3Com MSR 50 Series Configuration Manual page 1117

As IGMPv1 does not specifically define a Leave Group mechanism, upon leaving a
multicast group, an IGMPv1 host stops sending reports with the destination
address being the address of that multicast group. If no member of a multicast
group exists on the subnet, the IGMP router will not receive any report addressed
to that multicast group, so the routers will delete the multicast forwarding entries
for that multicast group after a period of time.
Enhancements in Compared with IGMPv1, IGMPv2 has introduced a querier election mechanism
IGMPv2 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 on the same subnet.
In IGMPv2, an independent querier election mechanism is introduced. The querier
election process is as follows:
1 Initially, every IGMPv2 router assumes itself as the querier and sends IGMP general
query messages (often referred to as general queries) to all hosts and routers on
the local subnet (the destination address is 224.0.0.1).
2 Upon hearing 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 wins 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 an IGMP query from the querier before the timer expires, it resets
this timer; otherwise, it assumes 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 router. The multicast router relies on host response timeout to
know whether a group no longer has members. This adds to the leave latency.
In IGMPv2, when a host leaves a multicast group:
1 This host sends a Leave Group message (often referred to as leave message) to all
routers (the destination address is 224.0.0.2) on the local subnet.
2 Upon receiving the leave message, the querier sends a configurable number of
group-specific queries to the group being left. The destination address field and
group address field of the message are both filled with the address of the
multicast group being queried.
3 Up receiving a group-specific query, one of the other members of that group, if
any, will respond with a membership report within the maximum response time
set in the query.
4 If the querier receives a membership report from any member of the group within
the maximum response time, it will maintain the memberships of the group;
otherwise, the querier will assume that the group has no longer any member on
the subnet and will stop maintaining the memberships of the group.
IGMP Overview 1117