Igmp Snooping; Overview; Principle Of Igmp Snooping; Igmp Snooping Related Ports - 3Com Baseline 2928 PWR Plus User Manual

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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
flooded to all devices at Layer 2. However, 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 1-1 Multicast forwarding before and after IGMP snooping runs
Multicast packet transmission
without IGMP Snooping
Source
Host A
Receiver
Multicast packets

IGMP Snooping Related Ports

As shown in Figure
and Switch B, Host A and Host C are receiver hosts (namely, multicast group members).
1-1, when IGMP snooping is not running on the switch, multicast packets are
Multicast router
Layer 2 switch
Host C
Receiver
Host B
1-2, Router A connects to the multicast source, IGMP snooping runs on Switch A
Multicast packet transmission
when IGMP Snooping runs
Multicast router
Source
Layer 2 switch
Host A
Receiver
Host B
1-1
Host C
Receiver