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IGMP snooping configuration
Overview
Internet Group Management Protocol (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 211, when IGMP snooping is not running on the switch, multicast packets are 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 211 Multicast forwarding before and after IGMP snooping runs
Multicast packet transmission
without IGMP Snooping
Source
Multicast router
Host A
Receiver
Host B
Host C
Receiver
Multicast packets
Layer 2 switch
Multicast packet transmission
when IGMP Snooping runs
Source
Multicast router
Host A
Receiver
Host B
Host C
Receiver
Layer 2 switch
IGMP snooping related ports
As shown in Figure 212, 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).
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