Allied Telesis AT-S62 - Manual (Page 301)

AT-S62 Menus Interface User’s Guide
Section II: Advanced Operations 301
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. 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 the 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 AT-8500 Series switch supports all
three IGMP versions. 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