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

Operation Manual – STP
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. Generally, if the switch does not receive the STP packets from the upstream
switch for 3 times of hello time, the switch will decide the upstream switch is dead and
will recalculate the topology of the network. Then, in a steady network, the recalculation
may be caused when the upstream is busy. In this case, user can redefine the timeout
interval to a longer time to avoid this kind of meaningless recalculation.
Chapter 1 MSTP Region-configuration
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