What does stratum 16 mean?
In the Network Time Protocol (NTP), stratum counts how many hops a clock is from a reference clock such as a GPS receiver or an atomic clock. Valid strata run from 1 to 15. Stratum 16 is a special value meaning the clock is unsynchronized: it has no trustworthy source, so its time can't be relied on, and NTP clients won't sync to a server that reports it. It is the opposite of an accurate time source.
Why a device shows stratum 16
- It just started. Until its first successful sync, a time server has no valid stratum to offer; ntpd, for example, reports 16.
- It lost its upstream source, for example after an internet outage or when its configured time server was retired.
- UDP port 123 is blocked by a firewall between the device and its time server.
- Configuration errors, such as a misspelled server name or DNS failing to resolve it.
- Its source is unusable: unsynchronized itself, or already at stratum 15.
Checking time sync
| System | Command | What to look for |
|---|---|---|
| Windows | w32tm /query /status | The stratum and source. A source of Local CMOS Clock or Free-running System Clock means Windows isn't syncing from a time server. |
| Windows | w32tm /resync | Forces a sync attempt once the problem is fixed |
| Linux with chrony | chronyc tracking, chronyc sources | The stratum, and the source marked * as the one in use |
| Linux with ntpd | ntpq -p | Peers showing 16 in the st column (often with .INIT. as the refid) haven't been reached |
| Linux with systemd | timedatectl | Whether the system clock is synchronized |
Why it matters
Unsynchronized clocks cause real problems. In an Active Directory domain, Kerberos authentication fails when a computer's clock is more than 5 minutes off from the domain controller (the default limit), so users can't sign in. Certificates can look expired or not yet valid, and log entries from different devices stop lining up, which makes troubleshooting and security investigations harder.
For the CompTIA A+ exam
Core 1 (220-1201) objective 2.3 lists NTP as a server role, and this course's NTP lesson covers the stratum levels. Know that NTP uses UDP port 123, that stratum 15 is the farthest valid level, and that stratum 16 marks an unsynchronized device that shouldn't be used as a time source.