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Overview of Multicast Snooping

Network devices such as routers operate mainly at the packet level, or Layer 3. Other network devices such as bridges or LAN switches operate mainly at the frame level, or Layer 2. Multicasting functions mainly at the packet level, Layer 3, but there is a way to map Layer 3 IP multicast group addresses to Layer 2 MAC multicast group addresses at the frame level. For more information about this translation procedure, see Layer 2 Frames and Multicast.

Routers are able to handle both Layer 2 and Layer 3 addressing information because the frame and its addresses must be processed to access the encapsulated packet inside. It is easy for routers to run Layer 3 multicast protocols such as PIM or IGMP and determine where to forward multicast content or when a host on an interface joins or leaves a group. However, bridges and LAN switches, as Layer 2 devices, are not supposed to have access to the multicast information inside the packets their frames carry.

How then are bridges and other Layer 2 devices to determine when a device on an interface joins or leaves a multicast tree, or whether a host on an attached LAN is interested in receiving the content of a particular multicast group or not?

The answer is for the Layer 2 device to implement a series of procedures known as multicast snooping. Multicast snooping is a general term and applies to the process of a Layer 2 device “snooping” at the Layer 3 packet content to determine which actions should be taken to process or forward a frame. There are more specific forms of snooping, such as IGMP snooping or PIM snooping. In all cases, snooping involves a device configured to function at Layer 2 having access to normally “forbidden” Layer 3 (packet) information. However, snooping makes multicasting more efficient in these devices.

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