User guide

14
Forwarding modes
In essence, the job of a layer 2 switch is
to transfer as fast as possible, data packets
arriving at one port out to another port
as determined by the destination address.
This is known as data forwarding and
most switches offer a choice of methods
to achieve this. Choosing the most
appropriate forwarding method can often
have a sizeable impact on the overall speed
of switching:
• Store and forward is the original
method and requires the switch to
save each entire data packet to buffer
memory, run an error check and
then forward if no error is found (or
otherwise discard it).
• Cut-through was developed to address
the latency issues suffered by some
store and forward switches. The switch
begins interpreting each data packet as
itarrives.Oncetheinitialaddressing
information has been read, the switch
immediately begins forwarding the
data packet while the remainder is still
arriving.Onceallofthepackethasbeen
received, an error check is performed
and, if necessary, the packet is tagged
as being in error. This checking ‘on-the-
fly’ means that cut-through switches
cannot discard faulty packets themselves.
However, on receipt of the marked
packet, a host will carry out the discard
process.
• Fragment-free is a hybrid of the above
twomethods.Itwaitsuntiltherst64
bits have been received before beginning
to forward each data packet. This way
the switch is more likely to locate and
discard faulty packets that are fragmented
due to collisions with other data packets.
• Adaptive switches automatically choose
between the above methods. Usually
they start out as a cut-through switches
and change to store and forward or
fragment-free methods if large number of
errors or collisions are detected.
Sowhichonetochoose?TheCut-through
method has the least latency so is usually
the best to use with AdderLink
Innityunits.However,ifthenetwork
componentsand/orcablinggeneratealot
of errors, the Store and forward method
shouldprobablybeused.Onhigherend
store and forward switches, latency is
rarely an issue.
Glossary