138 CHAPTER 17: MSTP REGION-CONFIGURATION
Configuring the Time
Parameters of a Switch
The switch has three time parameters, Forward Delay, Hello Time, and Max Age.
Forward Delay is the switch state transition mechanism. The spanning tree will be
recalculated upon link faults and its structure will change accordingly. However,
the configuration BPDU recalculated cannot be immediately propagated
throughout the network. The temporary loops may occur if the new root port and
designated port forward data right after being elected. Therefore the protocol
adopts a state transition mechanism. It takes a Forward Delay interval for the root
port and designated port to transit from the learning state to forwarding state.
The Forward Delay guarantees a period of time during which the new
configuration BPDU can be propagated throughout the network.
The switch sends Hello packet periodically at an interval specified by Hello Time to
check if there is any link fault.
Max Age specifies when the configuration BPDU will expire. The switch will
discard the expired configuration BPDU.
You can use the following command to configure the time parameters for the
switch.
Perform the following configuration in system view.
Every switch on the switching network adopts the values of the time parameters
configured on the root bridge of the CIST.
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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.
Tabl e 113 Configure the time parameters of a switch
Operation Command
Configure Forward Delay on the switch stp timer forward-delay centi-seconds
Restore the default Forward Delay of the
switch
undo stp timer forward-delay
Configure Hello Time on the switch stp timer hello centi-seconds
Restore the default Hello Time on the switch undo stp timer hello
Configure Max Age on the switch stp timer max-age centi-seconds
Restore the default Max Age on the switch undo stp timer max-age
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