Cisco TelePresence Administrator's Manual page 98

Device authentication
More information about each authentication mechanism is contained in Authenticating devices deployment
guide.
Note: accurate timestamps play an important part in authentication, helping to guard against replay attacks.
For this reason, if you are using device authentication, both the VCS and the endpoints must use an NTP
server to synchronize their system time.
Cisco VCS Administrator Guide (X7.1) Page 98 of 479