IGMP Protocol OverviewIGMP has three versions. Version 3 obsoletes and is backwards-compatible with version 2; version 2 obsoletesversion 1.IGMP Version 2IGMP version 2 improves on version 1 by specifying IGMP Leave messages, which allows hosts to notifyrouters that they no longer care about traffic for a particular group.Leave messages reduce the amount of time that the router takes to stop forwarding traffic for a group to asubnet (leave latency) after the last host leaves the group. In version 1 hosts quietly leave groups, and therouter waits for a query response timer several times the value of the query interval to expire before it stopsforwarding traffic.To receive multicast traffic from a particular source, a host must join the multicast group to which the sourceis sending traffic. A host that is a member of a group is called a receiver. A host may join many groups, andmay join or leave any group at any time. A host joins and leaves a multicast group by sending an IGMPmessage to its IGMP Querier. The querier is the router that surveys a subnet for multicast receivers andprocesses survey responses to populate the multicast routing table.IGMP messages are encapsulated in IP packets, as shown in the following illustration.Figure 42. IGMP Messages in IP PacketsInternet Group Management Protocol (IGMP) 478
