To support multiple network layer protocols, the Internet Engineering Task Force (IETF) extends BGP-4 to Multiprotocol Extensions for BGP-4 (MP-BGP). RFC 4760 defines the MP-BGP standard. MP-BGP is forward compatible. Specifically, CX devices supporting MP-BGP can communicate with the CX devices that do not support MP-BGP.
BGP-4 Update packets carry three IPv4-related attributes: NLRI (Network Layer Reachable Information), Next_Hop, and Aggregator. Aggregator contains the IP address of the BGP speaker that performs route aggregation.
To carry information about multiple network layer protocols in NLRI and Next_Hop, MP-BGP introduces the following route attributes:
- MP_REACH_NLRI: indicates the multiprotocol reachable NLRI. It is used to advertise a reachable route and its next hop.
- MP_UNREACH_NLRI: indicates the multiprotocol unreachable NLRI. It is used to delete an unreachable route.
The preceding two attributes are optional non-transitive. Therefore, the BGP speakers that do not support MP-BGP will ignore the information carried in the two attributes and do not advertise the information to other peers.
The Address Family Information field consists of a 2-byte Address Family Identifier (AFI) and a 1-byte Subsequent Address Family Identifier (SAFI).
BGP uses address families to distinguish different network layer protocols. For the values of address families, see RFC 3232 (Assigned Numbers). The CX600 supports multiple MP-BGP extension applications, such as VPN extension and IPv6 extension, which are configured in their respective address family views.