For router A to send multiple paths per NLRI to peer B for a particular address family (AFI x, SAFI y), the BGP capability advertisement during session setup must indicate that router A must send multiple paths for (AFI x, SAFI y), and peer B is willing to receive multiple paths for (AFI x, SAFI y).
By default, unless changed through configuration, all paths for a particular NLRI in the LOC-RIB are advertised to all add-path peers with which the send capability has been negotiated. All such advertisements (and any subsequent withdrawals) include a path identifier. Each advertised path for a specific NLRI must have a unique path identifier. When a path is reflected or propagated from one peer to another, the path identifier is expected to change, even if there has been no change in the next-hop. A BGP Update message sent to an add-path peer may advertise and withdraw more than one NLRI belonging to one or more address families. In this case, the add-path may be supported for some address families and not others, and the path identifiers associated with different NLRI in the Update message may be the same or different.
In the current implementation, the add-path is only supported by the iBGP sessions it forms with other add-path capable peers. The add-path capability is not supported for eBGP sessions or for native IPv4 and IPv6 routes (that is, IPv4 and IPv6 routes advertised without a label) in iBGP sessions.The ability to receive multiple paths per prefix from an add-path peer is configurable per route type. The following route types are supported:
label-IPv4
label-IPv4