Allied Telesis AR400 manual IGMP overview, Queriers and Snoopers

IGMP overview > Queriers and Snoopers

IGMP overview

Clients in an IP subnetwork use IGMP to indicate that they are interested in receiving a multicast. IGMP then ensures that routers and switches forward multicast packets out the appropriate ports to the interested clients.

IGMP is very flexible, as the examples in this How To Note show, but the basic operation is simple. When a client wants to start receiving a multicast—which is also called joining a multicast group—the client sends an IGMP Membership Report message. When a router or switch is running IGMP and receives a Report message, it starts forwarding traffic for the relevant multicast group to the client.

IGMP periodically polls clients by sending General Query messages, to check that the clients still belong to their multicast groups.

To leave a group, a client sends an IGMP Leave message to indicate that it no longer needs to receive the group traffic.

Note that IGMP does not exchange multicast routing information between subnets. The multicast routing protocols PIM and DVMRP do this.

Queriers and Snoopers

It is neither necessary nor desirable for every router or switch in an IP subnetwork to coordinate multicast traffic flows. Instead, a single router or switch does this and is called the Querier or the Designated Router. The Querier generates Query messages to check group membership, and processes Membership Reports and Leave messages.

However, other routers and switches in the network need to know whether to send multicasts out each of their ports. They find out this information by becoming Snoopers. Each Snooper checks IGMP messages before forwarding them to and from the Querier, and uses the information in the messages to determine which ports to send multicasts out of.

IGMP terms:

Multicast or Multicast stream

A flow of information—usually video or audio—that can go from one source to many destination clients.

Group

A multicast stream that clients can join. Groups have IP addresses in the 224.0.0.0/4 network.

Group member

A client that belongs to a particular multicast group.

IGMP Querier or Designated Router

A device in a subnetwork that is the coordinator for all multicast streams and IGMP membership information.

Each subnetwork has only one Querier (see "Multiple potential IGMP queriers" on page 15). The Querier generates Membership Query messages to check which clients are group members, and processes Membership Reports and Leave messages.

IGMP Snooper

A device that spies on IGMP messages to create flow efficiencies by ensuring that multicast data streams are only sent to interested ports. A Snooper can decide on the best path to send multicast packets at Layer 2 but it cannot alter those packets or generate its own IGMP messages.

IGMP Proxy

A device that passes membership reports upstream and multicast streams and queries downstream. The proxy acts on behalf of clients and servers by altering packets.

Page 5 AlliedWare™ OS How To Note: IGMP