NTP Hierarchy

Reference clocks are stratum 0.

Stratum 1 NTP servers get their time from reference clocks.

Stratum 2 NTP servers get their time from stratum 1 NTP servers.

Stratum 3 NTP servers get their time from stratum 2 NTP servers.

Stratum 15 is the maximum. Anything above that is considered unreliable.

Devices can also 'peer' with devices at the same stratum to provide more accurate time.

An NTP client can sync to multiple NTP servers.

Stratum 1 NTP servers which get their time directly from reference clocks are also called primary servers

Stratum 2 and 3 NTP servers which get their time from other NTP servers are called secondary servers. They operate in server mode and client mode at the same time.