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"clock save interval" seems to break ntp

Someone asked me how close to "reality" ntp would keep a router clock and I didn't know so started graphing the "offset" on an 877 running 15.1(4)M6.

I enclose the first few days worth of this as "ntp-week.png" and just the last 24 hours in "ntp-day.png".

As one can see the offset is mostly a few tens of milliseconds but there are spikes up to 1.4s from time to time. After a few days I noticed that when these happened, they seemed to be 8 hours apart or sometimes multiples of 8 hours. I later discovered that some of the spikes are less than 5 minutes long so MRTG never sees them and in fact they seem to be every 8 hours as far as I can tell.

It occurred to me that I had the command "clock save interval 8" in the config, as seems to be recommended when using ntp on a device with no battery-backed clock. So I removed it, and since then the "offset" has never gone over 6ms.

Has anyone else experienced "clock save interval" apparently causing problems for ntp like this?

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