User guide

12
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layer2,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 can result in bridging
loops.
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 standardised way to categorise
the necessary functions of any standard
network.
TherearesevenlayersintheOSImodel
andthesedenethestepsneededtoget
the data created by you (imagine that
you are Layer 8) reliably down onto the
SowhyareLayer2andLayer3of
particular importance when discussing
AdderLinkInnity?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
(the collection of layered data), strip away
onlytherstfewwrapperstodiscover
the intended destination then rewrap the
packet and send it in the correct direction.
Insimplied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 more 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.
Glossary
LAYER
7L
AYER 7
LAYER
6L
AYER 6
LAYER
5L
AYER 5
LAYER
4L
AYER 4
LAYER
3L
AYER 3
LAYER
2L
AYER 2
LAYER
1L
AYER 1
Application
Presentation
Session
Transport
Network
Data Link
Physical
Network connection
transmission medium (the cable, optical
bre,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 (plus
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nallyonlythe
original data are left.
continued