ing. With the store-and-forward switching method, the LAN switch copies
the entire frame onto its onboard buffers and computes the cyclic redun-
dancy check (CRC). Because it copies the entire frame, latency through the
switch varies with frame length.
including the CRC). If the frame doesn't contain any errors, the LAN switch
looks up the destination hardware address in its forwarding or switching
table and determines the outgoing interface. It then forwards the frame
toward its destination. This is the mode used by the Catalyst 5000 series
switches, and it cannot be modified on the switch.
method, the LAN switch copies only the destination address (the first 6 bytes
following the preamble) onto its onboard buffers. It then looks up the hard-
ware destination address in the MAC switching table, determines the outgo-
ing interface, and forwards the frame toward its destination. A cut-through
switch provides reduced latency because it begins to forward the frame as
soon as it reads the destination address and determines the outgoing
interface.
they automatically change over to store-and-forward mode so they will stop
forwarding the errors. When the error rate on the port falls below the thresh-
old, the port automatically changes back to cut-through mode.
switch waits for the collision window (64 bytes) to pass before forwarding.
If a packet has an error, it almost always occurs within the first 64 bytes.
FragmentFree mode provides better error checking than the cut-through
mode, with practically no increase in latency. This is the default switching
method for the 1900 switches.