The two main types of switching methods are store-and-forward and
cut-through as shown in Figure .
Depending upon the switch, this may be a configurable parameter on
each individual port.
Store-and-forward
Store-and-forward is typically the default method on most switches.
Before a switch forwards an Ethernet frame out another interface (by
looking up the destination MAC address in the switch’s Source
Address Table) it copies the entire frame into its buffers, checks the
FCS (Frame Check Sequence) against its own calculations. If the FCS
and its own calculations match, the frame is forwarded out the proper
port. If the FCS does not match its own calculations, then the frame
is dropped. Checking the FCS takes time, i.e. causes additional
latency in the switch, but all errors are filtered. 
Cut-through
There are two types of cut-through switching available on most
Cisco switches:
- Fast-forward
- Fragment-free
Fast-forward switching begins to forward a frame out the proper
switch port immediately after reading the layer 2 destination address
and looking that address up that address in the switch’s Source
Address Table. The frame will begin to be forwarded out that interface
before the rest of the frame is copied into the switch. The FCS is not
checked with fast-forward switching therefore there is no error
checking.
Fragment-free switching performs like cut-through, but waits until
the first 64 bytes of the frame has been received before forwarding
the first bytes of the frame out of the outgoing switch port.
According to Ethernet and 802.3 specifications, collisions should be
detected during the first 64 bytes of a frame. Just like fast-forward
switching, in fragment-free switching the FCS is not checked, so again
there is no error checking. Fragment-free switching is faster (less
latency) than store-and-forward but slower (more latency) than
fast-forward.
|