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Catalyst 3550 Multilayer Switch Software Configuration Guide
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Chapter6 Administering the Switch
Managing the System Time and Date
Managing the System Time and Date
You can manage the system time and date on your switch using automatic, such as the Network Time
Protocol (NTP), or manual configuration methods.
Note For complete syntax and usage information for the commands used in this section, refer to the Cisco
IOS Configuration Fundamentals Command Reference for Release 12.1.
This section contains this configuration information:
•Understanding the System Clock, page 6-32
•Understanding Network Time Protocol, page 6-32
•Configuring NTP, page 6-34
•Configuring Time and Date Manually, page 6-41

Understanding the System Clock

The heart of the time service is the system clock. This clock runs from the moment the system starts up
and keeps track of the current date and time.
The system clock can then be set from these sources:
•Network Time Protocol
•Manual configuration
The system clock can provide time to these services:
•User show commands
•Logging and debugging messages
The system clock keeps track of time internally based on U nive rsal Time Coor dina ted (UT C), al so
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 correctly displayed 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. For configuration information, see the “Configuring Time and
Date Manually” section on page 6-41.

Understanding Network Time Protocol

The NTP is designed to time-synchron i z e a network of devices. NTP runs over Us er Datagram Protocol
(UDP), which runs over IP. NTP is documented in RFC 1305.
An NTP network usually gets its time from an authoritative tim e sourc e, su ch as a r adi o cl ock 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