To configure an NTP enabled router to require authentication when other devices connect to it, use the following commands: NTP_Server(config)#ntp authentication-key 2 md5 securitytut NTP_Server(config)#ntp authenticate NTP_Server(config)#ntp trusted-key 2 Then you must configure the same authentication-key on the client router: NTP_Client(config)#ntp authentication-key 2 md5 securitytut NTP_Client(config)#ntp authenticate NTP_Client(config)#ntp trusted-key 2 NTP_Client(config)#ntp server 10.10.10.1 key 2 Note: To configure a Cisco device as a NTP client, use the command ntp server <IP address>. For example: Router(config)#ntp server 10.10.10.1. This command will instruct the router to query 10.10.10.1 for the time.