How a frame is switched to the destination port is a trade off between latency and reliability. A switch can start to transfer the frame as soon as the destination MAC address is received. Switching at this point is called cut-through switching and results in the lowest latency through the switch. However, no error checking is available. At the other extreme, the switch can receive the entire frame before sending it out the destination port. This gives the switch software an opportunity to verify the Frame Check Sum (FCS) to ensure that the frame was reliably received before sending it to the destination. If the frame is found to be invalid, it is discarded at this switch rather than at the ultimate destination. Since the entire frame is stored before being forwarded, this mode is called store-and-forward.

When using cut-through methods of switching, both the source port and destination port must be operating at the same bit rate in order to keep the frame intact. This is called synchronous switching. If the bit rates are not the same, the frame must be stored at one bit rate before it is sent out at the other bit rate. This is known as asynchronous switching. Store-and-forward mode must be used for asynchronous switching. 

Asymmetric switching provides switched connections between ports of unlike bandwidths, such as a combination of 100 Mbps and 1000 Mbps. Asymmetric switching is optimized for client/server traffic flows in which multiple clients simultaneously communicate with a server, requiring more bandwidth dedicated to the server port to prevent a bottleneck at that port.

Interactive Media Activity Drag and Drop: Switch Modes After completing this activity, the student will be able to identify the three types of switch modes.

   

Web Links LAN Switching http://www.cisco.com/univercd/cc/td/doc/ cisintwk/ito_doc/lanswtch.ht