In practice, an address family is associated with a specific network layer protocol, such as IPv4, IPv6, IPX or AppleTalk. There are also Address Family Identifier (AFI) numbers for tunneling mechanisms such as VPNs and MPLS. A Subsequent Address Family Identifier (SAFI) further specifies unicast versus multicast or more tunneling options.
When routers make a BGP connection, they exchange the AFIs and SAFIs they’re prepared to exchange BGP routing information for. If the two routers agree on those, they’ll send UPDATE messages containing the regular path attributes with the prefixes (also known as Network Layer Reachability Information, NLRI) and next hops encoded in an MP_REACH_NLRI attribute. Withdrawn routes are encoded in an MP_UNREACH_NLRI attribute.
