17Internet Group Management Protocol(IGMP)Internet group management protocol (IGMP) is supported on the Z9000 platform.Multicast is premised on identifying many hosts by a single destination IP address; hosts represented bythe same IP address are a multicast group. IGMP is a Layer 3 multicast protocol that hosts use to join orleave a multicast group. Multicast routing protocols (such as protocol-independent multicast [PIM]) usethe information in IGMP messages to discover which groups are active and to populate the multicastrouting table.IGMP Implementation Information• Dell Networking Operating System (OS) supports IGMP versions 1, 2, and 3 based on RFCs 1112, 2236,and 3376, respectively.• Dell Networking OS does not support IGMP version 3 and versions 1 or 2 on the same subnet.• Dell Networking systems cannot serve as an IGMP host or an IGMP version 1 IGMP Querier.• Dell Networking OS automatically enables IGMP on interfaces on which you enable a multicastrouting protocol.IGMP Protocol OverviewIGMP has three versions. Version 3 obsoletes and is backwards-compatible with version 2; version 2obsoletes version 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 toa subnet (leave latency) after the last host leaves the group. In version 1 hosts quietly leave groups, andthe router waits for a query response timer several times the value of the query interval to expire before itstops forwarding traffic.To receive multicast traffic from a particular source, a host must join the multicast group to which thesource is sending traffic. A host that is a member of a group is called a receiver. A host may join manygroups, and may join or leave any group at any time. A host joins and leaves a multicast group by sendingan 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.Internet Group Management Protocol (IGMP) 299
