H3C S5120-SI Series Operation Manual page 162

Configuration procedure
Follow these steps to configure the timers of MSTP:
To do...
Enter system view
Configure the forward delay
timer
Configure the hello timer
Configure the max age timer
These three timers set on the root bridge of the CIST apply on all the devices on the entire switched
network.
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 converge. 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 as a link failure and trigger a new spanning tree calculation 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
calculations and may take network congestion as 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 calculations,
thus reducing the auto-sensing capability of the network. We recommend that you use the default
setting.
The settings 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
Max age ú 2 × (hello time + 1 second)
We recommend that you specify the network diameter with the stp root primary command and let
MSTP automatically calculate optimal settings of these three timers.
Use the command...
system-view
stp timer forward-delay
centi-seconds
stp timer hello centi-seconds
stp timer max-age
centi-seconds
1-22
Remarks
—
Optional
1,500 centiseconds (15
seconds) by default
Optional
200 centiseconds (2 seconds)
by default
Optional
2,000 centiseconds (20
seconds) by default