3Com WX3000 Igmp Snooping Configuration, Igmp Snooping Overview, Principle of Igmp Snooping

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

Multicast packet transmission	Multicast packet transmission
without IGMP Snooping	when IGMP Snooping runs

Source

Multicast router

Layer 2 switch

Multicast router

Source

Layer 2 switch

Host A	Host C
Receiver	Receiver

Host A	Host C
Receiver	Receiver

Host B Host B

Multicast packets

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).

2-1