Configuring The Timeout Time Factor - 3Com 4500 Configuration Manual

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.
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.
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)
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).
<Sysname> system-view
[Sysname] stp timer forward-delay 1600
[Sysname] stp timer hello 300
[Sysname] stp timer max-age 2100

Configuring the Timeout Time Factor

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
for 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 number to avoid such cases. Normally, the
timeout time can be four or more times of the hello time. For a steady network, the timeout time can be
five to seven times of the hello time.
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