! 1. Enable authentication:
R(config)# ntp authenticate
! 2. Configure the MD5 key:
R(config)# ntp authentication-key KEY-NUMBER md5 KEY
! 3. Configure the key as trusted. Only NTP packets from servers that use the trusted keys will be accepted
R(config)# ntp trusted-key KEY-NUMBER
! 4. On clients only, define the key for each server.
R(config)# ntp server IP-ADDRESS key KEY-NUMBER