Interlogix GE-DSSG-244-POE User Manual - Manual (Page 134)

IFS NS3601-24P/4S GE-DSSG-244 and 244-POE User Manual
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following outlines what is communicated between a multicast router and a multicast group member using IGMP.
A host sends an IGMP “report” to join a group
A host will never send a report when it wants to leave a group (for version 1).
A host will send a “leave” report when it wants to leave a group (for version 2).
Multicast routers send IGMP queries (to the all-hosts group address: 224.0.0.1) periodically to see whether any group members
exist on their sub networks. If there is no response from a particular group, the router assumes that there are no group members on
the network.
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.