[ntp:questions] Re: NTP to sync clocks on an Isolated Local Network (UTC not relevant)

Harlan Stenn stenn at ntp.isc.org
Fri Sep 1 21:15:59 UTC 2006

>>> In article <1157112288.931694.6980 at i3g2000cwc.googlegroups.com>, duncan.perrett at elekta.com writes:

duncan> are connected together but not to the wider internet.  I am ONLY
duncan> interested in synchronisation and NOT the accuracy of the network
duncan> time relative to UTC.

You may soon discover that this assumption is wrong.

duncan> Assuming all the linux boxes are identical, I just pick one box's
duncan> Local clock to be the master clock.

Be sure to pick the right box.  ntpd can handle 500ppm of "difference", and
if the machine you pick is 300ppm slow and your other machines are 300ppm
fast, ntpd will not be able to handle it.

You will also need to be Careful that you are not losing clock interrupts.

Please see http://ntp.isc.org/bin/view/Support/TroubleshootingNTP, at least
the first two entrise (known HW and OS issues).

duncan> I use NTP in the Undisciplined Local Clock mode with the chosen
duncan> linux box acting as a server broadcasting its Local clock's time.
duncan> The other boxes act as clients in broadcastclient mode.

I recommend a chain of servers at least 3 deep, increasing the stratum of
each server by 2.  Let all of your machines peer with each other.

duncan> My server ntp.conf will NOT mention a driftfile as it would be
duncan> irrelevant (?).

Possibly not a good idea - the driftfile is what lets machines sync up
faster after a restart.

duncan>  Is the client's ntp.conf line - "disable auth"
duncan> necessary?  Any errors/omissions?

duncan> server's ntp.conf file :- *************************** server
duncan> prefer # use the local system clock as the reference

I'm not certain that 'prefer' is the way to go here.  The driver1.html page
has more information on this.

If your "primary master" is at a good enough stratum, it will be picked and
followed without the 'prefer' keyword.

duncan> clock fudge stratum 10 # 10 seems to be a standard
duncan> choice for this but I think it is # irrelevant on an isolated
duncan> network (?)

If you are going to go with 3 masters, you may want to use 9, 11, and 13 as
the stratum numbers.  Using 10, 12, and 14 may caue trouble if the first 2
fail, as you would find all of your other machines at s15 with nowhere to go
below that.

duncan>  broadcast # broadcast time to local
duncan> subnet

I recommend you use peer lines to your machines, not broadcast.

duncan> clients' ntp.conf files :- ************************** disable auth #
duncan> is this correct ?  broadcastclient

See the previous comment.


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