Cisco Catalyst 9200 Configuration Manual page 17

Administering the Device
Figure 1: Typical NTP Network Configuration
If the network is isolated from the Internet, Cisco's implementation of NTP allows a device to act as if it is
synchronized through NTP, when in fact it has learned the time by using other means. Other devices then
synchronize to that device through NTP.
When multiple sources of time are available, NTP is always considered to be more authoritative. NTP time
overrides the time set by any other method.
Several manufacturers include NTP software for their host systems, and a publicly available version for
systems running UNIX and its various derivatives is also available. This software allows host systems to be
time-synchronized as well.
NTP Stratum
NTP uses the concept of a stratum to describe how many NTP hops away a device is from an authoritative
time source. A stratum 1 time server has a radio or atomic clock directly attached, a stratum 2 time server
receives its time through NTP from a stratum 1 time server, and so on. A device running NTP automatically
chooses as its time source the device with the lowest stratum number with which it communicates through
NTP. This strategy effectively builds a self-organizing tree of NTP speakers.
NTP avoids synchronizing to a device whose time might not be accurate by never synchronizing to a device
that is not synchronized. NTP also compares the time reported by several devices and does not synchronize
to a device whose time is significantly different than the others, even if its stratum is lower.
NTP Associations
The communications between devices running NTP (known as associations) are usually statically configured;
each device is given the IP address of all devices with which it should form associations. Accurate timekeeping
is possible by exchanging NTP messages between each pair of devices with an association. However, in a
LAN environment, NTP can be configured to use IP broadcast messages instead. This alternative reduces
System Management Configuration Guide, Cisco IOS XE Gibraltar 16.10.x (Catalyst 9200 Switches)
NTP Stratum
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