NTP Facts Free Study Material

NTP Facts Free Study Material 8.7.2 NTP Facts The Network Time Protocol (NTP) is a protocol designed to time-synchronize a network of machines. You should be familiar with the following facts about NTP: Factor Description NTP Service NTP Host Roles NTP Terminology NTP Stratum When implementing NTP on a Cisco router, be aware of the following protocol characteristics: NTP runs over UDP port 123. NTP is an efficient protocol. Only one packet per minute is necessary to synchronize two machines to an accuracy of within a millisecond of one another. NTP corrects the time, keeps track of consistent time variations, and automatically adjusts for time drift on the client. By default, NTP services are disabled on all interfaces on a Cisco router. NTP is enabled globally when any NTP commands are entered. NTP packets are received on all interfaces when NTP is enabled. Network hosts that are configured to use the NTP protocol to synchronize time function in the following roles: An NTP time provider provides time to clients. An NTP time client seeks time from a time provider. A host can be an NTP time provider and an NTP time client at the same time, receiving time from one host while providing time to another host. An NTP client can be configured to synchronize time from any NTP time provider. Several NTP configurations are commonly used: All NTP clients on the network can be configured to synchronize time with a public NTP source on the Internet. ****CLIENT/SERVER MODE A master NTP time provider can be configured on one internal host that synchronizes time with a public time provider on the Internet, creating an internal authoritative time source. All other NTP clients on the network are configured to get time from this host. If the network is isolated from the Internet, a master NTP time provider can be configured on one internal host that gets time from its own internal clock, creating an internal authoritative time source. All other NTP clients are configured to get time from this host. NTP uses the following terminology that you should be familiar with: Skew measures the difference (in hertz) between the actual frequency of a clock and its frequency to keep more accurate time. Slam refers to immediately correcting the time of a clock. Slew refers to gradually adjusting the time of a clock until it displays the correct time. Dispersion measures how scattered the time offsets (in seconds) are from a given time server. Drift measures how quickly the skew of a clock changes (in Hertz per second). Jitter refers to small, rapid variations in a system clock. NTP uses the concept of stratum. Observe the following: The higher the stratum number, the lower it is in the stratum hierarchy, and the less precedence it has in relation to other time providers with a lower number. Stratum 1 is the highest level in the hierarchy. Stratum 1 time providers adjust their time using an external reference time source (such as a GPS, an atomic clock, o Stratum 2 providers synchronize their time from stratum 1 providers. Stratum 3 providers synchronize their time from stratum 2 providers, and so on until the stratum level of 15 is reached (the maximum allowed). Use the following commands to implement NTP: Command Action Router(config)#feature ntp Enables NTP on the device. Router(config)#ntp server [address] Allows the software clock to be synchronized with the specified NTP time server. Router(config)#ntp master [stratum-value] Configures the device as an NTP master clock to which peers synchronize themselves when an external NTP source is not available: stratum-value is an optional number ranging from 1 to 15. The value indicates the number of hops away a device is from an authoritative time source. This command will override valid time sources, especially if a low stratum number is configured.