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Chapter 4: Web-Based Management
74 GE-DS-242-PoE Managed Ethernet Switch User Manual
Default Spanning-Tree Configuration
Feature Default Value
Enable state STP disabled for all ports
Port priority 128
Port cost 0
Bridge Priority 32,768
NOTE: The Hello Time cannot be longer than the Max. Age. Otherwise, a configuration
error will occur.
Observe the following formulas when setting the above parameters:
• Max. Age _ 2 x (Forward Delay - 1 second)
• Max. Age _ 2 x (Hello Time + 1 second)
Illustration of STP
A simple illustration of three switches connected in a loop is depicted in the below
diagram. In this example, you can anticipate some major network problems if the STP
assistance is not applied.
If switch A broadcasts a packet to switch B, switch B will broadcast it to switch C, and
switch C will broadcast it to back to switch A and so on. The broadcast packet will be
passed indefinitely in a loop, potentially causing a network failure. In this example,
STP breaks the loop by blocking the connection between switch 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 switch A broadcasts a packet to switch C, then switch 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.