2-1
2 IGMP Snooping Configuration
IGMP Snooping Overview
Internet Group Management Protocol Snooping (IGMP Snooping) is a multicast constraining
mechanism that runs on Layer 2 devices to manage and control multicast groups.
Principle of IGMP Snooping
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 Figure 2-1, 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 trans m ission
without IGMP Snooping
Source
Multicast router
Host A
Receiver
Host B
Host C
Receiver
Multicast packets
Layer 2 switch
Multicast packe t transmissio n
when IGMP Snooping runs
Source
Multicast route
r
Host A
Receiver
Host B
Host C
Receiver
Layer 2 switch
Basic Concepts in IGMP Snooping
IGMP Snooping related ports
As shown in Figure 2-2, 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).