Cisco IOS Cookbook, 2nd Edition

Changing NTP Synchronization Periods

Problem

You want to adjust how often routers send NTP packets to verify clock synchronization.

Solution

NTP does not allow poll rates to be changed manually; however, there are algorithms that adaptively shape this parameter within the NTP protocol.

Discussion

NTP is an extremely efficient protocol that actively monitors all aspects of network timing to adjust its configuration accordingly. Upon initialization of NTP on a router, it sets its poll cycle to about once every 64 seconds. As the local clock becomes synchronized and stable, the router will adaptively back off the poll cycle to a maximum of 1,024 seconds (roughly 17 minutes):

Router>show ntp associations

      address         ref clock     st  when  poll reach  delay  offset    disp
*~172.25.1.1       130.207.244.240   2   440  1024  377     1.6   -3.23     5.6
+~172.25.1.3       204.152.184.72    2   829  1024  377     1.7    8.06     0.9
* master (synced), # master (unsynced), + selected, - candidate, ~ configured
Router>

Notice that the poll cycle for the two configured servers above has been throttled back to 1024 seconds. This indicates that the router is in a stable network and the time services are consistently accurate. This also illustrates NTP’s remarkable efficiency.

See Also

Recipe 14.5