6.3.b Implement and troubleshoot network time protocol

6.3.b [ii] NTP Authentication

You can configure the device to authenticate the time sources to which the local clock is synchronized . When you enable NTP authentication, the device synchronizes to a time source only if the source carries one of the authentication keys specified by the ntp trusted-key command. The device drops any packets that fail the authentication check and prevents them from updating the local clock. NTP authentication is disabled by default.

Adam, Paul (2014-07-12). All-in-One CCIE V5 Written Exam Guide (Kindle Locations 5886-5888).  . Kindle Edition.

http://www.ine.com/resources/ntp-authentication.htm