFI, SAFI> to a peer unless the BGP speaker advertises the ADD-PATH Capability to the peer indicating its ability to send multiple paths for the <AFI, SAFI>, and also receives the ADD-PATH Capability from the peer indicating its ability to receive multiple paths for the <AFI, SAFI>, in which case the speaker MUST generate a route update for the <AFI, SAFI> based on the combination of the address prefix and the Path Identifier, and use the extended NLRI encodings specified in this document. The peer SHALL act accordingly in processing an UPDATE message related to a particular <AFI, SAFI>.

A BGP speaker SHOULD include the best route [RFC4271] when more than one path is advertised to a neighbor, unless it is a path received from that neighbor.

As the Path Identifiers are locally assigned, and may or may not be persistent across a control plane restart of a BGP speaker, an implementation SHOULD take special care so that the underlying forwarding plane of a "Receiving Speaker" as described in [RFC4724] is not affected during the graceful restart of a BGP session.