Allied Telesis AT-GS950/48, User Manual

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Chapter 11: IGMP Snooping
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Overview
IGMP enables IPv4 routers to create lists of nodes that are members of
multicast groups. (A group of end nodes that receive multicast packets
from a multicast application is defined as a multicast group.) The router
creates a multicast membership list by periodically sending out queries to
the local area networks connected to its ports.
A node that wants to become a member of a multicast group responds to a
query by sending a
report which 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 continually issue reports on a continuous basis 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 from 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 from the port 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.
The IGMP snooping feature on the AT-GS950/48 switch supports IGMP
versions 1 and 2. The switch monitors the flow of queries from a router
and checks report and leave messages from host nodes to build its own
multicast membership lists. It uses the lists to forward multicast packets
only to its own 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 ports connected to
host nodes.