IGMPv2 enhancements
Backwards-compatible with IGMPv1, IGMPv2 has introduced a querier election mechanism and a leave-group mechanism.
Querier election mechanism
In IGMPv1, the DR elected by the Layer 3 multicast routing protocol (such as PIM) serves as the querier among multiple routers that run IGMP on the same subnet.
IGMPv2 introduced an independent querier election mechanism. The querier election process is as follows:
Initially, every IGMPv2 router assumes itself to be the querier and sends IGMP general query messages (often called "general queries") to all hosts and routers on the local subnet. The destination address is 224.0.0.1.
After receiving a general query, every IGMPv2 router compares the source IP address of the query message with its own interface address. After comparison, the router with the lowest IP address becomes the querier and all the other IGMPv2 routers become non-queriers.
All the non-queriers start a timer, known as an "other querier present timer." If a router receives an IGMP query from the querier before the timer expires, it resets this timer. Otherwise, it considers the querier to have timed out and initiates a new querier election process.
"Leave group" mechanism
In IGMPv1, when a host leaves a multicast group, it does not send any notification to the multicast routers. The multicast routers determine whether a group has members by using the maximum response time. This adds to the leave latency.
In IGMPv2, when a host leaves a multicast group, the following process occurs:
The host sends a leave message (with the destination of 224.0.0.2) to all routers on the local subnet.
After receiving the leave message, the querier sends a configurable number of group-specific queries to the group that the host is leaving. Both the destination address field and the group address field of the message are the address of the multicast group that is being queried.
One of the remaining members (if any on the subnet) of the group should send a membership report within the maximum response time advertised in the query messages.
If the querier receives a membership report for the group before the maximum response time expires, it maintains the memberships for the group. Otherwise, the querier assumes that the local subnet has no member hosts for the group and stops maintaining the memberships for the group.