Configuring The Timeout Factor - 3Com 4500G Family Configuration Manual

To do...
Configure the max age timer
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 bridge-diameter command and let
MSTP automatically calculate optimal settings of these three timers based on the network diameter.

Configuring the Timeout Factor

The timeout factor is a parameter used to decide the timeout time, as shown in the following formula:
Timeout time = timeout factor × 3 × hello time.
After the network topology is stabilized, each non-root-bridge device forwards configuration BPDUs to
the downstream devices at the interval of hello time to check whether any link is faulty. Typically, if a
device does not receive a BPDU from the upstream device within nine times the hello time, it assumes
that the upstream device has failed and starts a new spanning tree calculation process.
Sometimes a device may fail to receive a BPDU from the upstream device because the upstream
device is busy. A spanning tree calculation that occurs in this case not only is unnecessary, but also
wastes the network resources. In a very stable network, you can avoid such unwanted spanning tree
calculations by setting the timeout factor to 5, 6, or 7.
Follow these steps to configure the timeout factor:
Use the command...
stp timer max-age time
1-22
Remarks
Optional
2,000 centiseconds (20
seconds) by default