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Fat AP and Cloud AP V200R008C00 CLI-based Configuration Guide

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Introduction to IGMP Snooping

Introduction to IGMP Snooping

This section describes the definition and purposes of IGMP Snooping.


Internet Group Management Protocol Snooping (IGMP snooping) is a Layer 2 IPv4 multicast protocol. The IGMP snooping protocol maintains information about the outbound interfaces of multicast packets by snooping multicast protocol packets exchanged between the Layer 3 multicast device and user hosts. The IGMP snooping protocol manages and controls the forwarding of multicast packets at the data link layer.


In most cases, particularly in a LAN, multicast packets have to pass through Layer 2 devices. As shown in Figure 7-106, multicast packets have to pass through a Layer 2 fat AP located between multicast users and the Layer 3 multicast device, Router.

Figure 7-106  IGMP snooping networking

After receiving multicast packets from Router, fat AP forwards the multicast packets to the multicast receivers. The destination address of the multicast packets is a multicast group address. fat AP cannot learn multicast MAC address entries, so it broadcasts the multicast packets in the broadcast domain. All hosts in the broadcast domain will receive the multicast packets, regardless of whether they are members of the multicast group. This wastes network bandwidth and threatens network security.

IGMP snooping solves this problem. After IGMP snooping is configured, the Layer 2 multicast device can snoop and analyze IGMP messages between multicast users and the upstream router. The Layer 2 multicast device sets up Layer 2 multicast forwarding entries to control forwarding of multicast data. In this way, multicast data is not broadcast on the Layer 2 network.

Updated: 2019-01-11

Document ID: EDOC1000176006

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