Operation Manual – MulticastH3C S5600 Series Ethernet Switches Chapter 3 IGMP Configuration3-35) Through the above-mentioned query/report process, the IGMP routers learn thatmembers of G1 and G2 are attached to the local subnet, and generate (*, G1) and(*, G2) multicast forwarding entries, which will be the basis for subsequentmulticast forwarding, where * represents any multicast source.6) When the multicast data addressed to G1 or G2 reaches an IGMP router, becausethe (*, G1) and (*, G2) multicast forwarding entries exist on the IGMP router, therouter forwards the multicast data to the local subnet, and then the receivers onthe subnet receive the data.As IGMPv1 does not specifically define a Leave Group message, upon leaving amulticast group, an IGMPv1 host stops sending reports with the destination addressbeing the address of that multicast group. If no member of a multicast group exists onthe subnet, the IGMP routers will not receive any report addressed to that multicastgroup, so the routers will delete the multicast forwarding entries corresponding to thatmulticast group after a period of time.3.1.3 Enhancements Provided by IGMPv2Compared with IGMPv1, IGMPv2 provides the querier election mechanism and LeaveGroup mechanism.I. Querier election mechanismIn IGMPv1, the DR elected by the Layer 3 multicast routing protocol (such as PIM)serves as the querier among multiple routers on the same subnet.In IGMPv2, an independent querier election mechanism is introduced. The querierelection process is as follows:1) Initially, every IGMPv2 router assumes itself as the querier and sends IGMPgeneral query messages (often referred to as general queries) to all hosts androuters on the local subnet (the destination address is 224.0.0.1).2) Upon hearing a general query, every IGMPv2 router compares the source IPaddress of the query message with its own interface address. After comparison,the router with the lowest IP address wins the querier election and all otherIGMPv2 routers become non-queriers.3) All the non-queriers start a timer, known as “other querier present timer”. If a routerreceives an IGMP query from the querier before the timer expires, it resets thistimer; otherwise, it assumes the querier to have timed out and initiates a newquerier election process.II. “Leave group” mechanismIn IGMPv1, when a host leaves a multicast group, it does not send any notification tothe multicast router. The multicast router relies on IGMP query response timeout toknow whether a group no longer has members. This adds to the leave latency.In IGMPv2, on the other hand, when a host leaves a multicast group:
