[ntp:questions] Routers and NTP Timing loops
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.
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