Black Box iPATH ACR1000A-CTL-24 User Manual page 76

E.4 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:
Š Store and forward is the original method and requires the switch to save each entire data packet to buffer memory, run an error Š
check and then forward if no error is found (or otherwise discard it).
Š Cut-through was developed to address the latency issues suffered by some store and forward switches. The switch begins Š
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.
Š Fragment-free is a hybrid of the above two methods. It waits until the first 64 bits have been received before beginning to Š
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.
Š Adaptive switches automatically choose between the above methods. Usually they start out as a cut-through switches and Š
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 Agility 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.
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