Forward and Filter Decision Switching

Forward and filter decision switching is a type of switching method used in LAN switches. It is also referred to as transparent bridging or store-and-forward switching.

In forward and filter decision switching, the switch reads the destination MAC address of the incoming frame and looks up the address in its forwarding table. If the destination MAC address is not in the table, the switch will forward the frame to all ports except the port on which the frame was received. This flooding process is known as the “unknown unicast” or “broadcast” domain.

Once the switch determines which port the destination MAC address is located on, it forwards the frame only to that specific port. This filtering process is known as the “known unicast” domain.

The switch updates its forwarding table with the source MAC address and the incoming port of the frame. If the source MAC address is already in the table, the switch updates the incoming port for that MAC address.

This method of switching has the advantage of reducing unnecessary traffic on the network and providing fast, efficient communication between devices on the same LAN segment.

Switch forwards Ethernet frames based on MAC address in the frame. Different categories of Ethernet MAC addresses are unicast (a single MAC address), broadcast (addressed to all devices on the LAN) and multicast ( a subset of devices on LAN). Switching methods determine how a switch receives, processes, and forwards a Layer 2 frame. It includes

Store and Forward Switching

Switch copies each complete frame into the switch memory and computes a Cyclic Redundancy Check (CRC) for errors. If error is found, the frame is dropped else, forward the frame to the destination device. Store and Forward switching can cause delay in switching since CRC is calculated for each frame.

Cut-through Switching

Switch copies into its memory only the destination MAC address (first 6 bytes of the frame) of the frame before making a switching decision. A switch operating in cut-through switching mode reduces delay because the switch starts to forward the frame as soon as it reads the destination MAC address and determines the outgoing switch port. Problem related with cut-through switching is that the switch may forward bad frames.

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 frame before making a switching decision. The switches operating in fragment-free switching read at least 64 bytes of the frame before switching it to avoid forwarding Ethernet runt frames (Ethernet frames smaller than 64 bytes).