3com DUA1750-2BAA01 Manual De Usuario

3Com Switch 8800 Configuration Guide 

Chapter 24  IGMP Snooping Configuration

 

24-1 

IGMP Snooping (Internet Group Management Protocol Snooping) is a multicast control 

mechanism running on the Layer 2 Ethernet switch and it is used for multicast group 

management and control.  

IGMP Snooping runs on the link layer. 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: 

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

 

Figure 24-1 Multicast packet transmission without IGMP Snooping 

When IGMP Snooping runs, the packets are multicast rather than broadcasted on 

Layer 2. See the following figure: