H3C Technologies H3C S12500-X Series Switches - Manual (Page 91)

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devices. The network diameter is a parameter that indicates the network size. A bigger network diameter
indicates a larger network size.
Based on the network diameter you configured, the system automatically sets an optimal hello time,
forward delay, and max age for the device. Each MST region is considered a device and the configured
network diameter is effective only on the CIST (or the common root bridge) but not on other MSTIs.
To configure the network diameter of a switched network:
Step Command Remarks
1. Enter system view. system-view N/A
2. Configure the network
diameter of the switched
network. stp bridge-diameter diameter The default setting is 7.
Configuring spanning tree timers
The following timers are used for spanning tree calculation:
• Forward delay—Delay time for port state transition. To prevent temporary loops on a network, the
spanning tree feature sets an intermediate port state (the learning state) before it transits from the
discarding state to the forwarding state. The feature also requires that the port transit its state after
a forward delay timer to make sure the state transition of the local port stays synchronized with the
peer.
• Hello time—Interval at which the device sends configuration BPDUs to detect link failures. If the
device receives no configuration BPDUs within the hello time interval, it recalculates the spanning
tree.
• Max age—In the CIST of an MSTP network, the device uses the max age timer to determine if a
configuration BPDU received by a port has expired. If it has, a new spanning tree calculation
process starts. The max age timer does not take effect on other MSTIs except the CIST.
To avoid frequent network changes, make sure the timer settings meet the following formulas:
• 2 × (forward delay – 1 second) ≥ max age
• Max age ≥ 2 × (hello time + 1 second)
H3C recommends not manually setting the spanning tree timers. H3C recommends specifying the
network diameter and letting spanning tree protocols automatically calculate the timers based on the
network diameter. If the network diameter uses the default value, the timers also use their default values.
Configure the timers only on the root bridge. The timer settings on the root bridge apply to all devices on
the entire switched network.
Configuration restrictions and guidelines
• The length of the forward delay timer is related to the network diameter of the switched network. The
larger the network diameter is, the longer the forward delay time should be. If the forward delay
timer is too short, temporary redundant paths might occur. If the forward delay timer is too long,
network convergence might take a long time. H3C recommends using the default setting.
• An appropriate hello time setting enables the device to promptly detect link failures on the network
without using excessive network resources. If the hello time is too long, the device mistakes packet