Unlike other time synchronization protocols, NTP does not attempt to synchronize the internal clocks of the remote time servers to each other. Rather, NTP synchronizes the clocks to universal standard time, using the best available time source and transmission paths to that time source.
Use the show NTP statistics command to verify the NTP synchronization status. NTP uses the following criteria to determine the best available time server:
The time server with the lowest stratum.
The time server closest in proximity to the primary time server (reduces network delays).
The time server that offers the highest claimed precision.
NTP accesses several (at least three) servers at the lower stratum level because it can apply an agreement algorithm to detect a problem on the time source.