Frame Forwarding Methods on Cisco Switches

Cisco switches support different Switching(frame forwarding) Methods. Switching Methods are determined how a switch receives, processes, and forwards a Layer 2 Ethernet frame. Important Switching methods are following:-

• Store-and-forward switching


• Cut-through switching

Store-and-forward switching

In Store and Forward switching, Switch copies each complete incoming Ethernet frame into the switch memory. During the storage process, the switch analyzes the frame for information about its destination. Switch also computes trailer for  Cyclic Redundancy Check (CRC) for errors. If a Cyclic Redundancy Check (CRC) error is found. Then Ethernet frame is dropped and if there is no Cyclic Redundancy Check (CRC) error, the switch forwards the Ethernet frame to the destination device.

Store and Forward switching can cause a delay in switching since Cyclic Redundancy Check (CRC) is calculated for each Ethernet frame. CRC uses a mathematical formula, based on the number of bits (1s) in the frame, to determine whether the received frame has an error. After confirming the reliability of the frame, the frame is forwarded out to the right port, toward its destination. Discarding frames with errors reduces the amount of bandwidth consumed by corrupt data. Store-and-forward switching is required for Quality of Service (QoS)

Cut-Through Switching

In cut-through switching, the switch copies into its memory only the destination MAC address the frame before making a switching decision, to which port to forward the data. The destination MAC address is located in the first 6 bytes of the frame following the preamble. The switch looks up the destination MAC address in its switching table, determines the outgoing interface port, and forwards the frame onto its destination through the designated switch port. The switch does not perform any error checking on the frame. switch operating in cut-through switching mode reduces delay because the switch starts to forward the Ethernet frame as soon as it reads the destination MAC address. Problem-related with cut-through switching is that the switch may forward bad frames. Cut-through switching is the predominant switching method used on Cisco switches. There are two variants of cut-through switching:

Fast-forward switching

Fast-forward switching gives the lowest level of latency because switch immediately forwards a packet after reading the destination address. for the reason that fast-forward switching starts forwarding before the entire packet has been received, there may be times when packets are relayed with errors. This occurs infrequently, and the destination network adapter discards the faulty packet upon receipt. In fast-forward mode, latency is measured from the first bit received to the first bit transmitted. Fast-forward switching is the typical cut-through method of switching.

Fragment-free switching

Fragment-free switching is an advanced form of cut-through switching. The switches operating in cut-through switching read only up to the destination MAC address field in the Ethernet frame before making a switching decision. The switches operating in fragment-free switching read and store at least 64 bytes of the Ethernet frame before switching it to avoid forwarding Ethernet runt frames (Ethernet frames smaller than 64 bytes).  Fragment-free switching can be viewed as a compromise between store-and-forward switching and fast-forward switching. The reason fragment-free switching stores only the first 64 bytes of the frame is that most network errors and collisions occur during the first 64 bytes.Fragment-free switching tries to improve fast-forward switching by performing a small error check on the first 64 bytes of the frame.  Fragment-free switching is a compromise between the high latency and high integrity of store-and-forward switching; and the low latency and reduced integrity of fast-forward switching.

Some switches are configured to perform cut-through switching on a per-port basis until a user-defined error threshold is reached; and then they automatically change to store-and-forward. When the error rate falls below the threshold, the port automatically changes back to cut-through switching.