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)#ntp master 2
NTP client configuration:
Router(config)#ntp server 10.0.0.2