[c-nsp] Link State Periodic SPF

[Oliver Boehmer (oboehmer)]

On 11/01/2013 03:02, "Dhamija Amit" <amiitdhamija at yahoo.com> wrote:

>Hi 
>
>I would like to know about Link state Periodic SPF's , I have seen in
>Cisco IOS-XR and Juniper M/T -Series ( Junos) by default Periodic SPF
>runs every 15 minutes.
>
>Going to concept of SPF whenever there is change in topology router runs
>SPF based on SPF interval configured. Also when LSP/LSA refreshes after
>certain period then during that time also router executes SPF's.Then what
>is the purpose of periodic SPF's . Can we disable the same , What kind of
>impact will be there ??

ISIS standard (7.3.18) asks for at least one SPF every
maximumLSPGenerationInterval (which is, by default 15 mins). On today's
routers, SPF takes a few msecs, so there is not concern running this every
15 min.. XR allows to disable the period SPF (privileged/cisco-support
"spf periodic disable" command in ISIS AF mode), but I don't see a reason
why someone would need it.
As far as I know, a refreshed LSP does NOT trigger an SPF unless it
contains changed topology information.

	oli