Per Cisco ...
ntp trusted-key
Specifies one or more keys (defined in Step 2) that a time source must provide in its NTP packets in order for the device to synchronize to it. The range for trusted keys is from 1 to 65535.
This command provides protection against accidentally synchronizing the device to a time source that is not trusted.
See URL: http://www.cisco.com/c/en/us/td/docs/switches/datacenter/sw/5_x/nx-os/system_management/configuration/guide/sm_nx_os_cg/sm_3ntp.html
