MPREACHNLRI

MPREACHNLRI, short for MP_REACH_NLRI, is a BGP path attribute used in Multiprotocol Extensions for BGP (MP-BGP). It carries reachability information for a specific address family, enabling the distribution of routes for multiple AFI/SAFI combinations beyond traditional IPv4 unicast.

The attribute is defined to convey, for a given AFI/SAFI, the necessary next-hop information along with a list of network layer reachability information (NLRI). Its payload includes the address family identifier (AFI), the subsequent address family identifier (SAFI), the length of the Next Hop field, the next-hop address(es), and the NLRI data (the actual prefixes). The Next Hop field can contain one or more addresses appropriate to the specified AFI.

MP_REACH_NLRI is used when advertising routes for non-default address families, such as IPv6 unicast routes, VPNv4 routes, or other specialized AFI/SAFI combinations, across MP-BGP sessions. It is complemented by MP_UNREACH_NLRI, the attribute used to withdraw previously advertised routes for a given AFI/SAFI.

In practice, a BGP speaker processes MP_REACH_NLRI to install or update routes in its routing table for the indicated AFI/SAFI and to set the correct next-hop for those routes. Proper handling of the AFI/SAFI and next-hop information is essential for correct route propagation and avoidance of instability across multi-protocol BGP deployments. MP_REACH_NLRI is one of the core components enabling scalable routing across diverse address families in modern BGP networks, as documented in RFC 4760 alongside the MP-BGP framework.