[ntp:questions] Ignore one server except in extreme conditions?

A C agcarver+ntp at acarver.net
Wed Nov 16 05:47:10 UTC 2011

Is there a way to configure ntpd to ignore a particular clock unless 
there is no other choice?  What I'm thinking is to ignore the GPS 
receiver NMEA data and use only the PPS plus Internet servers for most 
of the synchronization.  But if the Internet servers go down, accept the 
GPS NMEA data for seconds numbering and let the tick be controlled by PPS.

I guess I'm asking for something opposite to the "prefer" flag, a "not 
unless you have to" flag.  The GPS does get selected among the various 
clock sources but it's the worst of the bunch with respect to jitter and 
offset.  Those two wander all over while the rest of the Internet 
servers and the PPS are stable.  It is almost never rejected as an 
outlier or false ticker even if it's way out of line with the other sources.

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