Dell PowerSwitch S4820T Configuration Guide for the S4820T System 9.112.1P1 - Page 357
Join a Multicast Group, Responding to an IGMP Query, Sending an Unsolicited IGMP Report
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leaves a multicast group by sending an IGMP message to its IGMP Querier. The querier is the router that surveys a subnet for multicast receivers and processes survey responses to populate the multicast routing table. IGMP messages are encapsulated in IP packets, as shown in the following illustration. Figure 44. IGMP Messages in IP Packets Join a Multicast Group There are two ways that a host may join a multicast group: it may respond to a general query from its querier or it may send an unsolicited report to its querier. Responding to an IGMP Query The following describes how a host can join a multicast group. 1 One router on a subnet is elected as the querier. The querier periodically multicasts (to all-multicast-systems address 224.0.0.1) a general query to all hosts on the subnet. 2 A host that wants to join a multicast group responds with an IGMP Membership Report that contains the multicast address of the group it wants to join (the packet is addressed to the same group). If multiple hosts want to join the same multicast group, only the report from the first host to respond reaches the querier and the remaining hosts suppress their responses (For how the delay timer mechanism works, refer to Adjusting Query and Response Timers). 3 The querier receives the report for a group and adds the group to the list of multicast groups associated with its outgoing port to the subnet. Multicast traffic for the group is then forwarded to that subnet. Sending an Unsolicited IGMP Report A host does not have to wait for a general query to join a group. It may send an unsolicited IGMP Membership Report, also called an IGMP Join message, to the querier. Leaving a Multicast Group The following describes how a host can leave a multicast group. 1 A host sends a membership report of type 0x17 (IGMP Leave message) to the all routers multicast address 224.0.0.2 when it no longer cares about multicast traffic for a particular group. 2 The querier sends a Group-Specific Query to determine whether there are any remaining hosts in the group. There must be at least one receiver in a group on a subnet for a router to forward multicast traffic for that group to the subnet. Internet Group Management Protocol (IGMP) 357
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