Forwarding modes

Posted on August 15, 2009. Filed under: CCNA, ICND1 break down | Tags: , , , |


After the switch decides to forward a frame, there are four forwarding modes to choose from:

  1. cut-through — In cut-through mode, the switch sends the frame to the destination as soon as the first 14 bytes are read. This delay is minimal and the frame reach the destination address in the shortest posssible time. The data flow is continuous and the transmit and receive rates are always the same. Switches have no error check in this mode. In summary, cut-through offers the fastest possible forwarding at the expense of error checking. Cut-through switching is good for networks with light traffic and mulitimedia applications which need minimum delays.
  2. Store-and-forward — In this mode. switch stores the entire frame and checks the CRC field for error before forwarding it. Store-and-forward switching is the best forwarding mode to prevent errors at the expense of forwarding speed.
  3. fragment-free — In this mode, switch check the first 64 bytes of incoming frames for errors. If there are no errors, the forwarding process begins. Fragment-free switching offers a compromise between cut-through and store-and-forward.
  4. Adaptive — If in this mode, the switch starts in one of the above-mentioned three modes pre-selected by the sysadmin. Depending on the network situations such as CRC errors, the mode changes to the “best” of the other two switching modes. Adaptive switching mode maximize the efficiency of the switch dynamically.

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