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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 first 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 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.
LAYER 2 AND LAYER 3: THE OSI MODEL
When discussing network switches, the terms Layer 2 and Layer 3 are very often used. These refer to parts of the Open System
Interconnection (OSI) model, a standardized way to categorize the necessary functions of any standard network.
There are seven layers in the OSI model and these define the steps needed to get the data created by you (imagine that you are
Layer 8) reliably down onto the transmission medium (the cable, optical fiber, radio wave, etc.) that carries the data to another
user; to complete the picture, consider the transmission medium is Layer 0. In general, think of the functions carried out by the
layers at the top as being complex, becoming less complex as you go lower down.
As your data travel down from you towards the transmission medium
(the cable), they are successively encapsulated at each layer within a
new wrapper (along with a few instructions), ready for transport. Once
transmission has been made to the intended destination, the reverse
occurs: Each wrapper is stripped away and the instructions examined
until finally only the original data are left.
So why are Layer 2 and Layer 3 of particular importance when discussing
Agility? Because the successful transmission of data relies upon fast and
reliable passage through network switches – and most of these operate
at either Layer 2 or Layer 3.
The job of any network switch is to receive each incoming network packet, strip away only the first few wrappers to discover the
intended destination then rewrap the packet and send it in the correct direction.
In simplified terms, the wrapper that is added at Layer 2 (by the sending system) includes the physical address of the intended
recipient system, i.e. the unique MAC address (for example, 09:f8:33:d7:66:12) that is assigned to every networking device at
manufacture. Deciphering recipients at this level is more straightforward than at Layer 3, where the address of the recipient is
represented by a logical IP address (e.g. 192.168.0.10) and requires greater knowledge of the surrounding network structure. Due
to their more complex circuitry, Layer 3 switches are more expensive than Layer 2 switches of a similar build quality and are used
more sparingly within installations.