3-3Enhancements Provided by IGMPv2Compared with IGMPv1, IGMPv2 provides the querier election mechanism and 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.“Leave group” mechanismIn IGMPv1, when a host leaves a multicast group, it does not send any notification to the multicastrouter. The multicast router relies on IGMP query response timeout to know whether a group no longerhas members. This adds to the leave latency.In IGMPv2, on the other hand, when a host leaves a multicast group:1) This host sends a Leave Group message (often referred to as leave message) to all routers (thedestination address is 224.0.0.2) on the local subnet.2) Upon receiving the leave message, the querier sends a configurable number of group-specificqueries to the group being left. The destination address field and group address field of messageare both filled with the address of the multicast group being queried.3) One of the remaining members, if any on the subnet, of the group being queried should send amembership report within the maximum response time set in the query messages.4) If the querier receives a membership report for the group within the maximum response time, it willmaintain the memberships of the group; otherwise, the querier will assume that no hosts on thesubnet are still interested in multicast traffic to that group and will stop maintaining thememberships of the group.Enhancements Provided by IGMPv3Built upon and being compatible with IGMPv1 and IGMPv2, IGMPv3 provides hosts with enhancedcontrol capabilities and provides enhancements of query and report messages.Enhancements in control capability of hostsIGMPv3 has introduced source filtering modes (Include and Exclude), so that a host can specify a list ofsources it expect or does not expect multicast data from when it joins a multicast group:z If it expects multicast data from specific sources like S1, S2, …, it sends a report with theFilter-Mode denoted as “Include Sources (S1, S2, …).
