OSPF SPF Scheduling and throttling timers

The OSPF SPF scheduling and throttling feature is used to keep an OSPF topology stable in the event that there is a continuous inflow of LSAs, causing continuous recalculation of the SPF algorithm. This feature uses the following timers and parameters:

  • The spf-start value is the the initial time interval to wait before running the SPF algorithm upon recipt of the first updated LSA.
  • The spf-hold value is the initial amount of time to wait between one SPF calculation and the next. Every time a new LSA is received before sfp-hold has elapsed, this value doubles, so it waits longer before running the SPF calculation again. You can see this clearly in the diagrams in the lesson.
  • Now imagine that you keep getting LSAs all the time. This spf-hold value will continue to double. It could potentially continue to double forever, reaching values of several minutes or hours!! This doubling must have a limit, and that's where the spf-max-wait value comes in. The spf-hold value, as it doubles, will hit a maximum of spf-max-wait, and will not double further, but will remain at that maximum. The original spf-hold value is reset only if spf-max-wait has elapsed without any new incoming LSAs.

In addition to throttling, it is also possible to configure OSPF update pacing timers.