18Internet Group Management Protocol(IGMP)Internet group management protocol (IGMP) is a Layer 3 multicast protocol that hosts use to join or leavea multicast group.Multicast is premised on identifying many hosts by a single destination IP address; hosts represented bythe same IP address are a multicast group. Multicast routing protocols (such as protocol-independentmulticast [PIM]) use the information in IGMP messages to discover which groups are active and topopulate the multicast routing 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.• IGMP on Dell Networking OS supports 95 interfaces on S4810 and S4820 and an unlimited number ofgroups on all other platforms.• 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.354 Internet Group Management Protocol (IGMP)
