1-31) The hosts send unsolicited IGMP reports to the addresses of the multicast groups that they want tojoin, without having to wait for the IGMP queries from the IGMP querier.2) The IGMP querier periodically multicasts IGMP queries (with the destination address of 224.0.0.1)to all hosts and routers on the local subnet.3) 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 membership for G1.Assume it is Host B that sends the report message. Upon hearing the report from Host B, Host C,which is on the same subnet with Host B, suppresses its own report for G1, because the IGMProuters (Router A and Router B) already know that at least one host on the local subnet isinterested in G1. This mechanism, known as IGMP report suppression, helps reduce traffic on thelocal 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 the multicast routing protocol (PIM for example)running on the routers generates (*, G1) and (*, G2) multicast forwarding entries, which will be thebasis for subsequent multicast forwarding, where * represents any multicast 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 to the address of the multicast group it listened to. If no member ofa multicast group exists on the subnet, the IGMP router will not receive any report addressed to thatmulticast group, so the routers will delete the multicast forwarding entries for that multicast group after aperiod of time.Enhancements in IGMPv2Compared with IGMPv1, IGMPv2 has introduced a querier election mechanism and a leave-groupmechanism.Querier election mechanismIn IGMPv1, the DR elected by the Layer 3 multicast routing protocol (such as PIM) serves as the querieramong multiple routers on the same subnet.In IGMPv2, an independent querier election mechanism is introduced. The querier election process isas follows:1) Initially, every IGMPv2 router assumes itself as the querier and sends IGMP general querymessages (often referred to as general queries) to all hosts and routers on the local subnet (thedestination address is 224.0.0.1).2) Upon hearing a general query, every IGMPv2 router compares the source IP address of the querymessage with its own interface address. After comparison, the router with the lowest IP addresswins the querier election and all other IGMPv2 routers become non-queriers.3) All the non-queriers start a timer, known as “other querier present timer”. If a router receives anIGMP query from the querier before the timer expires, it resets this timer; otherwise, it assumes thequerier to have timed out and initiates a new querier election process.
