Network Time Protocol (NTP)
Also known as: ntp
The Network Time Protocol (NTP) is a networking protocol that synchronizes the clocks of devices across a network to a common time reference. It runs over UDP port 123 and organizes time sources into a hierarchy of strata.
NTP keeps every device on a network agreeing on what time it is. Each client repeatedly exchanges timestamped messages with one or more servers, measures the round-trip delay, and gradually steers its own clock toward the server's — typically adjusting in small increments rather than jumping, so that time does not run backward. It uses UDP port 123, chosen because the low overhead and lack of retransmission suit frequent, small, time-sensitive exchanges.
Sources are organized into strata. Stratum 0 is a reference clock such as a GPS receiver or atomic clock; a server directly attached to one is stratum 1; a server synchronizing to a stratum 1 server is stratum 2, and so on. Each hop adds a small amount of error, so lower stratum numbers are more authoritative. On a Cisco network a router or switch is typically configured as an NTP client of an internal or public server, and may also act as an NTP server for other devices; authentication can be enabled so devices only accept time from trusted sources.
Accurate time matters more than it first appears. Log entries and syslog messages from different devices cannot be correlated during an incident if their clocks disagree. Digital certificates are validated against the current time, so a badly skewed clock breaks TLS. Kerberos authentication rejects tickets outside a tight time window, scheduled jobs fire at the wrong moment, and time-based access control lists apply at the wrong hours.
The CCNA exam covers NTP in its network services section, and questions focus on the configuration commands for client and server mode, the stratum concept, verification with show commands, and why NTP uses UDP rather than TCP. Pair it with the other infrastructure services — DHCP, DNS, and syslog — since they are tested together.
Key takeaways
- NTP synchronizes device clocks across a network and runs over UDP port 123.
- Time sources are ranked by stratum, with stratum 0 reference clocks at the top and each hop adding error.
- Network devices can act as NTP clients, NTP servers, or both, and can authenticate their time sources.
- Synchronized clocks are required for correlating logs, validating certificates, Kerberos authentication, and time-based access control.
- The CCNA tests NTP configuration, verification, stratum levels, and its use of UDP within network services.