The Time-to-Live (TTL) field of query messages is set to 1 so that the queries will not be forwarded to
other sub networks.
IGMP version 2 introduces some enhancements such as a method to elect a multicast queried for each
LAN, an explicit leave message, and query messages that are specific to a given group.
The states a computer will go through to join or to leave a multicast group are shown below:
Figure 4-8-4: IGMP State Transitions
IGMP Querier –
A router, or multicast-enabled switch, can periodically ask their hosts if they want to receive
multicast traffic. If there is more than one router/switch on the LAN performing IP multicasting,
one of these devices is elected “querier” and assumes the role of querying the LAN for group
members. It then propagates the service requests on to any upstream multicast switch/router to
ensure that it will continue to receive the multicast service.
Multicast routers use this information, along with a multicast routing
protocol such as DVMRP or PIM, to support IP multicasting across the
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