3Com 8800 Configuration Manual page 555

3Com Switch 8800 Configuration Guide Chapter 38 MSTP Region-configuration
Caution:
The Forward Delay configured on a switch depends on the switching network diameter.
Generally, the Forward Delay is supposed to be longer when the network diameter is
longer. Note that too short a Forward Delay may redistribute some redundant routes
temporarily, while too long a Forward Delay may prolong the network connection
resuming. The default value is recommended.
A suitable Hello Time ensures the switch to detect the link fault on the network but
occupy moderate network resources. The default value is recommended. If you set too
long a Hello Time, when there is packet dropped over a link, the switch may consider it
as a link fault and the network device will recalculate the spanning tree accordingly.
However, for too short a Hello Time, the switch frequently sends configuration BPDU,
which adds its burden and wastes the network resources.
Too short a Max Age may cause the network device frequently calculate the spanning
tree and mistake the congestion as a link fault. However, if the Max Age is too long, the
network device may not be able to discover the link fault and recalculate the spanning
tree in time, which will weaken the auto-adaptation capacity of the network. The default
value is recommended.
To avoid frequent network flapping, the values of Hello Time, Forward Delay and
Maximum Age should guarantee the following formulas.
2 x (forward-delay – 1 second) >= maximum-age
maximum-age >= 2 x (hello time + 1 second)
You are recommended to use the stp root primary command to specify the network
diameter and Hello Time of the switching network, and then MSTP will automatically
calculate and give the rather desirable values.
By default, Forward Delay is 15 seconds, Hello Time is 2 seconds, and Max Age is 20
seconds.
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