Configuring Spanning Tree Timers; Configuration Restrictions And Guidelines; Configuration Procedure - HP 6125G Configuration Manual

Configuring spanning tree timers

The following timers are used for spanning tree calculation:
Forward delay
•
It is the delay time for port state transition. To prevent temporary loops on a network, the spanning
tree sets an intermediate port state, the learning state, before it transitions from the discarding state
to the forwarding state, and requires that the port transitions its state after a forward delay timer to
make sure that the state transition of the local port keeps synchronized with the peer.
• Hello time
The device detects whether a link failure has occurred with the hello time interval. The spanning
tree sends a configuration BPDU every hello time interval. 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 parameter to determine whether a
configuration BPDU received by a port has expired. If a port receives a configuration BPDU that
has expired, that MSTI must be re-calculated. The max age timer is ineffective for MSTIs.
To avoid frequent network changes, be sure that the settings of the hello time, forward delay and
max age timers meet the following formulas:
2 × (forward delay – 1 second) ≥ max age
Max age ≥ 2 × (hello time + 1 second)
HP does not recommend you to manually set the spanning tree timers. Instead, you can specify the
network diameter and let 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 on the root bridge only, and 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. HP recommends you to use the default setting.
An appropriate hello time setting enables the device to quickly detect link failures on the network
•
without using excessive network resources. If the hello time is too long, the device will mistake
packet loss as a link failure and trigger a new spanning tree calculation process. If the hello time is
too short, the device will frequently send the same configuration BPDUs, which adds the device
burden and wastes network resources. HP recommends you to use the default setting.
• If the max age timer is too short, the device will frequently begin spanning tree calculation and
might mistake network congestion as a link failure. If the max age timer is too long, the device might
fail to quickly detect link failures and begin spanning tree calculations, reducing the auto-sensing
capability of the network. HP recommends you to use the default setting.

Configuration procedure

To configure the spanning tree timers:
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