Allied Telesis AT-S60 - Manual (Page 325)

Chapter 18: IGMP Snooping
Section II: Local and Telnet Management 324
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 want
to receive 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 node wanting to become a member of a particular multicast group
responds to a query 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. Any multicast packets belonging to
that multicast group are then forwarded by the router out the port. 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 out 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.
IGMP snooping enables the Fast Ethernet switch to monitor the flow of
queries from a router and reports from host nodes to build its own
multicast membership lists. The switch uses the lists to forward multicast
packets only to switch ports where there are host nodes that are
members of multicast groups. This improves switch performance and
network security by restricting the flow of multicast packets only to
those switch ports connected to host nodes.