IGMP

Stands for: Internet Group Management Protocol

Internet Group Management Protocol: how IPv4 hosts tell the routers on their link which multicast groups they want to receive.

Hosts send membership reports, and one querier per link polls them with general queries to 224.0.0.1, so a router keeps a group only while someone on the link still wants it. IGMPv3, now RFC 9776 (2025), adds source filtering. Switches snoop the same messages to send multicast only to ports that asked; PIM builds the trees between routers.

Version 1 came with RFC 1112; version 2, RFC 2236 (1997), added the Leave Group message for low leave latency and querier election by lowest IP address; version 3 adds source filtering, interest in a group only from some sources or from all but some. RFC 9776, an Internet Standard of March 2025, obsoletes RFC 3376 as the IGMPv3 specification and still updates RFC 2236, since IGMPv3 must handle version 2 reports and leaves. IGMPv3 reports go to 224.0.0.22, where all IGMPv3 routers listen.

The defaults are a robustness variable of 2, a query interval of 125 seconds and a query response interval of 10 seconds. A router forgets a group when no report arrives within the Group Membership Interval: 2 times 125 plus 10, or 260 seconds, under RFC 2236, and 2 times 125 plus 2 times 10, or 270 seconds, under RFC 9776, which changed the formula to resolve an erratum. The five minutes often quoted matches neither. A silent querier is replaced after the Other Querier Present Interval, 255 seconds.

A snooping switch, in RFC 4541's Informational advice, forwards reports only towards router ports, found from queries whose source is not 0.0.0.0, and sends group traffic only to ports that reported, but forwards 224.0.0.0/24 on all ports. With no querier, entries time out and the switch either floods the group as unregistered traffic or stops delivering it, as its policy dictates. IPv6 does the same with MLD: MLDv2, RFC 9777, is a translation of IGMPv3, with reports sent to ff02::16.

Also known as: igmp, igmp snooping, membership report, querier, igmpv2, igmpv3

Sources

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