Network Time Protocol (NTP) is a networking protocol designed to time-synchronize device with in a network. NTP time server works within the TCP/IP suite and uses User Datagram Protocol (UDP) port 123 as its transport protocol. An NTP network usually receives its time from an authoritative time resource, such as an atomic clock or a radio clock attached to a time server and distributes this time across the network.
NTP servers are normally dedicated NTP devices that use a single time reference to which they can synchronize a network. This time reference is a Coordinated Universal Time (UTC) source, a global time scale distributed by atomic clocks over the internet. The dedicated NTP servers are required for Security, Accuracy, Protection, Legality, and Control.
Unlike PCs or servers, Cisco network devices specifically need to run NTP to synchronize the time and date. That's because most Cisco devices don't have an internal clock. An NTP client synchronizes the time and date with an NTP server. The NTP server should be a reliable source.
To configure NTP on your IOS router, follow the steps given below:
1. Choose the NTP server your Cisco router/switches will use.
2. Find out the IP address for this server. It could be an external source such as NIST or internal.
3. Enter the following commands on the IOS device:
Router# configure terminal
Router(config)# ntp server <IP address of NTP Server>
4. Verify the association with the server using the show ntp status and show ntp associations commands.
The NTP service can be activated by entering any ntp command. When you use the ntp broadcast client command, the NTP service is activated (if it has not already been activated) and the device is configured to receive NTP broadcast packets on the specified interface simultaneously.
The command "ntp broadcast client" Allows the system to receive NTP broadcast packets on the specified interface.
Following is the sample output from "show ntp association"
The device with ip address 192.168.7.1 is NTP master. The first entry shows 192.168.7.1. This indicates that the local machine has synced with itself. Generally, only an NTP master syncs with itself.
The third column shows how many hops that the master clock is away from the local machine. It is called stratum in NTP terminology. 192.168.13.33 is 3 hops away as seen in the output.