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User-Changeable STP Parameters
The factory default setting should cover the majority of installations. However, it is advisable to keep
the default settings as set at the factory, unless it is absolutely necessary. The user changeable
parameters described below are configured using the menus located in the Spanning Tree folder of
the web manager. The CLI Reference Manual contains a listing of Spanning Tree Commands in its
own section.
•
Bridge Hello Time
The Bridge Hello Time can be from 1 to 10 seconds. This is the interval between two transmissions of
BPDU packets sent by the Root Bridge to tell all other Switches that it is indeed the Root Bridge. If you
set a Hello Time for your Switch, and it is not the Root Bridge, the set Hello Time will be used if and
when your Switch becomes the Root Bridge. The Hello Time cannot be longer than the Max. Age.
Otherwise, a configuration error will occur.
•
Bridge Max Age
The Bridge Maximum Age Timer can be from 6 to 40 seconds. At the end of the Max. Age, if a BPDU
has still not been received from the Root Bridge, the Switch will start sending its own BPDU to all other
switches for permission to become the Root Bridge. If it turns out the Switch has the lowest Bridge
Identifier, it will become the Root Bridge.
•
Bridge Forward Delay
The Bridge Forward Delay can be from 4 to 30 seconds. This is the time any port on the Switch
spends in the listening state while moving from the blocking state to the forwarding state.
•
Bridge Priority
Bridge Priority for the switch can be set from 0 to 65535. 0 is equal to the highest Priority.
•
Forwarding BDPU
Enabled by default. This may be disabled, in which case BDPU packets (also called Hello messages)
are no longer forwarded.
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)
-
Port Priority. A Port Priority can be from 0 to 255. The lower the number, the greater the probability
the port will be chosen as the Root Port.*
-
Port Cost. A Port Cost can be set from 1 to 65535. The lower the number, the greater the
probability the port will be chosen to forward packets.*
* Port Cost and Port Priority can be configured for individual ports using the STP Port settings menu
(page 64) or use the CLI command group
Illustration of STP
A simple illustration of three Bridges (or three switches) connected in a loop is depicted in Figure 5.5.
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 5.6. 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.
Switch Management
DES-7000/DES-7100 Layer 2 Switch User's Guide
config
stp_ports.
39
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