NTP uses a hierarchical structure of hosts, such as computers and routers, that form client-server and peer associations. An NTP client synchronizes with an NTP server, which in turn synchronizes with another time source. If two hosts provide synchronization for each other, they are peers.
Primary or stratum 1 servers synchronize directly with an accurate time source, such as a radio clock or an atomic clock. Secondary or stratum n servers synchronize with other servers, and are n hops from an accurate time source.
To obtain high precision and reliability with NTP, clients typically synchronize with several NTP servers at different physical locations. Peer associations, especially for stratum 1 and 2 servers, provide redundancy for the network.
Hosts synchronize by exchanging NTP messages through UDP. NTP uses the IP and UDP checksums to confirm data integrity.
By default, the router is an NTP client. You must configure NTP client parameters to start NTP client operation. You can also configure the router as an NTP server, whether or not you configure NTP client parameters.
Figure 30 shows an example of an NTP hierarchy.
Figure 30: Example of an NTP Hierarchy
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