Understanding the network.pdf - Back to Home

Cut-through uses the same technology as a standard transparent
bridge. The switch only looks at the frame up to the destination MAC
address, which results in a very low latency. Total latency consists of
the time it takes to examine the frame enough to get the destination
address, perform the SAT lookup and whatever interframe spacing
exists between the incoming frames. Cut-through switches are limited
to switching between similar LAN media types (Ethernet to Ethernet,
FDDI to FDDI, and so on) because the frame is merely looked at, then
forwarded. Cut-through switches also filter frames like transparent
bridges, so traffic that has destination addresses associated with the
port on which the traffic was received is discarded. The downside to
cut-through switching is that no error checking is performed, so
misformed frames and collision fragments get forwarded on the same
as "normal" frames.
The store-and-forward method is a more complicated and
process-intensive forwarding scheme, and it consequently has a
higher latency. A cut-through switch only looks at the frame up to the
destination address, but a store-and-forward switch uses a four-step
process to handle packets:
1. The switch reads the destination address of the frame.
2. The switch stores the frame in the port's frame buffer.
3. The address is compared against the SAT table so the
destination port can be located.
4. Before the switch forwards the frame, it performs a Cyclic
Redundancy Check (CRC) on the frame. If the frame is good, it
is sent to the destination port; if the frame fails, it is dropped.
Although the same frame handling rules used by a cut-through are
followed, the storing of the frame provides the facilities for greater
control of frame flow. Because the frames are stored, the data within
them can now be examined, and the switch can be directed to handle
certain types of data one way and other types another. Misformed
frames, runts (frames smaller than the minimum frame size), jabbers
(frames that exceed the maximum frame size), and collision
fragments are all discarded by a store-and-forward type switch. After
the packet is filtered and deemed valid, it can then be forwarded on to
its destination. Where the packet processing latency on a cut-through
switch is defined largely on the time it takes to perform its SAT lookup,
the frame processing latency on a store-and-forward switch varies
depending on the size of the packet. Commonly, a store-and-forward
switch's latency rating is measured using a 64Kbps packet size.
Store-and-forward switching is the basis for many of the proprietary
virtual LAN (VLAN) technologies. The store-and-forward method is