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Configuring IGMP snooping
Overview
IGMP snooping runs on a Layer 2 device as a multicast constraining mechanism to improve
multicast forwarding efficiency. It creates Layer 2 multicast forwarding entries from IGMP packets
that are exchanged between the hosts and the router.
As shown in Figure 10, when IGMP snooping is not enabled, the Layer 2 switch floods multicast
packets to all hosts in a VLAN. When IGMP snooping is enabled, the Layer 2 switch forwards
multicast packets of known multicast groups to only the receivers.
Figure 10 Multicast packet transmission without and with IGMP snooping
IGMP snooping ports
As shown in Figure 11, IGMP snooping runs on Switch A and Switch B, and Host A and Host C are
receivers in a multicast group. IGMP snooping ports are divided into member ports and router ports.
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
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