3-2Figure 3-1 Joining multicast groupsQueryReportDRHost A(G2)Host B(G1)Host C(G1)EthernetRouter A Router BIP networkAssume that Host B and Host C are expected to receive multicast data addressed to multicast group G1,while Host A is expected to receive multicast data addressed to G2, as shown in Figure 3-1. The basicprocess that the hosts join the multicast groups is as follows:1) The IGMP querier (Router B in the figure) periodically multicasts IGMP queries (with thedestination address of 224.0.0.1) to all hosts and routers on the local subnet.2) Upon receiving a query message, Host B or Host C (the delay timer of whichever expires first)sends an IGMP report to the multicast group address of G1, to announce its interest in G1. Assumeit is Host B that sends the report message.3) Host C, which is on the same subnet, hears the report from Host B for joining G1. Upon hearing thereport, Host C will suppress itself from sending a report message for the same multicast group,because the IGMP routers (Router A and Router B) already know that at least one host on the localsubnet is interested in G1. This mechanism, known as IGMP report suppression, helps reducetraffic over the local subnet.4) At the same time, because Host A is interested in G2, it sends a report to the multicast groupaddress of G2.5) Through the above-mentioned query/report process, the IGMP routers learn that members of G1and G2 are attached to the local subnet, and generate (*, G1) and (*, G2) multicast forwardingentries, which will be the basis for subsequent multicast forwarding, where * represents anymulticast source.6) When the multicast data addressed to G1 or G2 reaches an IGMP router, because the (*, G1) and(*, G2) multicast forwarding entries exist on the IGMP router, the router forwards the multicast datato the local subnet, and then the receivers on the subnet receive the data.As IGMPv1 does not specifically define a Leave Group message, upon leaving a multicast group, anIGMPv1 host stops sending reports with the destination address being the address of that multicastgroup. If no member of a multicast group exists on the subnet, the IGMP routers will not receive anyreport addressed to that multicast group, so the routers will delete the multicast forwarding entriescorresponding to that multicast group after a period of time.
