What Is IGMP Snooping and IGMP Snooping Switch?


IGMP snooping switches are gradually entering the core of the network. Maybe some people are still not familiar with the fresh terms such as IGMP snooping, IGMP snooping switch, etc. What is IGMP snooping? What is IGMP snooping switch?

What Is IGMP Snooping?

IGMP snooping, short for Internet Group Management Protocol Snooping, is a multicast constraint mechanism that runs on layer 2 devices to manage and control multicast groups. As shown in the following figure, this Layer 2 device is usually a data switch. The Layer 2 device (switch) running IGMP snooping analyzes the received IGMP messages and establishes a mapping relationship between the Layer 3 (router) and receivers. Then the layer 2 device (switch) forwards the multicast data according to the mapping relationship. When the layer 2 device (switch) is not running IGMP snooping, the multicast data is broadcast on the second layer. After the layer 2 device (switch) runs IGMP snooping, the multicast data of the multicast group is not broadcast on the second layer but among the known receivers on the second layer.
IGMP snooping model diagram
Enable IGMP Snooping: Before and After

Types of IGMP Snooping

There are two types of IGMP snooping. One is passive and the other is active. The passive mode only parses multicast packets and does not filter packets. The active mode filters out multicast packets that are meaningless to the router. For example, two receivers under the switch using the passive mode are added to the same group, so that there is an entry in the router above the switch. If a receiver wants to leave the group, it sends a leave message to the router. However, even if the router receives the leave message, it does not delete the entry because there is another receiver in the multicast group. Different from the passive mode, the active mode enables the switch to filter out such packets.

What Is IGMP Snooping Switch?

A data switch with IGMP snooping is called an IGMP snooping switch. This kind of switch can parse the IGMP packets passing through it and establish a multicast forwarding mapping relationship. In this way, the switch can forward multicast data according to the multicast mapping relationship. If this function is disabled, the switch can only broadcast multicast data. The practical application of IGMP snooping switch is to reduce the processing load caused by receivers on the network receiving unnecessary multicast packets. Most IGMP snooping switches use the active mode, so they can snoop the IGMP messages and leave messages and forward them only when necessary to the connected IGMP routers.


Today, IGMP snooping has become very common and advanced, and the current network supports multicast transmission. Besides, for the convenience and saving the load of the server, using the IGMP snooping switch is required. Although there are some shortcomings, the future of this kind switch is promised.

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