[ntp:questions] Toggling time sync between two servers

vwvr6vw brian.l.hostetler at lmco.com
Sat Mar 25 18:41:54 UTC 2006

I'm new to ntp, but have been asked to debug an issue with our NTP
configuration.  First, here is our setup.  We have two processors which
each have a third party application reading time off a serial line from
two different sources and then disciplining the internal clock on each
processor with the "adjtime" system call.  xntpd runs on each of these
processors with only internal time as a source.  One processor is
fudged at stratum 0 and the other at stratum 2.  Currently, xntp is
configured with "enable pll".  My first change will be to change
"enable pll" to "diable pll" as I believe that we shouldn't have two
different applications(our third party app and xntp) discipling the
internal clock at the same time.

These two processors act as the NTP servers to all other processors on
our network.  I've read here that having two servers is a bad situation
and that it is better to have four, but that just about
anything(including just 1) is better than having two.  I believe we our
finding that out first hand, so I work to resolve that as well.

When I checked the ntp logs for some client processors on our network,
I see that occasionaly there are large(upto 1 second) differences
between our two servers.  On top of this, the clients can not decide
which one to sync with and we get stuck in a loop as follows.

restart min polling
step time/sync to server1 at stratum 0
restart min polling due to step
step time/sync to server2 at stratum 2

This exact sequence repeats over and over on appx 5 minute intervals
due to the min polling time.  I realize that I have many problems here,
but I would like to know why ntp toggles between syncing/stepping to
server1 and server2 after each polling period.  I would have thought
that the resultant action would be the same after each polling period.
But instead it is consistent that the client steps time to the opposite
server that it stepped to last time, like so.

25 Mar 16:36:58 xntpd[7752]: time reset (step) 0.421856 s
25 Mar 16:41:55 xntpd[7752]: synchronized to, stratum=1
25 Mar 16:41:54 xntpd[7752]: time reset (step) -0.421930 s
25 Mar 16:46:33 xntpd[7752]: synchronized to, stratum=3
25 Mar 16:46:34 xntpd[7752]: time reset (step) 0.419369 s
25 Mar 16:51:31 xntpd[7752]: synchronized to, stratum=1
25 Mar 16:51:30 xntpd[7752]: time reset (step) -0.419446 s
25 Mar 16:56:09 xntpd[7752]: synchronized to, stratum=3

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