IGMP

showstack / protocols / igmp

Also called Internet Group Management Protocol, IGMP snooping, IGMP querier.

How a host tells the network it wants a multicast group, and how a switch learns which ports to send that group to. On a show network it is the difference between sACN and Dante behaving, and every port on the switch receiving every universe.

Network transportOpenIETFReported

Management Housekeeping, no deadline

Discovery, monitoring, configuration, backups, updates. Useful, and none of it has a deadline of its own.

What kills it. Nothing kills it. That is the problem: with no deadline it has no manners, and it will happily starve everything that does have one. Putting it on a show VLAN is the commonest self-inflicted wound in the industry.

Why this one, and not what the category suggests. Housekeeping that decides where multicast media is allowed to go — management traffic with an outsized effect on media.

The four flows, and why the model is worth having →

What goes wrong

Snooping without a querier is worse than no snooping. With no querier the switch ages out its group memberships and multicast traffic starts and stops on a timer, which reads as random dropouts.

Exactly one querier per VLAN. Two switches both acting as querier is a common cause of intermittent multicast on a network that looks correct.

Version mismatch matters: an IGMPv2 device on a v3-only network may never join a group at all.

Turning snooping off "to make it work" floods every port. It does often make it work, right up to the point where the link fills.

Typical use

Any network carrying sACN, Dante, AES67, NDI or ST 2110 multicast
Keeping multicast traffic off ports that did not ask for it

Sources

RFC 3376 — Internet Group Management Protocol, Version 3 — IETF primary
RFC 2236 — Internet Group Management Protocol, Version 2 — IETF primary

Understand the mechanism

Show networks How a one gets down a wire

Something wrong, or missing?

Known gaps on this entry: default_ports, first_published, implementations. If you can source one of them, that is a ten minute pull request.

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