[ntp:questions] NTP servers redundancy

Steve Kostecke kostecke at ntp.org
Tue Jan 19 20:36:11 UTC 2010

On 2010-01-18, jthu at free.fr <jthu at free.fr> wrote:

> for a ship project, we've got a LAN with two NTP servers synchronised
> by a masterclock, itself synchronized by GPS. The idea was to have
> redundancy between the 2 servers, to ensure continuous synchronization
> to the clients in case one of the server went down.
> I wonder now if the dual NTP source architecture is the good one :
> Is the "clock hopping" resulting of this dual architecture a real
> problem ??

That depends on your acceptable error margin and whether or not you are
able to maintain your servers' relative offsets within this margin.

> Does the NTP algorithm enable that when a server go down (or is no
> more synchronized), its clients switch easily to the second one,
> without any visible synchronization interruption ??

If both servers are within 128ms of each other, the clients will slew
toward the time seen from the new time server. If the server are more
than 128ms "apart", the clients will step to the time seen from the new
time server.

> Will it be "transparent" for the clients when the first server will be
> back ?

Chances are the clients will continue to follow the "second" server
after the "first" server is reachable.

Steve Kostecke <kostecke at ntp.org>
NTP Public Services Project - http://support.ntp.org/

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