SPF calculations occur at the interval set by the timers throttle spfcommand. The wait interval indicates the amount of time to wait until the next SPF calculation occurs. Each wait interval after that calculation is twice as long as the previous one until the wait interval reaches the maximum wait time specified.
The SPF timing can be better explained using an example. In this example the start interval is set at 5 milliseconds (ms), the wait interval at 1000 milliseconds, and the maximum wait time is set at 90,000 milliseconds.
timers throttle spf 5 1000 90000
The figure below shows the intervals at which the SPF calculations occur so long as at least one topology change event is received in a given wait interval.
Figure 1. SPF Calculation Intervals Set by the timers throttle spf Command
Notice that the wait interval between SPF calculations doubles when at least one topology change event is received during the previous wait interval. Once the maximum wait time is reached, the wait interval remains the same until the topology stabilizes and no event is received in that interval.
If the first topology change event is received after the current wait interval, the SPF calculation is delayed by the amount of time specified as the start interval. The subsequent wait intervals continue to follow the dynamic pattern.
If the first topology change event occurs after the maximum wait interval begins, the SPF calculation is again scheduled at the start interval and subsequent wait intervals are reset according the parameters specified in the timers throttle spfcommand. Notice in the figure below that a topology change event was received after the start of the maximum wait time interval and that the SPF intervals have been reset.
Figure 2. Timer Intervals Reset After a Topology Change Event