System Clock - Cisco Catalyst 4500 Series Software Configuration Manual

Managing the System Time and Date

System Clock

The core of the time service is the system clock, which monitors the date and time. This clock starts when
the system starts.
The system clock can provide time to these services:
•
•
The system clock keeps track of time internally based on Universal Time Coordinated (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 is correct for the local time zone.
The system clock keeps track of whether the time is authoritative or not (whether it was 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. For configuration information, see the
Manually" section on page
Understanding Network Time Protocol
The NTP is designed to 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 have been 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 associate. 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 be configured to send or receive
broadcast messages; however, 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.
Catalyst 4500 Series Switch, Cisco IOS Software Configuration Guide - Cisco IOS XE 3.9.xE and IOS 15.2(5)Ex
4-2
User show commands
Logging and debugging messages
4-11.
Chapter 4 Administering the Switch
"Configuring Time and Date