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Chapter 4: Web-Based Management
94 GE-DS-242-PoE Managed Ethernet Switch User Manual
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 4-54: 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.
NOTE: Multicast routers use this information, along with a multicast routing protocol
such as DVMRP or PIM, to support IP multicasting across the Internet.