MP-BGP

BGP-4 carries only IPv4 unicast routing information. IETF extended BGP-4 by introducing Multiprotocol Extensions for BGP-4 (MP-BGP). MP-BGP can carry routing information for multiple address families, including IPv4 multicast, IPv6 unicast, IPv6 multicast, and VPNv4.

MP-BGP is backward compatible with BGP.

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• Unfeasible route prefixes in the withdrawn routes field.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.

The current MP-BGP implementation supports multiple protocol extensions, including VPN, IPv6, and multicast. For more information about VPN, see MPLS Configuration Guide.

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.