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Adding External NTP Server Reference Clocks

SR and SC Series NTP Servers Help and Support: Adding External NTP Server Reference Clocks

Models that this article applies to:
SR Series: SR9210, SR9750, SR9850, SR9860D
SC Series: SC9205, SC9705

The SR and SC series of GPS-referenced NTP servers can utilise external NTP servers as additional reference clocks. External NTP servers can be added as either peered servers or reference servers.

NTP Peering

NTP uses peering to agree a common time between a number of NTP servers on a network. Peering allows a number of NTP servers to communicate together to provide a network with the same agreed time. Peering can be used to provide a high degree of redundancy.

Only servers with known good clocks should be peered together. The IP address or fully qualified domain name of peered NTP servers can be entered into the External Time Server field of the SR/SC Series NTP Configuration Menu.

Peering is only generally available to synchronize a number of local NTP servers. Internet based NTP servers do not generally allow peering, they must be specified as servers in their own right.

External Reference NTP Servers

If GPS and LF radio time sources are unavailable, external reference NTP servers, such as internet based NTP servers, can be configured and used to maintain time.
To configure an external NTP server that does not allow peering, you will need to use the NTP daemons ‘server’ command in the NTP configuration file. You can append commands to the NTP configuration file using the ‘Edit NTP Configuration’ option in the ‘NTP Configuration Menu’. The ‘server’ command has a single operand, the IP address or fully qualified domain name of the server to be used, see below.

server # Domain name of external NTP time server
server # IP address of external NTP time server

In the event of failure of the primary synchronization references, the backup servers will be used for synchronization. In this event the stratum of the NTP server will be one less than the stratum of the synchronization server. i.e. If the synchronization server is stratum 2, the device will become stratum 3. The NTP server will only revert to stratum 1 operation when the primary synchronization reference (GPS or LF radio) comes back on line.


The SR/SC series web page ‘Log’ tab displays the NTP daemons logging output. This will indicate the current time reference in use.

Alternatively, you can use the NTP distribution querying utility, ‘ntpq’. This is pre-installed on most Linux machines. A windows version can be downloaded from here:

The utility can be run from a command line, the format of the command is as follows:
ntpq -p x.x.x.x # where x.x.x.x is the IP address of the NTP server appliance.

You should see something like:

C:> ntpq -p

remote refid st t when poll reach delay offset jitter
LOCAL(0) .INIT. 16 l 23 64 377 0.000 0.000 0.244
SHM(0) .GPS. 0 l – 64 0 0.000 0.000 4000.00
SHM(1) .LFa. 0 l – 64 0 0.000 0.000 4000.00
* .ACTS. 1 u 70 128 377 99.302 -1.174 14.813

The asterisk indicates which reference clock that the device is synchronized to.