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Chapter 10: IGMP Snooping
138 Section II: Menus Interface
IGMP Snooping Overview
The Internet Group Management Protocol (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 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 becoming 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—versions 1, 2, and 3. One of the
differences between the versions is how a host node signals that it no
longer wants to be a member of a multicast group. In version 1 it 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 receiving a leave request from a host node, the router
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.
Version 3 adds the ability of host nodes to join or leave specific sources in
a multicast group through the use of Group-Source report and Group-
Source leave messages.
The IGMP snooping feature on the switch supports IGMP versions 1 and
2. It enables the switch to monitor the flow of queries from a router and
reports and leave messages 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