User's Manual

Chapter 13: Setting Internet Group Management Protocol (IGMP) Snooping
146
Overview
IGMP snooping allows the switch to control the flow of multicast packets
from its ports. It enables the switch to forward packets of multicast groups
to those ports that have host nodes.
IGMP is used by IPv4 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 that wants to become a member of a multicast group responds to a
query by sending a report. A report indicates that an end node wants to
become a member of a multicast group. Nodes that join a multicast group
are referred to as host nodes. After joining 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 from 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 the multicast packets
only to router ports where host nodes are located.