[ntp:questions] Routers and NTP Timing loops

Maarten Wiltink maarten at kittensandcats.net
Wed Jun 16 14:16:36 UTC 2010

"Jacobs, Kevin J." <kjacobs at mitre.org> wrote in message
news:70189427BD8CE046B781F28D29C21AEE0C9BD78EA6 at IMCMBX4.MITRE.ORG...

> I  had an interesting experience with a Cisco router as "ntp master 9"
> using itself as an NTP source and it's neighboring switch as the only
> other  NTP source.

> The switch had the exact same configuration right back at the router!.
> So what happens? Is NTP smart enough to know that this is a bad idea?
> If not, what goofy things happen if you establish an NTP timing loop?
> I know that if this were frequency references and stratum 3 clocks,
> they would ultimately peg at the end of their frequency range (not good)
> but I don't know what happens from a time perspective. I had some
> interesting date-time issues and this is a potential root cause. Just
> looking to see if anyone has had similar experience and have done some
> in-depth research in this area.

The reference implementation gets a refid with every association to
prevent exactly this. If the refid of a server is recognised as that of
the client itself, the server is not considered a viable time source.

I don't think it breaks cycles of three or more machines.

Maarten Wiltink

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