Switching Methods and Types Explained

A switch has many ports. It forwards an incoming frame only from the port connected to the destination device of the frame. A switch uses a relatively simple concept to forward a frame. It finds the destination MAC address of the incoming frame in the CAM table.

If the CAM table has an entry for the destination MAC address, it forwards the frame from the port mentioned in the entry. If not, it forwards the frame from all ports except the one it arrived on. A switching method explains how a switch initiates forwarding a frame. There are three switching methods: store-and-forward, fragment-free, and cut-through.

The store-and-forward switching method

In the store-and-forward method, the switch waits till all fields of the frame are received.

After receiving all fields of the frame, the switch verifies whether the received frame is error-free. If the received frame is error-free, the switch forwards the frame from the selected port or ports. If the received frame contains errors, the switch discards the frame.

To determine the condition of a frame, the switch uses the frame's FCS field. The FCS field contains a CRC value. The CRC value allows a receiving device to determine whether the frame is in its original state or has been damaged or tampered with in transit.

After creating a frame, the sender or the source device runs the Cyclic Redundancy Check algorithm on it. The value produced by this algorithm is known as the CRC value. The CRC value is stored in the FCS field of the frame. After storing the CRC value, the sender device loads the frame on the media.

Upon receiving the frame, the receiver or destination device runs the same CRC algorithm on the frame and compares the result with the CRC value stored in the FCS field. If the result and the CRC value match, the frame is considered error-free. If not, the frame is considered damaged.

In the store-and-forward switching method, a switch forwards only error-free frames. To determine the state of a frame, it pulls the entire frame, runs the CRC algorithm, and compares the result with the value stored in the FCS field. It forwards the frame only if both results match.

If both results do not match, the frame is discarded.

This method provides the highest level of accuracy but at the cost of speed. If you compare all three methods of switching, this method ranks first in accuracy and last in speed.

The cut-through switching method

In the cut-through method, the switch starts forwarding as soon as the forwarding port is determined. An Ethernet frame stores the destination MAC address in the first field. To forward a frame, a switch only needs the frame's destination address. Since the destination address occurs very early in the Ethernet frame, a switch can start forwarding the received bits of the frame before receiving all bits of the frame.

In this method, the switch does not check the frame's condition before forwarding it. This reduces latency but also propagates errors. This is the fastest switching method. But it provides speed at the cost of having forwarded some frames that contain errors.

The fragment-free switching method

In the fragment-free method, after determining the forward port, the switch waits till the first 64 bytes of the frame are received.

The 64 bytes is the minimum legal size of an Ethernet frame. An Ethernet frame that is shorter than 64 bytes is called a runt frame. A runt frame is a corrupt frame.

This method is the modified version of the cut-through switching method. This method reduces the number of runt frames that are being switched. Sometimes this method is also known as the modified cut-through or runtless switching method. Compared with the remaining two methods, it offers moderate speed and accuracy.

Conclusion

Switches use three main forwarding methods: store-and-forward, cut-through, and fragment-free. Store-and-forward offers the highest accuracy by checking for errors, but it is slower. Cut-through is the fastest, but it may forward damaged frames. Fragment-free provides a compromise between speed and reliability. Understanding these methods helps you select the right switch method for your network.