NTP server and client and the master command
When you configure a network device, say Router A with an IP address of 10.10.10.1, to operate as an Network Time Protocol (NTP) server, you don’t actually have to explicitly configure it as a server. When an NTP client such as Router B is configured with the command
ntp server 10.10.10.1 this makes Router A the NTP server, and Router B the NTP client.
Router A in turn can also be an NTP client. For example, the
ntp server pool.ntp.org command actually makes Router A an NTP client to the pool.ntp.org server, but at the same time it retains the role of NTP server for Router B.
master command is simply used to let a device know to consider its own clock as the NTP source, and it is also used to manually set a stratum number for this NTP source. This way, clients of this device will adopt their stratum number based on the locally configured one. This type of configuration means that no external NTP sources are used for synchronization.