Manual

Chapter 15: IGMP Snooping
Section II: Advanced Operations 204
IGMP Snooping Overview
IGMP snooping is best explained by first defining IGMP. This protocol
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. A report indicates an end
node’s desire to become a member of a multicast group. Nodes that join
a multicast group are referred to as host nodes. After it has become a
member of a multicast group, a host node must continue to periodically
issue reports to remain a member.
After 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 three versions of IGMP. The AT-8524M switch supports IGMP
Version 1 and Version 2. One of the differences between the two
versions is how a host node signals 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, it 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. After a router receives a leave request from a host node, it
promptly removes the node from appropriate membership list. The
router also stops sending multicast packets out the port to which the
node is connected if it determines there are no further host nodes on the
port.
IGMP snooping enables the switch to monitor the flow of queries from a
router and reports from host nodes to build its own multicast
membership lists. It 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.