MPUNREACHNLRI

MPUNREACHNLRI, formally MP_UNREACH_NLRI, is a BGP path attribute defined for Multiprotocol Extensions for BGP-4 (MP-BGP). It is used to withdraw previously announced NLRI for a given address family, identified by AFI (address family identifier) and SAFI (subsequent address family identifier). This attribute appears in BGP UPDATE messages that carry MP-BGP information and is part of the mechanism that allows a router to remove routes for specific families without affecting others.

The payload of MP_UNREACH_NLRI begins with the AFI (2 octets) and SAFI (1 octet), followed by a list of NLRI prefixes that are being withdrawn for that AFI/SAFI. Each withdrawn NLRI is encoded using the standard NLRI format used in BGP updates (prefix length in bits, followed by the prefix). The attribute does not carry next-hop information; that aspect is handled by MP_REACH_NLRI when routes are announced.

MP_UNREACH_NLRI is defined as an optional, transitive BGP path attribute. In practice, it is used in updates to remove routes for a specific MP-enabled address family, often in combination with MP_REACH_NLRI, which advertises newly reachable NLRI for the same AFI/SAFI. The two attributes together enable efficient management of multiple address families in MP-BGP.

See also MP_REACH_NLRI and RFC 4760, which formalize the multiprotocol extensions that underpin these attributes.