DES-3326S Layer 3 Fast Ethernet Switch User’s Guide
Switch Management and Operating Concepts 67
Illustration of STP
A simple illustration of three Bridges (or three switches)
connected in a loop is depicted below. In this example, you can
anticipate some major network problems if the STP assistance
is not applied. If Bridge A broadcasts a packet to Bridge B,
Bridge B will broadcast it to Bridge C, and Bridge C will
broadcast it to back to Bridge A ... and so on. The broadcast
packet will be passed indefinitely in a loop, potentially causing
a network failure.
STP can be applied as shown in Figure 2-4. In this example,
STP breaks the loop by blocking the connection between Bridge
B and C. The decision to block a particular connection is based
on the STP calculation of the most current Bridge and Port
settings. Now, if Bridge A broadcasts a packet to Bridge C,
then Bridge C will drop the packet at port 2 and the broadcast
will end there.
Setting-up STP using values other than the defaults, can be
complex. Therefore, you are advised to keep the default factory
settings and STP will automatically assign root bridges/ports
and block loop connections. Influencing STP to choose a
particular switch as the root bridge using the Priority setting,
or influencing STP to choose a particular port to block using
the Port Priority and Port Cost settings is, however, relatively
straight forward.
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