IGMP Snooping
Definition
IGMP snooping is a switch function that listens to IGMP (Internet Group Management Protocol) messages to learn which ports lead to receivers of each IPv4 multicast group. RFC 4541 notes that a switch without snooping forwards multicast traffic to all its ports, in the same way as broadcast. With snooping, the traffic of a group is sent only to the ports where a receiver has joined it and to the ports that lead to a multicast router or querier. Snooping depends on an IGMP querier on the VLAN to keep its membership tables current.
In the field
On a large sACN system without IGMP snooping, devices unrelated to lighting can show rising processor load or packet loss, because every universe sent in multicast reaches every port. Whether snooping is enabled by default depends on the switch model and firmware, so the setting, and the presence of a querier, are checked on each VLAN that carries multicast.
Related lessons
Sources
- RFC 4541 : sans snooping, le trafic multicast est relayé sur toutes les interfaces ; avec snooping, il suit les tables d'abonnement et est aussi relayé vers les ports routeur · rfc-editor.org
- RFC 3376 : IGMPv3, rôle du querier et requêtes générales (rendue obsolète par la RFC 9776, qui reprend ces mécanismes) · rfc-editor.org
- ANSI E1.31-2018 (sACN) : univers transmis en multicast, unicast également pris en charge · tsp.esta.org