CAM Table

A forwarding information base (FIB), also known as a forwarding table or MAC table, is most commonly used in network bridging, routing, and similar functions to find the proper interface to which the input interface should forward a packet. It is a dynamic table that maps MAC addresses to ports. It is the essential mechanism that separates network switches from network hubs. Content-addressable memory (CAM) is typically used to efficiently implement the FIB, thus it is sometimes called a CAM table.

The role of an Ethernet switch is to forward Ethernet frames from one port to another. The presence of a FIB is one attribute that separates a switch from a hub. Without a functional FIB, all frames received by a network switch would be echoed back out to all other ports, much like an Ethernet hub. A switch should only emit a frame on the port where the destination network device resides (unicast), unless the frame is for all nodes on the switch (broadcast) or multiple nodes (multicast).

Generally, the FIB is a system memory construct used by Ethernet switch logic to map a station's MAC address to the switch port the station is connected to. This allows switches to facilitate communications between connected stations at high speed regardless of how many devices are connected to the switch. The FIB is consulted to make the frame forwarding decision. Switches learn MAC addresses from the source address of Ethernet frames on the ports, such as response packets.

A data link layer technology, such as media access control (MAC) protocols on local area networks, has an address that has no significance beyond a single medium.

Besides Ethernet bridging based on MAC layer addresses, other data-link-layer technologies using forwarding tables include frame relay and Asynchronous Transfer Mode (ATM) switches, and (MPLS). ATM has both link-local addresses and addresses that have end-to-end significance in the ATM domain.

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