Specifying an Authentication Key for NTP on C Series Controllers (C-Web Interface)
Only time servers transmitting network time packets that contain one of the specified key numbers and whose key matches the value configured for that key number are eligible for synchronization. Other systems can synchronize with the local system without being authenticated.
To configure an NTP authentication key that is shared among systems that run NTP:
Click Configure, expand System, and then click NTP.
The NTP pane appears.
In the Create new list, select Authentication Key. Type the key number for the key. The key number must match on all systems using that particular key for authentication. Click OK.