Setting The Timeout Factor Of A Specific Bridge - H3C S9500 Series Operation Manual

Operation Manual – MSTP
H3C S9500 Series Routing Switches
Every switch on the switching network adopts the values of the time parameters
configured on the root bridge of the CIST.
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 Max
Age should guarantee the following formulas.
2 x (Forward-Delay – 1 second) >= Max Age
Max Age >= 2 x (Hello Time + 1 second)
You are recommended to use the stp bridge-diameter 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.

1.2.8 Setting the Timeout Factor of a Specific Bridge

A switch transmits hello packet regularly to the adjacent bridges to check if there is link
failure. Typically, if the switch does not receive the STP packets from the upstream
switch during 3 times the hello time, the switch considers that the upstream switch is
dead and initiates a new spanning tree calculation process. In a topologically stable
network, this kind of spanning tree recalculation may take place when the upstream is
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Chapter 1 MSTP Configuration