What is NLRI?

NLRI (Network Layer Reachability Information) is the list of prefixes a BGP UPDATE message advertises.

A BGP UPDATE has three parts: withdrawn routes (prefixes that are no longer reachable), path attributes, and NLRI. The attributes, such as ORIGIN, AS_PATH and NEXT_HOP, apply to every prefix in the NLRI, so one UPDATE can announce many prefixes that share the same path. Each prefix is encoded compactly as a length in bits followed by just enough bytes to hold it, so 203.0.113.0/24 takes four bytes. The original NLRI field at the end of the message only carries IPv4 unicast. Every other address family, including IPv6, VPN routes and EVPN, is carried in the multiprotocol attributes MP_REACH_NLRI and MP_UNREACH_NLRI (RFC 4760), each labelled with an address family and subsequent address family (AFI/SAFI) and carrying its own next hop.

You meet the term mostly in specifications, packet captures and vendor documentation; router output usually just says "prefixes" or "routes". It helps explain several everyday behaviours. Address families must be negotiated as capabilities when the session opens, so a peer only accepts the kinds of NLRI both sides agreed to, which is why an IPv6 or EVPN family has to be activated before any of its routes flow. The multiprotocol design is also what allows IPv4 NLRI to be sent with an IPv6 next hop. The NLRI itself is just reachability: the decision about which path is best comes from the attributes that travel with it. The BGP Fundamentals lesson covers the message types and attributes, and IPv4 over an IPv6-only core relies on the multiprotocol encoding.