IGMP Querier
Definition
An IGMP querier is the device, usually a router or a switch, that periodically sends IGMP general queries on a VLAN to ask hosts which multicast groups they still want to receive. When several candidates are present, the one with the lowest IP address becomes querier. Switches with IGMP snooping rely on the replies to keep their membership tables current. With the default IGMPv3 timers of RFC 3376, a query interval of 125 s and a robustness variable of 2, a membership expires 260 seconds after the last report. When no querier is active, switches let the entries expire and then either flood the multicast or stop forwarding it, depending on the model.
In the field
Multicast (sACN, NDI, AES67) that works after power-up and stops after a few minutes is a typical symptom of a VLAN without an IGMP querier, because the timing matches the expiry of the snooping tables, about 260 seconds with default timers, although some switches use shorter values. Enabling an IGMP querier on the core switch or on the router of each multicast VLAN resolves it.
Related lessons
Sources
- RFC 3376 : élection du querier (adresse IP la plus basse) ; Query Interval 125 s, Robustness Variable 2, Query Response Interval 10 s ; Group Membership Interval = Robustness Variable × Query Interval + Query Response Interval, soit 2 × 125 + 10 = 260 s · rfc-editor.org
- RFC 9776 (STD 100, 2025) : révision d'IGMPv3 qui rend obsolète la RFC 3376 et reprend ces temporisations · ietf.org
- RFC 4541, IGMP snooping : requêtes générales du querier relayées sans condition, rapports d'abonnement transmis aux routeurs multicast et au querier · rfc-editor.org