Supermicro SSE-F3548S/SSE-F3548SR Configuration User’s Guide
7 IGMP Snooping
Switches learn the source MAC addresses for unicast traffic and forward the unicast traffic only to the
required ports. However for multicast and broadcast traffic, switches forward the traffic to all ports
except for the port that received that traffic. This basic multicast switching function helps all hosts
connected to the switch receive the multicast traffic.
In practical deployments, all hosts connected to a switch may not run the same multicast applications.
The hosts that do not run multicast applications receive the multicast traffic unnecessarily. Similarly, the
multicast traffic is forwarded to other switches unnecessarily when there are no hosts connected to the
other switches expecting the multicast traffic.
Forwarding multicast traffic to unnecessary hosts and switches wastes network bandwidth and computing
resources. In IP TV and other similar multicast intensive deployments, this problem leads to considerable
underutilization of network and compute resources.
Figure IGS-1: Multicast Forwarding without IGMP Snooping
The IGMP snooping function helps switches to forward IPv4 multicast traffic to only the ports that require
IPv4 multicast traffic. This function saves network bandwidth by preventing the unnecessary flooding of
IPv4 multicast traffic.
A switch performs the IGMP snooping function by snooping Layer 3 IGMP packets and recognizes an IGMP
host’s connected ports by snooping the IGMP join messages sent from hosts. Similarly, a switch
recognizes an IGMP router’s connected ports by snooping the IGMP control messages sent by IGMP
routers. The switch maintains a multicast forwarding table based on the hosts joined and router
connected ports for every multicast group and updates the multicast forwarding table when hosts leave
multicast groups.
Switch B
Switch A
Receiver 1
Receiver 2
Multicast
Traffic
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