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Multicast Introduction & Concepts


Overview

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.


Just Add Power Implementation

In a Just Add Power system:

  • Each Transmitter (source device) is assigned a unique Channel that corresponds to a Multicast IP Address that identifies it on the network.
  • A command is sent to the Receiver (display device) to change the Channel to match the value of the Transmitter to be watched. The act of changing the Channel triggers IGMP join and leave requests so that the desired video is shown on the Receiver.
  • The switch manages the join and leave requests to forward the Channel to the ports that have Receivers connected, ensuring that only appropriate devices get audio/video while data devices are left alone.
  • The default multicast IP range is 239.92.0.0 - 239.92.100.0
  • Multicast traffic uses IGMPv2

References

  • https://tools.ietf.org/html/rfc2236
  • https://tools.ietf.org/html/rfc4541
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  • 06-Aug-2020
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