[ntp:questions] Two time sources with offset => fails to synchronize

Martin Burnicki martin.burnicki at meinberg.de
Tue Feb 12 10:19:10 UTC 2008

Hi Thierry,

Thierry MARTIN wrote:
> Hi all,
> I am running ntp 4.2.0 on FC5.
> Two time sources:
> - embedded GPS board(Meinberg board)
> - remote Radio clock synchronized ntp server (stratum 1)
> It seems that the Radio clock has 50ms offset compared to the GPS
> source, which leads a failure in synchronisation.
> I am looking for some documentation that explain why it fails.
> Which parameter(s) in the configuration triggers that behaviour?

2 time sources is the worst configuration you can have. If they don't agree
on the "right" time, ntpd (or even you) don't know whom to trust.

So 3 or 4 time sources would be basically a better choice. However, if you
have configured a local GPS clock and 3 external servers then the external
servers could "overvote" the GPS clock, i.e. if the 3 external servers
agree about a time which differs from the GPS time then the GPS clock might
be marked as falseticker.

This can easily happen if all the external NTP servers are reachable via an
ADSL line, in which case the packet propagation delay differs for the
transmitted and received packets, which results in a seemingly time offset
of a few milliseconds which is the same for all external servers.

> I used the "prefer" keyword in the GPS configuration line, with the same
> result. Is there a way to make it work better?

Select a set of time sources which agree on the same correct time. For some
hints how the "best" time source is determined by ntpd, see:

Martin Burnicki

Meinberg Funkuhren
Bad Pyrmont

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