3com WX3000 Manuel D’Utilisation

 

2-1 

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

mechanism that runs on Layer 2 devices to manage and control multicast groups. 

By analyzing received IGMP messages, a Layer 2 device running IGMP Snooping establishes 

mappings between ports and multicast MAC addresses and forwards multicast data based on these 

mappings.  

As shown in 

, when IGMP Snooping is not running on the switch, multicast packets are 

broadcast to all devices at Layer 2. When IGMP Snooping is running on the switch, multicast packets for 

known multicast groups are multicast to the receivers, rather than broadcast to all hosts, at Layer 2.  

Figure 2-1 Before and after IGMP Snooping is enabled on Layer 2 device 

Multicast packet transmission

 without IGMP Snooping

Host A

Host B

Host C

 Multicast packets

Multicast packet transmission

 when IGMP Snooping runs

Host A

Host B

Host C

 

 

As shown in 

, Router A connects to the multicast source, IGMP Snooping runs on Switch A 

and Switch B, Host A and Host C are receiver hosts (namely, multicast group members).