Cisco Catalyst 9200 Configuration Manual page 16

Administering the Device
Network Time Protocol
The system clock keeps track of time internally based on Coordinated Universal Time (UTC), also known as
Greenwich Mean Time (GMT). You can configure information about the local time zone and summer time
(daylight saving time) so that the time appears correctly for the local time zone.
The system clock keeps track of whether the time is authoritative or not (that is, whether it has been set by a
time source considered to be authoritative). If it is not authoritative, the time is available only for display
purposes and is not redistributed.
Network Time Protocol
The NTP is designed to time-synchronize a network of devices. NTP runs over User Datagram Protocol
(UDP), which runs over IP. NTP is documented in RFC 1305.
An NTP network usually gets its time from an authoritative time source, such as a radio clock or an atomic
clock attached to a time server. NTP then distributes this time across the network. NTP is extremely efficient;
no more than one packet per minute is necessary to synchronize two devices to within a millisecond of one
another.
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.
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.
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.
Cisco's 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.
The Figure shows a typical network example using NTP. Device A is the primary NTP, with the Device B,
C, and D configured in NTP server mode, in server association with Device A. Device E is configured as an
NTP peer to the upstream and downstream device, Device B and Device F, respectively.
System Management Configuration Guide, Cisco IOS XE Gibraltar 16.10.x (Catalyst 9200 Switches)
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