3Com Switch 4800G 24-Port Configuration Manual page 219

Configuring the Timeout
Factor
To do...
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.
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CAUTION:
The length of the forward delay time is related to the network diameter of the
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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
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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 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
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launch spanning tree calculation 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 calculation, 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) ¬¶ max age
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Max age ¬¶ 2 × (hello time + 1 second)
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We recommend that you specify the network diameter in the stp root primary
command and let MSTP automatically calculate an optimal setting of these three
timers.
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
After the network topology is stabilized, each non-root-bridge device forwards
configuration BPDUs to the surrounding devices at the interval of hello time to
check whether any link is faulty. Typically, if a device does not receive a BPDU from
Configuring the Root Bridge
Use the command...
stp timer max-age
centi-seconds
219
Remarks
Optional
2,000 centiseconds (20
seconds) by default