3com 8807 User Guide

Running on the link layer, IGMP Snooping is a multicast control mechanism on the 
Layer 2 Ethernet switch and it is used for multicast group management and 
control.

When receiving the IGMP messages transmitted between the host and router, the 
Layer 2 Ethernet switch uses IGMP Snooping to analyze the information carried in 
the IGMP messages. If the switch hears IGMP host report message from an IGMP 
host, it will add the host to the corresponding multicast table. If the switch hears 
IGMP leave message from an IGMP host, it will remove the host from the 
corresponding multicast table. The switch continuously listens to the IGMP 
messages to create and maintain MAC multicast address table on Layer 2. And 
then it can forward the multicast packets transmitted from the upstream router 
according to the MAC multicast address table.

When IGMP Snooping is disabled, the packets are broadcasted on Layer 2. See the 
following figure:

Figure 102   Multicast packet transmission without IGMP Snooping

When IGMP Snooping runs, the packets are multicast rather than broadcasted on 
Layer 2. See the following figure:

Internet / Intranet

Video stream

VOD Server

Layer 2 Ethernet Switch

Video stream

Multicast group member

Non-multicast 
group member

Multicast router

Video stream

Video stream

Video stream

Non-multicast
group member