MP-BGP

BGP-4 can only advertise IPv4 unicast routing information. Multiprotocol Extensions for BGP-4 (MP-BGP) can advertise routing information for the following address families:

• IPv6 unicast address family.

• IPv4 multicast and IPv6 multicast address families.

  PIM uses static and dynamic unicast routes to perform RPF check before creating multicast routing entries. When the multicast and unicast topologies are different, you can use MP-BGP to advertise the routes for RPF check. MP-BGP stores the routes in the BGP multicast routing table. For more information about PIM and RPF check, see IP Multicast Configuration Guide.

Prefixes and next hops are key routing information. BGP-4 uses update messages to carry the following information:

• Feasible route prefixes in the Network Layer Reachability Information (NLRI) field.

• Unfeasible route prefixes in the withdrawn routes field.

• Next hops in the NEXT_HOP attribute.

BGP-4 cannot carry routing information for multiple network layer protocols.

To support multiple network layer protocols, MP-BGP defines the following path attributes:

• MP_REACH_NLRI—Carries feasible route prefixes and next hops for multiple network layer protocols.

• MP_UNREACH_NLRI—Carries unfeasible route prefixes for multiple network layer protocols.

MP-BGP uses these two attributes to advertise feasible and unfeasible routes for different network layer protocols. BGP speakers not supporting MP-BGP ignore updates containing these attributes and do not forward them to its peers.

MP-BGP uses address families and subsequent address families to identify different network layer protocols for routes contained in the MP_REACH_NLRI and MP_UNREACH_NLRI attributes. For example, an Address Family Identifier (AFI) of 2 and a Subsequent Address Family Identifier (SAFI) of 1 identify IPv6 unicast routing information carried in the MP_REACH_NLRI attribute. For address family values, see RFC 1700.