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Page 5 | AlliedWare™ OS How To Note: IGMP
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.