IGMP

networking

Stands for: Internet Group Management Protocol

Internet Group Management Protocol - how a host raises its hand for a multicast group, and how the network learns which ports actually want the stream.

IGMP runs between hosts and their first-hop router: a membership report says 'I want group 239.x.x.x', a leave says 'done', and an elected querier polls periodically so state does not go stale (IGMPv3, RFC 3376, adds source filtering - the foundation of SSM). The switch-side twin is IGMP snooping: switches eavesdrop on the reports so multicast floods only the ports that asked, instead of the whole VLAN - the single setting whose absence explains most 'multicast melted my network' stories. IGMP handles the last hop only; getting the stream ACROSS the network between routers is PIM's job.

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

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