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- [Instructor] Multicast is a technique primarily used

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to send traffic from one host to many simultaneously.

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It's commonly used to distribute a single video stream

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to multiple hosts at the same time,

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but is also used for services

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like dynamic routing protocol communication.

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The Internet Group Management Protocol

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is used by hosts and adjacent routers

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to join multicast groups.

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IGMP is used exclusively by IPv4 hosts

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as this was supplanted by multicast listener discovery

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in the ICMP version six suite for IPv6.

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IGMP allows a host that wants

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to receive specific multicast traffic

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to solicit a local router

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to have that multicast traffic sent on an interface.

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There are currently three versions of IGMP.

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Version two added the ability for a host

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to exit a multicast group.

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Version three added support for source specific multicast.

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The vast majority of networks don't have

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hosts plugged directly into router ports.

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Instead they go through a switch.

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By default, when a switch receives a packet

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destined for a multicast address,

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it will flood that packet out all of its ports.

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I say destined because traffic should never be sourced

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from a multicast address.

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If multiple switches are connected,

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then the flooding continues.

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If a network has high volumes of multicast traffic,

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this could unnecessarily consume resources.

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Many switch manufacturers implement a system

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known as IGMP snooping to combat this.

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In effect, when IGMP snooping is enabled,

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the switch listens for any host sending IGMP messages.

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If the switch receives an IGMP message,

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it will enable forwarding of multicast

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for that particular group to that interface,

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then will elect the upstream router

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of the group membership requests.

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This way, the multicast traffic is only sent

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to the interfaces that have explicitly requested it.

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The switch can then block other multicast traffic.

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Multicast routing and distribution isn't terribly common,

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but IGMP can be quite useful if you find yourself doing so.