Configuration BPDUs sent by the regional root bridge always have a hop count set to the maximum value.
When a switch receives this configuration BPDU, it decrements the hop count by 1, and uses the new hop
count in the BPDUs that it propagates. When the hop count of a BPDU reaches 0, it is discarded by the
device that received it. Devices beyond the reach of the maximum hop can no longer participate in
spanning tree calculation, so the size of the MST region is limited.
Make this configuration on the root bridge only. All devices other than the root bridge in the MST region
use the maximum hop value set for the root bridge.
Follow these steps to configure the maximum number of hops of an MST region:
To do...
Use the command...
Remarks
Enter system view
system-view
—
Configure the maximum hops of
the MST region
stp max-hops
hops
Required
20 by default.
Configuring the network diameter of a switched network
Any two terminal devices in a switched network are connected through a specific path composed of a
series of devices. The network diameter is the number of devices on the path composed of the most
devices. The network diameter is a parameter that indicates the network size. A bigger network diameter
indicates a larger network size.
Make this configuration on the root bridge only.
Follow these steps to configure the network diameter of a switched network:
To do...
Use the command...
Remarks
Enter system view
system-view
—
Configure the network diameter of
the switched network
stp bridge-diameter
diameter
Required
7 by default.
NOTE:
•
Based on the network diameter you configured, MSTP automatically sets an optimal hello time, forward
delay, and max age for the device.
•
In MSTP mode, each MST region is considered as a device. The network diameter configuration is
effective only for the CIST (or the common root bridge), but not for MSTIs.
Configuring timers of MSTP
STP calculation involves the following timing parameters.
•
Forward delay: Determines the time interval of port state transition. To prevent temporary loops, a
port must go through an intermediate state, the learning state, before it transitions from the
discarding state to the forwarding state, and must wait a certain period of time (forward delay)
before it transitions from one state to another to keep synchronized with the remote device during
state transition.
•
Hello time: Used to detect link failures. STP sends configuration BPDUs at the interval of hello time.
If a device fails to receive configuration BPDUs within the hello time, a new spanning tree
calculation process will be triggered because of configuration BPDU timeout.
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