83The device detects whether a link failure has occurred with the hello time interval. The spanningtree sends a configuration BPDU every hello time interval. If the device receives no configurationBPDUs within the hello time interval, it recalculates the spanning tree.• Max ageIn the CIST of an MSTP network or each VLAN of a PVST network, the device uses the max ageparameter to determine whether a configuration BPDU received by a port has expired. If a portreceives a configuration BPDU that has expired, that MSTI must be re-calculated. The max agetimer does not take effect on MSTIs.To avoid frequent network changes, be sure that the settings of the hello time, forward delay andmax age timers meet the following formulas:{ 2 × (forward delay – 1 second) ≥ max age{ Max age ≥ 2 × (hello time + 1 second)H3C does not recommend you to manually set the spanning tree timers. Instead, you can specify thenetwork diameter and let spanning tree protocols automatically calculate the timers based on thenetwork 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 deviceson 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. Thelarger the network diameter is, the longer the forward delay time should be. If the forward delaytimer 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 you to use the default setting.• An appropriate hello time setting enables the device to quickly detect link failures on the networkwithout using excessive network resources. If the hello time is too long, the device will mistakepacket loss as a link failure and trigger a new spanning tree calculation process. If the hello time istoo short, the device will frequently send the same configuration BPDUs, which adds the deviceburden and wastes network resources. H3C recommends you to use the default setting.• If the max age timer is too short, the device will frequently begin spanning tree calculation andmight mistake network congestion as a link failure. If the max age timer is too long, the device mightfail to quickly detect link failures and begin spanning tree calculations, reducing the auto-sensingcapability of the network. H3C recommends you to use the default setting.Configuration procedureTo configure the spanning tree timers:Step Command Remarks1. Enter system view. system-view N/A2. Configure the forwarddelay timer.• In STP/RSTP/MSTP mode:stp timer forward-delay time• In PVST mode:stp vlan vlan-list timer forward-delaytimeOptional.Use one of the commands.The default setting is 15seconds.
