9-Building The MAC Table, Part 1
Switches use MAC addresses to direct network communications through the switch to the appropriate outbound port toward the destination. A switch is made up of integrated circuits and accompanying software that controls the data paths through the switch. For a switch to know which port to use to transmit a frame, it must first learn which devices exist on each port. As the switch learns the relationship of ports to devices, it builds a table called a MAC address table, or content addressable memory (CAM) table. CAM is a special type of memory used in high-speed searching applications.
LAN switches determine how to handle incoming data frames by maintaining the MAC address table. A switch builds its MAC address table by recording the MAC address of each device connected to each of its ports. The switch uses the information in the MAC address table to send frames destined for a specific device out the port, which has been assigned to that device.
An easy way to remember how a switch operates is the following saying: A switch learns on “source” and forwards based on “destination.” This means that a switch populates the MAC address table based on source MAC addresses. As frames enter the switch, the switch “learns” the source MAC address of the received frame and adds the MAC address to the MAC address table or refreshes the age timer of an existing MAC address table entry.
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