3Com 4210 PWR Configuration Manual page 164

162 C 14: MSTP C
HAPTER
Configuring the Timeout
Time Factor
ONFIGURATION
All switches in a switched network adopt the three time-related parameters
configured on the CIST root bridge.
c
CAUTION:
The forward delay parameter and the network diameter are correlated.
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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.
An adequate hello time parameter enables a switch to detect link failures in
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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.
As for the max age parameter, if it is too small, network congestion may be
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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)
You are recommended to specify the network diameter of the switched network
and the hello time by using the stp root primary or stp root secondary
command. After that, the three proper time-related parameters are determined
automatically.
Configuration example
# Configure the forward delay parameter to be 1,600 centiseconds, the hello time
parameter to be 300 centiseconds, and the max age parameter to be 2,100
centiseconds (assuming that the current switch operates as the CIST root bridge).
<4210> system-view
[4210] stp timer forward-delay 1600
[4210] stp timer hello 300
[4210] stp timer max-age 2100
When the network topology is stable, a non-root-bridge switch regularly forwards
BPDUs received from the root bridge to its neighboring devices at the interval
specified by the hello time parameter to check link failures. Normally, a switch
regards its upstream switch faulty if the former does not receive any BPDU from
the latter in a period three times of the hello time and then initiates the spanning
tree recalculation process.
Spanning trees may be recalculated even in a steady network if an upstream
switch continues to be busy. You can configure the timeout time factor to a larger