Most of the bridges operate in a transparent mode, while switches operate in different modes.
Cisco switches have three modes to control the latency in traffic. It depends upon how thoroughly you want the frame to be checked before it is passed on. If there are more checks, then more latency is created in the switch.
The three different modes are
- Store and forward
The cut-through switching mode provides the fastest switching functions with the lowest latency. The switch copies the destination MAC address to its memory and reads only the first 6 bytes of the frame. Once the frame reaches the destination, the switch then checks its MAC table in order for the port to forward the frame. It then sends it on its way. There is no error checking in the cut-through mode.
The fragment free mode is also referred to as ‘hybrid’ or ‘modified cut-through’. In this mode, the switch works like a cut-through mode with the exception that, it reads and store the first 64 bytes of the frame before it is forwarded to the destination.
Most of the network collision, error and fragment in frames occur during the first 64 bytes of a frame. The fragment-free switch inspects the first 64 bytes, and if no error is found, it passes the frame to its destination. If errors are found, it sends the frame to the transport layer in order to re-check the fragment. Latency is the medium in this mode.
Store and Forward
The store and forward mode is a default switching mode for the distribution layer switches. The entire frame is read and copied to memory. A complete cyclic Redundancy check (CRC) will take place to check for error in order to compare the frame’s FCS value with the CRC output value.
(CRC is an error-checking method that uses a mathematical formula, based on the number of bits (1s) in the frame, to determine whether the received frame has the error. If a CRC is found, the frame is discarded. If the frame is error-free, the switch forwards the frame out of the appropriate interface port.)