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Configuring IGMP snooping
Overview
Internet Group Management Protocol (IGMP) snooping is a multicast constraining mechanism that runs
on Layer 2 devices to manage and control multicast groups.
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 217, w
ithout IGMP snooping, a Layer 2 switch floods multicast packets out of all ports
but the incoming port. IGMP snooping enables a Layer 2 switch to forward multicast packets destined for
a known multicast group address out of only ports that have multicast receivers. This feature improves
bandwidth efficiency, enhances multicast security, and helps per-host accounting for multicast users.
Figure 217 Multicast forwarding before and after IGMP snooping runs
Basic concepts in IGMP snooping
This section lists the basic concepts in IGMP snooping.
IGMP snooping related ports
As shown in Figure 218, Router A connects to the multicast source, IGMP snooping runs on Switch A and
Switch B, Host A and Host C are receiver hosts as multicast group members.
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