3Com WX3000 operation manual Configuring the Timeout Time Factor

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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 device 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 devices 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 device operates as the CIST root bridge).

<device> system-view

[device] stp timer forward-delay 1600

[device] stp timer hello 300

[device] stp timer max-age 2100

Configuring the Timeout Time Factor

When the network topology is stable, a non-root-bridge device 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 device regards its upstream device 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 device 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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