leaves a multicast group by sending an IGMP message to its IGMP Querier. The querier is the router that surveys a subnet for multicastreceivers 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 PacketsJoin a Multicast GroupThere 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 unsolicitedreport to its querier.Responding to an IGMP QueryThe 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) ageneral 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 thegroup it wants to join (the packet is addressed to the same group). If multiple hosts want to join the same multicast group, only thereport from the first host to respond reaches the querier and the remaining hosts suppress their responses (For how the delay timermechanism 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 thesubnet. Multicast traffic for the group is then forwarded to that subnet.Sending an Unsolicited IGMP ReportA 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 IGMPJoin message, to the querier.Leaving a Multicast GroupThe 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 nolonger 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 leastone receiver in a group on a subnet for a router to forward multicast traffic for that group to the subnet.370 Internet Group Management Protocol (IGMP)
