Cisco® CCNA Exam Cram Notes : NTP

VII. IP connectivity and Services

6. NTP

NTP synchronizes time across devices so you can correlate system logs and other time-specific events from multiple network devices

  • NTP uses UDP as its transport protocol

  • An NTP server normally receives its time from an authoritative clock and distributes it across the network

  • NTP is very efficient. 1 packet per second is all that is needed to synchronize a client within a millisecond of the server

  • Stratum is used in NTP to describe the distance between a device and the time source

  • Stratum1 = time source is directly attached

  • Stratum2 = time is received from a Stratum1 source via NTP

  • Before synchronizing, NTP compares the time reported by several network devices and does not synchronize with one that is significantly different, even if it is a stratum 1.

  • Public NTP servers are available for use via the internet

  • The time kept on a device is a critical resource. Use access list-based restrictions and an encrypted authentication mechanism to avoid the accidental or malicious setting of incorrect time.

NTP Master: NTP master is the Authoritative NTP server that is configured as a Network Time Protocol (NTP) master clock to which peers synchronize themselves, when an external NTP server is not available.

NTP uses the concept of a stratum to describe how many NTP hops away a machine is from an authoritative time source. This stratum is a number between 1 and 15 that indicates how far is a server is from a reference clock.

Cisco routers and switches can be configured as both NTP clients and NTP servers. To configure your device as an NTP client, use the following command:

router(config)#ntp server <IP_ADDRESS>

To configure the device as NTP master use the following command

router(config)#ntp master <stratum-number>

The stratum-number can be from 1 to 15 and indicates the advertised accuracy of the local reference clock. Default stratum level of router’s internal clock is 7 .

Examples for NTP configuration

NTP server configuration:

Router#set clock 09:25:00 11 Dec 2018
Router#config term
Router(config)#ntp master 2

NTP client configuration:

Router#config term
Router(config)#ntp server
Router(config)#ntp logging

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