forwarding mode question

Unanswered Question
Oct 26th, 2007

One type of frame switching uses low-latency forwarding and then switches to a higher-latency error checking mode when an error threshold is exceeded. What is the name of this frame switching mode?

A. Store and forward

B. Fragment-free

C. Adaptive cut-through

D. Fast-forward

Answer: C

Explanation:

Adaptive cut-through is a modified form of cut-through switching in which the switch waits for the collision window (64 bytes) to pass before forwarding. It means that each frame will be checked into the data field to make sure no fragmentation has occurred.

Why not C? I read from ICND and INTRO that there are only three forwarding mode for cisco switch. Two primary are cut-through and store and forward, and fragment-free is kinda combine those two. what is this Adaptive cut-through? and what is the difference between Adaptive cut-through and fragment-free?

Thank you.

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Hi There

I would question the validity of the answer given to the stated question.

From my own CCNA studies, the 3 main switching methods are

1, Cut through

2, Store and forward

3, Fragment free

However apparently some Cisco switches also allow for "adaptive cut thorugh" switching, which according to the mention it gets in Todd Lammle's CCNA study Guide Delux Edition (page 361) is switching on a per port basis until a USER DEFINED threshold is reached, at which point the port(s) will automatically change to the store and forward method until the errors cease.

So IMHO the explanation given in the answer actually defines fragment free switching, which does check the first 64 bytes, when the question is clearly referencing "Adaptive Cut Through" switching.

I did find another CCNA study guide which also mentions adaptive cut through switching. Check the section on cut through at this link http://www.2000trainers.com/cisco-ccna-03/ccna-switching-methods/

Hope that helps some.

Best regards & Good luck with the studies

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