This tutorial explains the three methods of switching; cut-through, Fragment-free and store-and-forward. Learn the switching methods which are used in internal frame processing of a switch.
Before we discuss the switching methods in detail, let’s briefly understand what the internal frame processing is.
A switch has multiple ports. These ports are used to connect various devices with it. To know which device is connected to which port,
the switch stores the MAC addresses of all connected devices with their related ports in a table, known as the CAM table.
When a switch receives a frame on any of its port, it reads the destination address from the
frame and finds that destination address in the CAM table. If the CAM table has an entry for that destination address,
the switch forwards that frame from the port on which the destination address is connected. If the CAM table does not have an
entry for that destination address, the switch forwards that frame from all of its ports. This process is known as the internal frame processing.
Switching methods explain how early the switch starts the internal frame processing. There are three types of switching methods; the store-and-forward method, the fragment-free method, and the cut-through method. Let’s understand each of these methods in detail.
The store-and-forward switching method
In this method, the switch does not start the internal frame processing until the complete frame is received. After receiving the entire frame, switch verifies whether the received frame is in the good state or not. If the received frame is in the good state, only then the switch processes it. If the received frame is not in the good state, the switch discards it.
To know the condition of a frame, the switch uses the FCS (frame check sequence) field of the frame. The FCS field contains a value known as the CRC value. The CRC value allows any receiving device to know whether the frame is exactly same as the source packed it or it has been damaged in the middle.
After creating a frame, the sender or source device runs the CRC (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. The frame is the last layer of data packing. After storing the CRC value, the sender node loads that frame in the media.
Upon receiving this 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 both the result and the CRC value match, the frame is considered in the good state. If both values do not match, the frame is considered as the damaged frame.
The store-and-forward method processes only the frames which are in the good state. If we compare all three methods of switching, this method stands at the first and the last positions in terms of accuracy and speed respectively.
The cut-through switching method
In this method, the switch starts the frame processing as soon as it reads the destination MAC address of the frame. Ethernet frame stores destination MAC address in the third field. The following image shows a standard Ethernet frame.
To start the frame processing, a switch only needs the destination MAC address. Since the destination MAC address occurs very early in the Ethernet frame, a switch can start the frame processing before it receives all the bits of the frame.
Just after reading the destination address, it checks the CAM table and based on the match it starts the forwarding of the frame (even though the frame may still be coming into the switch).
This method, before forwarding a frame, does not check its condition. This reduces the latency, but it also propagates errors. From all three switching methods, this is the fastest method of switching. But it provides speed at the price of having forwarded some frame that contains errors.
The fragment-free switching method
In this method, the switch starts the frame processing after reading the first 64 bytes of the frame. 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 reduces the number of Ethernet runt frames that are being switched.
Basically, the fragment-free switching method is the modified version of the cut-through switching method.
Sometimes it is also known as the modified cut-through or runtless switching method.
The following table compares all three methods.
|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|
That’s all for this tutorial. If you like this tutorial, please don’t forget to share it with friends through your favorite social platform.
Prerequisites for 200-301
200-301 is a single exam, consisting of about 120 questions. It covers a wide range of topics, such as routing and switching, security, wireless networking, and even some programming concepts. As with other Cisco certifications, you can take it at any of the Pearson VUE certification centers.
The recommended training program that can be taken at a Cisco academy is called Implementing and Administering Cisco Solutions (CCNA). The successful completion of a training course will get you a training badge.
Full Version 200-301 Dumps