This means that although the switch cannot prevent the forwarding of invalid
frames since it begins forwarding prior to verifying the checksum, it can
keep track of the number of errored frames since it does eventually verify
the checksum. In other words, unlike store and forward switches who verify
the checksum before forwarding, by the time a cut-through switch realizes
the frame is bad, the damage is done. This is key for modes like
adaptive/dynamic cut-through or whatever marketing calls it these days,
where a switch can use cut-through until a specific error threshhold is
reached and then dynamically switch to store and forward mode. In practice
however, the latency variance between modes is so minimal that I believe
almost all switches use store and forward.
Pete
*********** REPLY SEPARATOR ***********
On 8/17/2001 at 6:39 AM Phil Barker wrote:
>Just reading "Layer 2 Switching and Bridging" by Leigh
>Anne Chisholm.
>
>Cut-Through
>
>"Since the port does not wait to receive the CRC at
>the end 'if/of' the frame, it cannot determine the
>integrity of the data received"
>
>Happy with that.
>
>"Cut-through switches CAN perform a CRC check as the
>frame passes through the switch, keeping track of the
>number of bad frames the port receives".
>
>I'm Confused.
>
>I'm guessing that some form of CRC checksum can be
>calculated on the first X-bits of the frame before the
>cut-through process is allowed rather than the entire
>frame.
>
>Any ideas ?
>
>Phil.
>
>
>
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