5-13To a BGP speaker that does not belong to any confederation, the sub-ASs of a confederation are awhole, and the information about the sub-ASs is invisible to the BGP speaker. The confederation ID,which is usually the corresponding AS number, uniquely identifies a confederation. In Figure 5-13, AS200 is a confederation ID.The disadvantage of confederation is that when an AS changes from non-confederation toconfederation, configurations are needed on the routers, and the topology changes.In a large-scale BGP network, router reflector and confederation can be used simultaneously.MP-BGPMP-BGP overviewBGP-4 can only process IPv4 routing information. It is not applicable to the applications using othernetwork layer protocols (such as IPv6) when inter-AS routing information exchange is required.To support multiple network layer protocols, IETF extends BGP-4 to MP-BGP. MP-BGP standard isdescribed in RFC2858, multiprotocol extensions for BGP-4.aMP-BGP is backward compatible. It enables communications to routers with BGP-4 employed.Extended attribute of MP-BGPOf different types of BGP-4 packets, all the information concerning to IPv4 are carried by Updatepackets. The information is hold by NLRI, NEXT_HOP (in the AS_PATH attribute), and AGGREGATOR(in the AS_PATH attribute). (The AGGREGATOR attribute contains the IP address of the BGP speakerthat generates a summarized route.)To support multiple network layer protocols, NLRI and NEXT_HOP need to hold the information aboutthe network layers. To achieve this, the following two path-related attributes are added in MP-BGP.z MP_REACH_NLRI, which stands for multiprotocol reachable NLRI and is used to advertisereachable routes and next hop information.z MP_UNREACH_NLRI, which stands for multiprotocol unreachable NLRI and is used to withdrawunreachable routes.The two attributes are all of the optional non-transitive type. Therefore, BGP speakers that do notsupport multiple protocols ignore the information carried in the two attributes and do not pass theinformation to their neighbors.Protocol StandardProtocol standards concerning BGP are:z RFC1771: A border gateway protocol 4 (BGP-4)z RFC2858: Multiprotocol extensions for BGP-4z RFC3392: Capabilities advertisement with BGP-4z RFC2918: Route refresh capability for BGP-4z RFC2439: BGP route flap dampingz RFC1997: BGP communities attributez RFC2796: BGP route reflectionz RFC3065: Autonomous system confederations for BGPOthers are still in draft, such as the graceful restart feature and the extended COMMUNITY attribute.
