H3C S9500 Series Operation Manual page 130

Operation Manual – MSTP
H3C S9500 Series Routing Switches
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) ú max age
Ma x age ú 2 × (hello time + 1 second)
We recommend that you specify the network diameter and the hello time using the stp
bridge-diameter command by preference and let MSTP automatically calculate an
optimal setting of the other two timers.
II. Configuration example
# Set the forward delay to 1,600 centiseconds, hello time to 300 centiseconds, and max
age to 2,100 centiseconds.
<Sysname> system-view
[Sysname] stp timer forward-delay 1600
[Sysname] stp timer hello 300
[Sysname] stp timer max-age 2100
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Chapter 1 MSTP Configuration