3Com Router 3000 Ethernet FamilyConfiguration Guide Chapter 6 MBGP Multicast Extension Configuration3Com Corporation6-1Chapter 6 MBGP Multicast ExtensionConfiguration6.1 MBGP Multicast Extension6.1.1 MBGP Multicast Extension OverviewAt present, the most widely used inter-domain unicast routing protocol is BGP-4.Because the multicast topology may be different from the unicast topology, BGP-4 mustbe modified in order to implement the transmission of inter-domain multicast routinginformation. Some routers in the network may only support unicast rather thanmulticast and may not forward multicast packets since the particular policy requires that.To construct inter-domain multicast trees, the knowledge of multicast-supporting partsof the network (or the multicast network topology) is also necessary, other than that ofthe unicast routing information.BGP-4 has been proved to be an effective and stable inter-domain unicast routingprotocol. Therefore, it is more rational to enhance and extend the BGP-4 protocol thanto construct a new protocol. RFC2858 specifies the methods of multi-protocol BGPextension. The extended BGP (Multi-protocol BGP, MBGP or BGP-4+) carries not onlythe IPv4 unicast routing information but also routing information of other network layerprotocols (such as multicast and IPv6 etc). Carrying the multicast routing information isonly one of the extension functions. This chapter describes mainly MBGP extension formulticast.MBGP enables unicast and multicast routing information to be exchanged through thesame process but stored in different routing tables. As MBGP is an enhanced version ofBGP-4, all the common policies and configuration methods that BGP-4 supports can beapplied to multicast.6.1.2 MBGP Extension Attributes for MulticastTo enable MBGP to support multicast, RFC2858 defines two new path attributesMP_REACH_NLRI (Multiprotocol Reachable NLRI) and MP_UNREACH_NLRI(Multiprotocol Unreachable NLRI) in UPDATE packets. They are all optionalnon-transitive attributes, that is, routers that do not support MBGP can ignore theinformation in the attributes and not forward the attributes.Among the information carried by MP_REACH_NLRI and MP_UNREACH_NLRI, AFI(Address Family Identifier) and SAFI (Subsequent Address Family Identifier) canidentify for which address family the information is. SAFI is a complement to NLRI
