3Com Switch 8800 Configuration Guide Chapter 31 MBGP Multicast Extension Configuration31-1Chapter 31 MBGP Multicast ExtensionConfiguration31.1 MBGP Multicast Extension Overview31.1.1 IntroductionAt 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 routing trees, you need to know the unicast routinginformation as well as the information of multicast-supporting parts of the network,namely, the multicast network topology.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 provisions the multi-protocol extension methodfor BGP. The extended BGP (MBGP, also written as BGP-4+) can not only carry IPv4unicast routing information but also the routing information of other network layerprotocols (such as multicast, IPv6). Carrying multicast routing information is only one ofthe extended functions. This chapter describes mainly MBGP extension for multicast.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.31.1.2 MBGP Extension Attributes for MulticastTo make MBGP support multicast, RFC2858 defines two new route atttributes in theUPDATE message: MP_REACH_NLRI (multiprotocol reachable NLRI) andMP_UNREACH_NLRI (multiprotocol unreachable NLRI). 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(Network Layer Reachability Information), with value 1 for the unicast mode of NLRI,and value 2 for the multicast mode of NLRI.
