Allied Telesis AT-S111, AT-GS950/48PS manual 136

Chapter 11: IGMP Snooping

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-GS950/48PS switch supports IGMP versions 1 and 2. The switch monitors 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 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.

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