[ntp:questions] 2xGPS and 4x peers setup - disturbed by 5th and 6th peer

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Mon Nov 3 22:00:21 UTC 2014


Martin Burnicki wrote:
> We have had a case where a customer had one local
> computing center and 2 ones at different remote locations.
> In each of the computing centers were 2 GPS
> controlled LANTIME NTP servers installed.
>
> In the local computing center there was also a Linux server
> running ntpd which had all 6 LANTIMEs configured as time sources.
>
> Unfortunately the internet connection of the local
> computing center seemed to have an asymmetry in the
> packet delay, so from the Linux client's point of view
> all 4 LANTIMEs in the remote locations seemed to have a
> time offset in the same range, a few milliseconds,
>  compared to the 2 local LANTIME.
>
> Even though all the 4 remote servers showed much more
>  jitter due to the long network path they were preferred
>  by the Linux client, and the 2 local LANTIMEs were marked
>  as "falsetickers" even though they showed much less jitter.
>
> If I remember correctly then the Linux client was
>  running 4.2.6p?, and a test with a -dev version of ntpd
>  showed that the newer ntpd preferred the 2 local
>  LANTIMEs over the 4 remote ones. This seems to indicate
>  that the weight put on different criteria
>  in the selection algorithm has changed over versions,
>  and the newer versions of ntpd act more like you'd expect.

TOS MinDist affects this.

e.g. in the case of serial nema and pps, sometimes the mindist
 needs to be increased from 1ms to perhaps 20ms,
 and I've seen as much as 400ms (fairly often);
 {which makes me wonder if the PPS is inverted?}

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