1036. 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 multicastdata to the local subnet, and then the receivers on the subnet receive the data.IGMPv1 does not specifically define a leave group message (often called a "leave message"). When anIGMPv1 host is leaving a multicast group, it stops sending reports to the address of the multicast groupthat it listened to. If no member exists in a multicast group on the subnet, the IGMP router will not receiveany report addressed to that multicast group. In this case, the router will delete the multicast forwardingentries for that multicast group after a period 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.IGMPv2 introduced an independent querier election mechanism. The querier election process is asfollows:1. Initially, every IGMPv2 router assumes itself as the querier and sends IGMP general querymessages (often called "general queries") to all hosts and routers on the local subnet. Thedestination address is 224.0.0.1.2. After receiving 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."Leave group" mechanismIn IGMPv1, when a host leaves a multicast group, it does not send any notification to the multicast router.The multicast router relies on the host response timeout timer to determine whether a group has members.This adds to the leave latency.In IGMPv2, when a host leaves a multicast group, the following steps occur:1. This host sends a leave message to all routers on the local subnet. The destination address is224.0.0.2.2. After receiving the leave message, the querier sends a configurable number of group-specificqueries to the group that the host is leaving. The destination address field and group address fieldof the message are both filled with the address of the multicast group that is being queried.3. One of the remaining members (if any on the subnet) of the group that is being queried shouldsend a membership report within the maximum response time set in the query messages.
