Planet GS-5220 Series User Manual page 257

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.
Figure 4-8-4: IGMP State Transitions
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User's Manual of GS-5220 LCD Series