NTP will automatically vary its polling time based on how well your device is doing relative to the time source(s). The polling interval can be as short as 64 seconds and as long as 1024 seconds. The way to tell is to look at the value of poll in the output below which indicates how often it polls that server. As NTP sees consistent and stable time, it will gradually adjust the polling interval upward. In this example, it polls every 1024 seconds.
router#show ntp associations
address ref clock st when poll reach delay offset disp
+~98.191.213.2 .GPSD. 1 53 1024 377 0.030 0.758 1.093
+~98.191.213.12 .GPSD. 1 468 1024 377 0.003 0.701 1.052
+~98.191.213.14 .GPSD. 1 665 1024 357 0.319 0.189 1.071
*~98.191.213.7 .GPS. 1 235 1024 377 0.054 0.768 1.045
+~192.168.213.7 .GPS. 1 725 1024 377 0.060 0.947 1.040
* sys.peer, # selected, + candidate, - outlyer, x falseticker, ~ configured
In the example below (non-Cisco device), it is polling every 64 seconds:
remote refid st t when poll reach delay offset jitter
==============================================================================
127.127.46.0 .GPSD. 0 l 37 64 377 0.000 13.134 3.128
o127.127.46.128 .GPSD. 0 l 37 64 377 0.000 0.340 0.046
+98.191.213.2 .GPSD. 1 u 59 64 377 0.329 0.693 0.095
+98.191.213.12 .GPSD. 1 u 17 64 377 0.601 0.449 0.112
The when field tells you when NTP will next poll that server, in seconds.
NTP takes 8 polling cycles to settle into a semi-stable state. You can see the list of responses by issuing show ntp associations detail.
The reach field is an octal representation of those 8 queries, using a single bit for each query/response. Each poll will bit shift the reach field to the left, adding a 1 for success or a zero for a failure. So, if the server returns 8 successful values, the reach field will go 1, 3, 7, 17, 37, 77, 177 and finally 377. Since the polling interval starts at 64 seconds, it takes 8*64=512 seconds to stabilize, which is 8.53 minutes.
In the first table, you can see that it missed a poll on 98.191.213.14 because the bit field is 357. After NTP runs a while, you want to see 377 in the reach field, indicating 8 successful polls and responses. You also want to configure more than one NTP server because if you have too many missed responses, NTP will automatically throw that time server out of its algorithm and pick another one.
