Cisco Catalyst 2960 Series, Configuration Manual

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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
configuration complexity because each device can simply be configured to send or receive broadcast messages.
However, in that case, information flow is one-way only.
NTP Security
The time kept on a device is a critical resource; you should use the security features of NTP to avoid the
accidental or malicious setting of an incorrect time. Two mechanisms are available: an access list-based
restriction scheme and an encrypted authentication mechanism.
NTP Implementation
Implementation of NTP does not support stratum 1 service; it is not possible to connect to a radio or atomic
clock. We recommend that the time service for your network be derived from the public NTP servers available
on the IP Internet.
Consolidated Platform Configuration Guide, Cisco IOS Release 15.2(4)E (Catalyst 2960-X Switches)
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Information About Administering the Switch