Based on the network diameter you configured, MSTP automatically sets an optimal hello time,
forward delay, and max age for the device.
The configured network diameter is effective for the CIST only, and not for MSTIs. Each MST
region is considered as a device.
The network diameter must be configured on the root bridge. Otherwise, it will not take effect.
Configuring Timers of MSTP
MSTP involves three timers: forward delay, hello time and max age. You can configure these three
parameters for MSTP to calculate spanning trees.
To prevent temporary loops on a network, MSTP sets an intermediate port state called learning
between the discarding state and the forwarding state, that is, before a port in the discarding state
can transit to the forwarding state, it needs to go through the learning state. Forward delay is the
delay time for port state transition. This is to ensure that the state transition of the local port and that
of the peer occur in a synchronized manner.
Hello time is the time interval at which a device sends configuration BPDUs to the surrounding
devices to ensure that the paths are fault-free. If a device fails to receive configuration BPDUs
within a certain period of time, it starts a new spanning tree calculation process.
MSTP can detect link failures and automatically restore blocked redundant links to the forwarding
state. A device on the CIST determines whether a configuration BPDU received by a port has
expired according to the max age parameter. If yes, it starts a new spanning tree calculation
process. The max age set for an MSTI does not take effect.
These three timers set on the root bridge of the CIST apply on all the devices on the entire switched
Make this configuration on the root bridge only.
Follow these steps to configure the timers of MSTP:
Enter system view
Configure the forward delay
Configure the hello timer
Use the command...
stp timer forward-delay time
stp timer hello time
1,500 centiseconds (15
seconds) by default
200 centiseconds (2 seconds)