R21xx-HP FlexFabric 11900 Layer 2 LAN Switching Configuration Guide

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.

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 takes effect only on the CIST (or MSTI 0) 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 (or MSTI 0).

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)

HP recommends not manually setting the spanning tree timers. HP 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.