Network Time Protocol ( NTP) is the protocol that is used to synchronize the clocks of various network devices in an enterprise network.
In NTP you have to configure one device in the network as the NTP server and other devices as the client. The NTP server provides the time information to the NTP Clients. The NTP Clients adjust their clock according to the time of the NTP server. The NTP server acts as the reference clock.
Example
In the above diagram, the router R1 is configured as the NTP Server and R2 is configured as NTP Client.
R1 provides the time information to R2 and R2 adjusts its time according to the R1's clock. As time passes the router will have the same time and their clocks will be synchronized.
The router R1 will be made as the NTP server by using the command ntp master. Once the ntp master command is configured on a router, it thinks and acts like a NTP server.
The ntp master command is shown below.
From the above, you can see that ntp master command uses a parameter called stratum number.
NTP stratum level tells us about the accuracy of the time source (NTP server).
The lesser the stratum number value, the more accurate is the clock source.
A stratum level of 0 is considered as the most accurate which receives its time from a satellite or radio.