OSPF SPF Scheduling and Throttling

This topic is to discuss the following lesson:

Hello Rene,
I have a question. I see that after the 2xspfhold and then second spf start there is a gap of 5 seconds(35 to 40). Could you please explain as to why is that so?

Hi Ravi,

There is no particular reason, the time from 35 to 40 seconds ( 5 seconds in total) is a period of inactivity, I made up that time.

Rene

i could not understand the SPF hold and spf-max-wait timer …??

Can u please help me with it ??

Hello Narad

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.

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.

I hope this has been helpful!

Laz