Setting Values of VBST Timers
Context
- Forward Delay: determines the interval for port status transition. On a network where a spanning tree algorithm is used, when the network topology changes, new BPDUs are transmitted throughout the network after a given period of time. During the period, the port that should enter the blocking state may be not blocked and the originally blocked port may be unblocked, causing temporary loops. To address this problem, set the Forward Delay during which all ports are blocked temporarily.
- Hello Time: is the interval at which Hello packets are sent. The switch sends BPDUs to neighboring devices at an interval of the Hello Time to check whether links are faulty. If the switch does not receive any BPDU within the timeout period (timeout period = Hello Time x 3 x Timer Factor), the switch recalculates the spanning tree due to BPDU timeout.
- Max Age: determines whether BPDUs expire. The switch determines whether the received BPDU expires based on this value. If the received BPDU expires, the spanning tree needs to be recalculated.
Devices on a ring network must use the same values of Forward Delay, Hello Time, and Max Age.
Generally, you are not advised to adjust values of the three parameters. This is because the three parameters are relevant to the network scale. It is recommended that the network diameter be adjusted so that the spanning tree protocol automatically adjusts the three parameters. When the default network diameter is used, the default values of the three parameters are used.
To prevent frequent network flapping, ensure that the values of Hello Time, Forward Delay, and Max Age conform to the following formulas:
2 x (Forward Delay - 1.0 second) ≥ Max Age
Max Age ≥ 2 x (Hello Time + 1.0 second)
Procedure
- Run system-view
The system view is displayed.
- Set values of Hello Time, Forward Delay,
and Max Age.
Run stp vlan { vlan-id1 [ to vlan-id2 ] } &<1-10> timer forward-delay forward-delay
The value of Forward Delay is set.
By default, the value of Forward Delay is 1500 centiseconds.
Run stp vlan { vlan-id1 [ to vlan-id2 ] } &<1-10> timer hello hello-time
The value of Hello Time is set.
By default, the value of Hello Time is 200 centiseconds.
Run stp vlan { vlan-id1 [ to vlan-id2 ] } &<1-10> timer max-age max-age
The value of Max Age is set.
By default, the value of Max Age is 2000 centiseconds.