Allied Telesis AT-S60 - Manual (Page 219)

AT-S60 Management Software User’s Guide
Section II: Advanced Features 219
IGMP Snooping Overview
IGMP enables routers to create lists of nodes that are members of
multicast groups. (A multicast group is a group of end nodes that
request multicast packets from a multicast application.) The router
creates a multicast membership list by periodically sending out queries
to the local area networks connected to its ports.
A user activates IGMP by selecting a multicast application such as a
radio, voice, or video application on their PC. This selection triggers a
series of message exchanges. A node responds to a query from the PC by
sending a report which indicates an end node’s intention to become a
member of a multicast group. Nodes that join a multicast group are
referred to as host nodes. Once a host node has been made a member of
a multicast group, it must continue to periodically issue reports to
remain a member.
Once the router has received a report from a host node, it notes the
multicast group that the host node wants to join and the port on the
router where the node is located. Then multicast packets belonging to
that multicast group are forwarded from the port by the router. If a
particular port on the router has no nodes that want to be members of
multicast groups, the router does not send multicast packets from the
port. This improves network performance by restricting multicast
packets only to router ports where host nodes are located.
There are two versions of IGMP, referred to as Version 1 and Version 2.
One of the differences between the two versions is how a host node
indicates that it no longer wants to be a member of a multicast group. In
Version 1, it simply stops sending reports. If a router does not receive a
report from a host node after a predefined length of time, referred to as
a time-out value, the router assumes that the host node no longer wants
to receive multicast frames and removes it from the membership list of
the multicast group.
In Version 2, a host node exits from a multicast group by sending a leave
request. Once a router receives a leave request from a host node, it
removes the node from the appropriate membership list. If it determines
there are no further host nodes on the port, the router also stops
sending out multicast packets from the port connected to the node.