[ntp:questions] Re: Cannot synchronize to server with local clock

David Woolley david at djwhome.demon.co.uk
Thu Jul 21 06:41:49 UTC 2005


In article <slrnddtjqe.4fk.serge.bets at laposte.net>,
Serge Bets <serge.bets at NOSPAM.laposte.invalid> wrote:

> So you removed LOCAL()? Without sync machines will diverge. Perhaps

This includes the machine with local clock as reference, so the
result is all diverge in a similar way, relative to true time.

> enough to take longer to sync during short online sessions. Or worse

Firstly, NTP is not intended to be used with intermittent connectivity.

In most cases, the delays will be very similar as the lowest stratum
machine will still have to lock onto the valid server before the 
others can start locking onto it.

> continue free wheeling but on a this anormal frequency. Finally a client
> (re)booted will have false time until next dialup, if ntpd has not died
> before on "no servers reachable". Not good.

This is a bit of a problem, but is what hardware clocks are for.

> My advice for you would be to set a 127.127.1.0 stratum 8 prefer on your
> server, and another one stratum 14 (but not prefer) on a given client.

Normal guideline is stratum 10, but local clock should always be a
conscious decision.  I don't believe it should be prefer here.  If
you include a local clock, you ensure you have enough real servers to
outvote it in the intersection algorithm.

Addition of a local clock here breaks a safety feature in NTP.  If that
safety feature weren't important, NTP would not drop to stratum 16 on
losing its server.

Local clock on a leaf node is never a good idea, as it serves no useful
purpose, but can confuse the the intersection algorithm.  A leaf node
will coast at exactly the same frequency with and without a local clock
configured.

> This way you have eternal sync, only one sync source at a time, and you
> can shutdown server *or* given client for maintenance.

You can, and probably should have, multiple local servers, but, in any
case, the client will coast through just the same with or without
a local clock configured.

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