Network Time Protocol

The NTP is designed to time synchronize a network of devices.

Runs over UDP.

NTP is documented in RFC 1305.

NTP uses the concept of a stratum to describe how many NTP hops away a device is from an
authoritative time source. A stratum 1 time server has a radio or atomic clock directly attached, a
stratum 2 time server receives its time through NTP from a stratum 1 time server, and so on. Cisco’s implementation of NTP does not support stratum 1 service.

 A device running NTP automatically chooses as its time source the device with the lowest stratum number with which it communicates through NTP.

NTP also compares the time reported by several devices and does not synchronize to a device whose time is significantly different than the others, even if its stratum is lower.

NTP can be configured to use IP broadcast messages. In that case, information flow is one-way only.

Two mechanisms are available for NTP: an access list-based restriction scheme and an encrypted authentication mechanism.

The system clock keeps an authoritative flag that shows whether the time is authoritative (believed to be accurate). If the system clock has been set by a timing source such as NTP, the flag is set. If the time is not authoritative, it is used only for display purposes. Until the clock is authoritative and the
authoritative flag is set, the flag prevents peers from synchronizing to the clock when the peers’ time is invalid. The symbol that precedes the show clock display has this meaning:
• *—Time is not authoritative.
• (blank)—Time is authoritative.
• .—Time is authoritative, but NTP is not synchronized.

NTP Version 4

An extension of NTPv3.

Supports both IPv4 and IPv6.

Backward-compatible with NTPv3.

NTPv4 protocol provides a security framework based on public key cryptography and standard X509 certificates.


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