Multicast describes network communication from one device to multiple selected devices.
Unlike unicast (one-to-one communication) and broadcast (one-to-everyone communication) which are relatively brainless processes, multicast communication must be constantly monitored by the switch to ensure that communication is passing only to subscribed devices.
Receiving devices send an Internet Group Management Protocol (IGMP) message to indicate to the network which source they want information from. The receiving device sends a join message to subscribe to a multicast stream, and a leave message to unsubscribe from a multicast stream.
Source devices send data to the network only once, regardless of the number of subscribed receiving devices. The switch is responsible for replicating the data so that each subscribed receiving devices gets a copy, without unnecessarily burdening devices that do not need the data.
IGMP snooping is the protocol that listens to IGMP traffic so that it can be properly managed. A switch with IGMP snooping watches join and leave requests, allows multicast on link that have a subscribed device, and can block multicast on links that do not have a subscribed device. This contributes to overall lower bandwidth by preventing devices from being overloaded with traffic that they do not want.
An IGMP querier - also referred to as a multicast router - is the device on the network that manages join and leave requests. It is responsible for recording subscription status and passing that information along to other switches on the network. This ensures that membership reporting is complete and accurate. Only one device in the network - often a core switch - is designated as the multicast router. All other switches are programmed to send join/leave requests to the multicast router.
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