Configuring MSTP 195

Configuring the Time Parameters of a Switch

The switch has three time parameters:

Forward delay

Hello time

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. The configuration BPDU recalculated cannot be immediately propagated throughout the network. Temporary loops can 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 a hello packet periodically to check if there is any link fault. The interval in which the hello packet is sent is specified by the hello timer.

Max age specifies when the configuration BPDU expires. 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.

Table 197 Configure the Time Parameters of a Switch

Operation

Command

 

 

Configure Forward Delay on the switch.

stp timer forward-delay

 

centiseconds

Restore the default Forward Delay of the

undo stp timer forward-delay

switch.

 

Configure Hello Time on the switch.

stp timer hello centiseconds

Restore the default Hello Time on the switch.

undo stp timer hello

Configure Max Age on the switch.

stp timer max-age centiseconds

Restore the default Max Age on the switch.

undo stp timer max-age

 

 

Every switch on the switching network adopts the values of the time parameters configured on the root switch of the CIST.

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 a forward delay that is too short can redistribute some redundant routes temporarily, while a forward delay that is too long can prolong the network connection resuming. The default value is recommended.

A suitable hello time ensures that the switch can detect the link fault on the network, but also occupy moderate network resources. The default value is recommended. If you set a hello time that is too long, when there is packet dropped over a link, the switch may consider it as link fault and the network device will recalculate the spanning tree accordingly. However, for a hello time

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