Multicast-Snooping

All devices with WLAN interfaces have a "LAN bridge" that transfers data between the Ethernet ports and the WLAN interfaces. The LAN bridge works like a switch in many respects. The central task of a switch is to forward packets only to the port to which the receiver is connected. To do this, the switch automatically forms a table from the incoming data packets in which the sender MAC addresses are assigned to the ports.

If a destination address of an incoming packet is found in this table, the switch can forward the packet specifically to the correct port. If the destination address is not found, the switch forwards the packet to all ports. This means that a switch can only forward a packet specifically if the destination address has already been received by it once as the sender address of a packet via a specific port. However, broadcast or multicast packets can never be entered as the sender address in a packet, which is why these packets are always "flooded" to all ports.

While this behavior is the correct action for broadcasts, since broadcasts should eventually reach all possible recipients, it is not necessarily the desired solution for multicasts. Multicasts are usually aimed at a specific group of recipients on a network, not all of them.

For example, video streams are often multicast, but not all stations on the network should receive a particular stream.

Various applications in the medical field use multicasts to transmit data to specific terminals that should not be viewed at all stations.

With a LAN bridge in the device, there will therefore also be ports to which no single receiver of the multicast is connected. The "unnecessary" sending of multicasts on ports without receivers is not a mistake, but it leads to performance problems, especially in WLAN networks. There, the unnecessary sending of multicasts can lead to a significant restriction of the available bandwidth, since multicasts in the WLAN—just like broadcasts—are sent at the lowest possible transmission rate so that they can be received by every WLAN subscriber.

With the Internet Group Management Protocol (IGMP) for IPv4 as well as Multicast Listener Discovery (MLD) for IPv6, the TCP/IP protocol family provides a protocol with which the network stations can inform the router to which they are connected of their interest in certain multicasts. To do this, the stations register with the routers for specific multicast groups from which you want to obtain the corresponding packets (multicast registration). IGMP uses special messages to register (join messages) and deregister (leave messages) for this purpose.

Multicast snooping makes use of these messages to decide to which port (i.e., also to which WLAN SSID) multicasts must be sent.