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IFS NS3502-8P-2S User Manual
166
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
Internet.