312 CHAPTER 18: MULTICAST P ROTOCOLFigure 79 Mapping between the multicast IP address and the Ethernet MAC addressOnly 23 bits of the last 28 bits in the IP multicast address are mapped to the MACaddress. Therefore, the 32 IP multicast addresses are mapped to the same MACaddress.IP Multicast Protocols Multicast uses the multicast group management protocol, and the multicast routingprotocol. The multicast group management protocol uses Internet GroupManagement Protocol (IGMP) as the IP multicast basic signalling protocol. It is usedbetween hosts and routers and enables routers to determine if members of themulticast group are on the network segment. The multicast routing protocol is usedbetween multicast routers to create and maintain multicast routes, enablinghighly-efficient multicast packet forwarding. Multicast routing protocols supported bythe Switch 5500 include PIM-SM and PIM-DM.Internet Group Management Protocol (IGMP)Internet Group Management Protocol (IGMP) is the only protocol that hosts can use.It defines the membership establishment and maintenance mechanism between hostsand routers, and is the basis of the entire IP multicast. Hosts report the groupmembership to a router through IGMP and inform the router of the conditions ofother members in the group through the directly connected host.If a user on the network joins a multicast group through IGMP declaration, themulticast router on the network will transmit the information sent to the multicastgroup through the multicast routing protocol. Finally, the network will be added tothe multicast tree as a branch. When the host, as a member of a multicast group,begins receiving the information, the router will query the group periodically to checkwhether members in the group are involved. As long as one host is involved, therouter receives data. When all users on the network quit the multicast group, therelated branches are removed from the multicast tree.Multicast Routing ProtocolA multicast group address has a virtual address. Unicast allows packets to be routedfrom the data source to the specified destination address. This is not possible formulticast. The multicast application sends the packets to a group of receivers (as withmulticast addresses) who are ready to receive the data but not only to one receiver (aswith unicast address).The multicast routing creates a loop-free data transmission path from one data sourceto multiple receivers. The task of the multicast routing protocol is to create adistribution tree architecture. A multicast router can use multiple methods to build upa path for data transmission, that is, the distribution tree.1110XXXX32bitsIPaddress48bitsMACaddress5bitsnotmapped Lower23bitsdirectlymappedXXXXXXXX XXXXXXXX XXXXXXXXXXXXXXXX XXXXXXXX XXXXXXXX XXXXXXXXXXXXXXXX XXXXXXXX
