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Some points to remember, clients adjust their time from what they hear from an NTP server, NTP server

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supply time to clients, 

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they don't adjust their time to the clients

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the clients adjust their time to the servers.

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An NTP client server device, such as router 2 in our example, got time from an NTP server.

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So it was synchronizing to router 3 and then provided time to router 1.

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NTP needs what is called a trusted clock source.

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Ideally, you would get this from a lower stratum device, such as a domain controller in your network

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or an NTP server on the Internet. In the lab

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We didn't have an NTP server that we were connecting to

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so I configured router 3 as an NTP master so that it trusted its own internal clock as the clock

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source.

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Once again, in the real world, that's not ideal.

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You should get your time from a better time source.

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A stratum level or stratum as some of you may pronounce it, defines the quality of the time

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source the lower the number, the better the time source.

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A stratum is a device that has an atomic clock or a GPS clock or radio clock.

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In other words, it's a very good reference for time.

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NTP is quite simple to configure once again to set up a server or stratum device, you would use the

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command NTP master to specify a loopback interface, which is better than using a physical interface

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because loopbacks don't go down.

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Use the NTP source loopback zero command to specify a server.

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Use the command NTP server and specify the server so the server command makes router 2 a client, the master

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command means that router 3 is the trusted source of time.

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Router 2 however, is also a server providing time to router 1 because router 1 is pointing

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to router 2 for its time.

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So we didn't configure router 2 as a server, it was simply configured as a client

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but it started providing time to router 1 when router 1 queried it.

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It can also use its own clock as a backup if it loses connection to router 3.