IGMP snooping: concept and use

To connect network devices to groups, IP nodes and routers use the IGMP management protocol. Internet Group Management Protocol manages multicast (group) data transmission in networks. It is located at the network level and connects the client computer to the local router in order to transfer data between them. Then, group traffic is routed to the other clients via the PIM protocol. It connects the local router to the remote one. Through the use of IGMP, the network resources of a number of applications (online games, streaming video) can be used more efficiently.

The decision to broadcast traffic to one or another interface allows the use of the IGMP snooping function. What it is? This is the process of tracking IGMP requests from consumers (hosts) to providers (group routers).

Concept and purpose of IGMP snooping

In English, snooping means "eavesdropping." When it is enabled, the intermediate network device (router or communicator) begins to analyze the transfer of all data packets between client computers connected to it and routers supplying group traffic. When a connection request is detected, the port to which the consumer is connected (client) is turned on, in the opposite situation (Leave request), the corresponding port is removed from the group list.

In most communicators, IGMP snooping is available, but requires pre-activation.

Why monitor network traffic?

Multicast-traffic can be transmitted including to computers that are not interested in it. This is called broadcast retransmission. To prevent this, IGMP snooping is used to reduce the load on the network. At the same time, this type of filtering requires additional memory and increases the load on the communicator. However, it is justified.

If the communicator starts broadcasting group traffic on all its ports, then:

• This process is useless;
• There may be problems in the operation of the ultimate recipient (network device), which has to process a large amount of unnecessary data.

To avoid such situations, there is an IGMP snooping function that significantly improves the performance of the entire network. It takes into account the needs at the network (third) level and thus optimizes the channel (second) level of data transmission.

Enabling the wiretap feature

In order to monitor multicast traffic, you must first enable IGMP snooping and configure it yourself. Consider how to do this on D-Link communicators when implementing a multicast scheme. Commands for activating the network listening:

To exclude a port from the network group, when the communicator receives a Leave request from the client, the IGMP Snooping Fast Leave function is used. It allows you to stop the transmission of unnecessary data streams over the network in order to more efficiently work. To activate this function, use the following command:

Applicable if you want to enable multicast filtering of the switch with a node connected to it that is involved in the data transfer.

Types of IGMP-wiretapping

The IGMP snooping function can be either passive or active. How is it shown?

1. Passive does not filter traffic, but simply tracks it.
2. Active - listen and filter data packets to reduce the load of the group router.

The second kind of implementation of the specified function is the most preferable, since it allows to minimize the amount of transmitted information by filtering requests for connection to the router and disconnecting from it.

The functionality of the IGMP snooping communicator helps reduce the load on the network by monitoring the communication processes between the providers (local routers) and the users (client computers) of the group traffic.