Ethernet Switches

Introduction #

Increasing the physical layer bandwidth as in Fast Ethernet was only one of the solutions to improve the performance of Ethernet LANs.

A second solution was to replace the hubs with more intelligent devices. As Ethernet hubs operate in the physical layer, they can only regenerate the electrical signal to extend the geographical reach of the network. From a performance perspective, it would be more interesting to have devices that operate in the data link layer and can analyze the destination address of each frame and forward the frames selectively on the link that leads to the destination. This would allow two hosts to communicate on one pair of interfaces while other pairs of interfaces can be simultaneously used for other communication, thereby improving communication efficiency. Such devices are usually called Ethernet switches. An Ethernet switch is a relay that operates in the data link layer.

MAC Address Tables #

An Ethernet switch understands the format of the Ethernet frames and can selectively forward frames over each interface. For this, each Ethernet switch maintains a MAC address table. This table contains, for each MAC address known by the switch, the identifier of the switch’s port over which a frame sent towards this address must be forwarded to reach its destination.

This is illustrated below with the MAC address table of switch 2.

• When the switch receives a frame destined to address B, it forwards the frame on its South port.
• If it receives a frame destined to address D, it forwards it only on its North port.