NTP timing based on GPS clock

Configure NTP using GPS time

You can configure the GPS time as the reference clock for NTP using the command ntp refclock gps.

The GPS time acts as a stratum 0 source, and the Cisco IOS NTP server acts as a stratum 1 device, which in turn provides clock information to its NTP clients (stratum 2 and 3).

Procedure

Step 1

Use the enable command to enable the router and enter the privileged EXEC mode.

Example:


						Router> 
						enable

Step 2

Use the configure terminal command to enter the global configuration mode.

Example:


                        Router# 
                        configure terminal

Step 3

Use the ntp refclock gps command to configure the NTP reference clock as GPS:

Example:


                        Router(config)#
                        ntp refclock gps

Step 4

Use the ntp refclock gps command to verify the configuration.

Example:


                        Router#
                        Sep 24 19:58:43.046 GMT: %PKI-6-AUTHORITATIVE_CLOCK: The system clock has been set.
                        Router#
                        show ntp status
                        Clock is synchronized, stratum 1, reference is .GPS.
                        nominal freq is 250.0000 Hz, actual freq is 249.9970 Hz, precision is 2**10
                        ntp uptime is 94000 (1/100 of seconds), resolution is 4016
                        reference time is E31778F3.0B851ED8 (19:58:43.045 GMT Thu Sep 24 2020)
                        clock offset is 11.0000 msec, root delay is 0.00 msec
                        root dispersion is 3950.55 msec, peer dispersion is 3938.47 msec
                        loopfilter state is 'CTRL' (Normal Controlled Loop), drift is 0.000011995 s/s
                        system poll interval is 64, last update was 7 sec ago.
                        Router#
                        Router#
                        Router#
                        show ntp associations
                        address ref clock st when poll reach delay offset disp
                        *~127.127.5.1 .GPS. 0 38 64 7 0.000 11.000 1938.8
                        * sys.peer, # selected, + candidate, - outlyer, x falseticker, ~ configured
                        Router#
                        Router#
                        show clock
                        20:00:43.660 GMT Thu Sep 24 2020
                        Router#

Step 5

Use the debug ntp refclock command to troubleshoot the configuration.

Example:


                        Router#
                        debug ntp ?
                        adjust NTP clock adjustments
                        all NTP all debugging on
                        core NTP core messages
                        events NTP events
                        packet NTP packet debugging
                        refclock NTP refclock messages
                        Router#
                        debug ntp re
                        Router#debug ntp refclock
                        *Sep 24 19:58:43.045 GMT: GPS: Poll Requested
                        *Sep 24 19:58:43.045 GMT: GPS (19:58:43.056 GMT Thu Sep 24 2020)
                        *Sep 24 19:58:43.045 GMT: Valid time rcvd from GPS: 2020/09/24 19:58:43.056 (frac = 0x0E560440)
                        *Sep 24 19:58:43.045 GMT: RTS poll timestamp (local clock) was 0xE31778F3.0B851ED8
                        *Sep 24 19:58:43.045 GMT: GPS timestamp is 0xE31778F3.0E560440
                        *Sep 24 19:58:43.045 GMT: NTP Core(NOTICE): ntpd PPM
                        *Sep 24 19:58:43.046 GMT: NTP Core(NOTICE): trans state : 5
                        *Sep 24 19:58:43.046 GMT: NTP Core(NOTICE): Clock is synchronized.