3Com Switch 8800 Advanced Software V5 Configuration Guide

Configuring the Root Bridge 115
Configuration procedure
Follow these steps to configure the timers of MSTP:
These three timers set on the root bridge of the CIST apply on all the devices on
the entire switched network.
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CAUTION:
■ The length of the forward delay time is related to the network diameter of the
switched network. Typically, the larger the network diameter is, the longer the
forward delay time should be. Note that if the forward delay setting is too
small, temporary redundant paths may be introduced; if the forward delay
setting is too big, it may take a long time for the network to resume
connectivity. We recommend that you use the default setting.
■ An appropriate hello time setting enables the device to timely detect link
failures on the network without using excessive network resources. If the hello
time is set too long, the device will take packet loss on a link for link failure and
trigger a new spanning tree computing process; if the hello time is set too
short, the device will send repeated configuration BPDUs frequently, which
adds to the device burden and causes waste of network resources. We
recommend that you use the default setting.
■ If the max age time setting is too small, the network devices will frequently
launch spanning tree computing and may take network congestion to a link
failure; if the max age setting is too large, the network may fail to timely detect
link failures and fail to timely launch spanning tree computing, thus reducing
the auto-sensing capability of the network. We recommend that you use the
default setting.
The setting of hello time, forward delay and max age must meet the following
formulae; otherwise network instability will frequently occur.
■ 2 × (forward delay - 1 second) ƒ max age
■ Ma x age ƒ 2 × (hello time + 1 second)
We recommend that you specify the network diameter and the hello time using
the stp root primary command by preference and let MSTP automatically
calculate an optimal setting of the other two timers.
To do... Use the command... Remarks
Enter system view system-view -
Configure the forward delay
timer
stp timer forward-delay
centi-seconds
Optional
1,500 centiseconds (15
seconds) by default
Configure the hello time timer stp timer hello centi-seconds Optional
200 centiseconds (2 seconds)
by default
Configuring the max age
timer
stp timer max-age
centi-seconds
Optional
2,000 centiseconds (20
seconds) by default