Cisco CCNA Certification: Cisco Switching ModesPage: 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 or Go To our Certification Articles Section
To pass the CCNA exam and earn that coveted certification,
you've got to know Cisco switches inside and out. Among the
many important details you've got to know are the three methods
that Cisco switches use to forward frames, and the differences
between the three.
The first switching method is Store-and-Forward. The name
is the recipe, because that's just what the switch does -
it stores the entire frame before beginning to forward it.
This method allows for the greatest amount of error checking,
since the Frame Check Sequence (FCS) can be run before the
frame is forwarded. As always, there is a tradeoff, since
this error checking process makes this the slowest of the
three frame forwarding methods.
The quickest method is Cut-Through, where only the destination
MAC address of the frame is examined before the forwarding
process begins. This means that the part of the frame is actually
being forwarded as it is still being received! The tradeoff
here is that the FCS does not run, so there is absolutely
no error checking with Cut-Through switching.
The middle ground between these two extremes is Fragment-Free,
so named since fragmented frames will not be forwarded. The
switch examines only the first 64 bytes of the frame for errors,
since that is the part of the frame that will be damaged in
case of a collision. There is error checking, but it is not
as thorough as Store-and-Forward.
Keeping these three switching schemes straight is vital to
your CCNA exam efforts, and it will help you in working with
Cisco switches in the real world as well. Keep studying!
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