R0106-HP MSR Router Series Layer 3 - IP Routing Configuration Guide(V7)

204
In large-scale BGP networks, you can use both route reflector and confederation.
MP-BGP
BGP-4 can only carry IPv4 unicast routing information. Multiprotocol Extensions for BGP-4 (MP-BGP) can
carry 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.
• VPNv4 and VPNv6 address families.
For more information about VPNv4 and VPNv6, see MPLS Configuration Guide.
• L2VPN address family.
For more information about L2VPN and VPLS, see MPLS Configuration Guide.
• IPv4 MDT address family.
MP-BGP advertises MDT information including the PE address and default group so that multicast
VPN can create a default MDT that uses the PE as the root on the public network. For more
information about multicast VPN, see IP Multicast Configuration Guide.
MP-BGP extended attributes
Prefixes and next hops are key routing information. BGP-4 uses update messages to carry the following
attributes:
• 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• Unfeasible route prefixes in the withdrawn routes field.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.
These attributes 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.
Address family
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.