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 the CAM table does not have an entry for the destination MAC address, it forward the frame from all ports except the port on which it arrived. A switching method explains how a switch starts the forwarding frame. There are three switching methods: the store-and-forward method, the fragment-free method, and the cut-through method.
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 know how switches learn MAC addresses and make frame-forwarding decisions, you can check the following tutorials.
To know the condition of a frame , the switch uses the FCS field of the frame. The FCS field contains a value known as the CRC value. The CRC value allows a receiving device to know whether the frame is exactly in the same state as the source packed it or has been damaged or tempered in the middle.
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 CRC algorithm on the frame and compares the result with the CRC value stored in the FCS field of the frame. If the result and the CRC value are the same, the frame is considered error-free. If they are not the same, the frame is considered as the damaged frame.
In the store-and-forward switching method, a switch forwards only error-free frames. To know the state of a frame, it pulls the complete frame, runs the CRC algorithm, and compares the result with the result stored in the FCS field. It forwards the frame only if both results match.
If both results do not match, it discards the frame.
This method provides the highest level of accuracy but at the cost of speed. If we compare all three methods of switching, this method respectively stands at the first and the last positions in terms of accuracy and 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 destination address of the frame. 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 condition of the frame before forwarding it. This reduces the latency, but it also propagates errors. Of all three switching methods, this is the fastest method of switching. 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 smaller than 64 bytes is known as the 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.
If we compare it with the remaining two methods, it provides moderate speed and accuracy.
Comparing switching methods
The following table compares all three methods.
| Methods/Compression | Store-and-forward | Fragment-free | Cut-and-through |
| Frame processing starts | After receiving the complete frame and running the CRC algorithm. | After receiving the first 64 bytes. | After receiving the first 8 bytes. |
| Position in term of speed | Third | Second | First |
| Position in term of accuracy | First | Second | Third |
