Introduction to Ethernet Switching
An Ethernet switch is designed to manage the flow of data between the various devices that are connected to its ports.
MAC Address Table
The heart of an Ethernet switch is the Media Access Control (MAC) address table. Every device that you attach to an Ethernet network has a MAC address. This address is assigned to the device by the device’s manufacturer. For example, each network interface card (NIC) that you install into your network computers has a MAC address that was assigned to it by the card’s manufacturer.
A switch’s MAC address table is a list of the MAC addresses of the devices that are connected to its ports. The switch uses this table to direct data frames to their appropriate destination end nodes, and in some cases, to discard frames that it receives. The switch creates the MAC address table by examining the frames that it receives on its ports. Each frame is examined for its source address; that is, the MAC address of the end node that sent the frame. The switch checks to determine whether the address is already in its MAC address table. If it is not, the switch adds the address to the table along with the port number on which the frame was received. The result is a table that contains a list of all the MAC addresses of end nodes that have sent frames to the switch and the ports on the switch to which the end nodes are connected.
The switch also checks the destination MAC address of each frame it receives. The destination address is the MAC address of the end node to which the frame is intended. If the address is in the table, the switch directs the frame directly to the port where the end node is located. This helps to ensure that end nodes will only receive traffic that is intended for them and not have to deal with traffic intended for other end nodes.
If the destination address is not in the MAC address table, the switch broadcasts the frame to all switch ports. When the destination node responds, the switch will be able to match the address to a port so that the next time a frame is destined to that particular end node, the switch will be able to forward the frame to the correct port instead of having to broadcast the frame to all ports.
In some cases a switch will even discard a frame. If the switch receives a frame that is destined to a node on the same port on which the frame was received, the switch discards the frame.
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