Switches typically are Layer 2 devices (some
switches now perform Layer 3 and higher functions).
According to the OSI model, the data unit
processed by a switch is called a frame. Switches
must balance speed and accuracy (no errors) when
processing frames, because typically they are measured
on both attributes.
The three primary frame switching modes are as
follows:
• Cut-through: Also known as fast-forward. The
switch checks only the destination address and
immediately begins forwarding the frame. This
can decrease latency but also can transmit
frames containing errors.
• Store-and-forward: The switch waits to receive
the entire frame before forwarding. The entire
frame is read, and a cyclic redundancy check
(CRC) is performed. If the CRC is bad, the
frame is discarded. Although this method
increases latency (processing time), it also tends
to minimize errors.
• Fragment-free (modified cut-through): The
switch reads the first 64 bytes before forwarding
the frame. 64 bytes is the minimum number of
bytes necessary to detect and filter out collision
frames.