3Com 4200G 24-Port - Configuring the MSTP Time-Related Parameters

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Configuring the MSTP Time-related Parameters
Three MSTP time-related parameters exist: forward delay, hello time, and max age. You can configure
the three parameters to control the process of spanning tree calculation.
Configuration procedure
Follow these steps to configure MSTP time-related parameters:
To do... Use the command... Remarks
Enter system view
system-view
—
Configure the forward delay
parameter
stp timer forward-delay
centiseconds
Required
The forward delay parameter
defaults to 1,500 centiseconds
(namely, 15 seconds).
Configure the hello time
parameter
stp timer hello centiseconds
Required
The hello time parameter defaults to
200 centiseconds (namely, 2
seconds).
Configure the max age
parameter
stp timer max-age
centiseconds
Required
The max age parameter defaults to
2,000 centiseconds (namely, 20
seconds).
All switches in a switched network adopt the three time-related parameters configured on the CIST root
bridge.
z The forward delay parameter and the network diameter are correlated. Normally, a large network
diameter corresponds to a large forward delay. A too small forward delay parameter may result in
temporary redundant paths. And a too large forward delay parameter may cause a network unable
to resume the normal state in time after changes occurred to the network. The default value is
recommended.
z An adequate hello time parameter enables a switch to detect link failures in time without occupying
too many network resources. And a too small hello time parameter may result in duplicated
configuration BPDUs being sent frequently, which increases the work load of the switches and
wastes network resources. The default value is recommended.
z As for the max age parameter, if it is too small, network congestion may be falsely regarded as link
failures, which results in frequent spanning tree recalculation. If it is too large, link problems may be
unable to be detected in time, which prevents spanning trees being recalculated in time and makes
the network less adaptive. The default value is recommended.
As for the configuration of the three time-related parameters (that is, the hello time, forward delay, and
max age parameters), the following formulas must be met to prevent frequent network jitter.
2 x (forward delay – 1 second) >= max age
Max age >= 2 x (hello time + 1 second)