User`s guide

Section II: Local and Telnet Management
178
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 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, 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. Once a router receives a leave request from a host node, it
removes the node from appropriate membership list. The router will also
stop sending out 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 Fast Ethernet 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.