The All-New Switch Book: The Complete Guide to LAN Switching Technology, Second Edition

4.3. Cut-Through Versus Store-and-Forward Operation

In all the previous descriptions of bridge and switch operation, we implied that each frame was received (stored) completely before any decisions were made regarding whether and where to forward the frame. This method of switch operation is called store-and-forward. A switch makes its time-critical filter/forward decision based on the Destination Address in each received frame. The Destination Address is usually one of the first fields present in the frame. (In Ethernet, it is the first field following the Start-of-Frame Delimiter.) Thus, the switch can start the table lookup and forwarding-decision process without waiting for the rest of the frame to arrive; it has all the information it needs once the Destination Address has been received.

It takes some time for a frame to be fully received; a maximum-length Ethernet frame has a duration of about 1.2 ms (at 10 Mb/s). However, the Destination Address has fully arrived after a maximum of 11.2 μs (including Preamble and Start-of-Frame Delimiter). What if the table lookup can be completed before the end of the frame has even arrived at the input? Once the lookup is complete, the switch knows to which port(s) (if any) the frame should be forwarded; assuming that the appropriate output port is available, the switch can begin transmitting the frame before the frame has even been fully received at the input. This method of switch operation is called cut-through, and is depicted in ...