NTP uses a client-server model with time messages sent via UDP on port 123. The current protocol is version 4 (NTPv4), which is a proposed standard as documented in RFC 5905.
NTP uses a hierarchical, semi-layered system of time sources. Each level of this hierarchy is termed a "stratum" and is assigned a number starting with zero at the top.
Stratum 0These are high-precision timekeeping devices such as atomic (cesium, rubidium) clocks, GPS clocks or other radio clocks.
Generally it is considered best practice for a client to poll three or more servers and determine the best source of time messages based on returned timestamps.
The Wireless LAN Controller (WLC) requires setting the system time. Once the time is set it will be maintained until a system reset. Using NTP to manage the time on the WLC is preferable. For lightweight APs the time is synchronised to the WLC time. For autonomous APs the time can be set manually.
How to configure time on Autonomous APs
How to configure NTP and SNTP on the Cisco AP