26
FUR
THER
INFORMA
TION
Spanning Tree Protocol (STP)
In order to build a robust network, it is necessary
to include certain levels of redundancy within the
interconnections between switches. This will help to
ensure that a failure of one link does not lead to a
complete failure of the whole network.
The danger of multiple links is that data packets, especially
multicast packets, become involved in continual loops as
neighbouring switches use the duplicated links to send and
resend them to each other.
To prevent such bridging loops from occurring, the
Spanning Tree Protocol (STP), operating at
, is
used within each switch. STP encourages all switches
to communicate and learn about each other. It prevents
bridging loops by blocking newly discovered links until it
can discover the nature of the link: is it a new host or a
new switch?
The problem with this is that the discovery process can
take up to 50 seconds before the block is lifted, causing
problematic timeouts.
The answer to this issue is to enable the portfast variable
for all host links on a switch. This will cause any new
connection to go immediately into forwarding mode.
However, take particular care not to enable portfast on
any switch to switch connections as this will result in
bridging loops.
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 ADDERLink
INFINITY 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 ADDERLink INFINITY? 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.
continued