When Cisco routers are configured to use the NTP servers placed in the Active Directory, the Cisco routers do not receive any NTP packets from the NTP server. This issue occurs because Cisco routers use NTP and Active Directory domains use W32Time service. W32Time uses Simple Network Time Protocol (SNTP), a subset of NTP, for time synchronization. SNTP and NTP use the same network-packet format. The main difference between SNTP and NTP is that SNTP does not provide the error-check and filtering functions that NTP provides. Cisco router and switches use NTP and allow for all error-checking and filtering functions provided by NTP v3.