|Re: [chrony-users] Chrony source selection algorithm documentation|
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On Thu, Mar 10, 2016 at 07:58:09PM +0100, Henrik Bieler wrote:
> I still would like to know a way of preferring the ntpd over the gps
> when they are online. And switching back to gps when they go offline.
> Maybe someone can explain how chrony selects sources or point to
What stratum do the NTP servers have? My guess is that the NTP sources
are considered as unsynchronized for having stratum larger than the
local clock. This is a requirement of the NTPv3 protocol which
prevents synchronization loops (see "test 7" in RFC 1305). It was
later removed in NTPv4 (implemented in chrony >= 2.0), which made it
more vulnerable to synchronization loops, but allows synchronization
to sources with larger stratum.
In your case, if chronyd (version 1.30) selected the GPS, it is
stratum 1 and will ignore sources with stratum 2 or larger for a
possibility they are directly or indirectly synchronized to it. You
can check the measurements log to see if TEST7 is failing.
In order to outvote the GPS source as a falseticker you would need at
least two stratum 1 servers. Considering that less than 10% of the
pool.ntp.org servers are stratum 1, I think it's quite unlikely to
happen. My suggestion would be to use two or three stratum 1 servers
with open access from the list of servers maintained at the NTP
website instead of the pool.
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