7-34
Catalyst 2950 Desktop Switch Software Configuration Guide
78-14982-01
Chapter 7 Administering the Switch
Managing the Syste m Ti m e and Date

Managing the System Time and Date

You ca n mana ge t he sy st em t ime and d ate o n y our sw it ch usi ng a utom a tic c on figurati on, su ch as the
Network Time Protocol (NTP) , or manual configurati on metho ds.
Note For comple te syntax an d usage infor mation for the commands us ed in this sectio n, refer t o the Cisco IOS
Configuration F undam en tals Co mma nd Re ference for Rel ease 12 .1 .
This se ctio n c onta ins thi s configu ratio n inf or mat ion:
•Understa ndin g the System C lock , pa ge 7- 34
•Understa ndin g Ne twork Time Protoc ol, page 7-34
•Configuring NTP, page 7-36
•Configuring Time and D ate M a nual ly, pag e 7- 43

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 tra ck of the da te and tim e.
The system clock can then be set from these sources:
•Network Time Protoc ol
•Manual configurat ion
The system clock can provide time to these services:
•User show commands
•Logging and de bugging m essag es
The syste m clock keeps tr ack of time internal ly based on Universal Ti me Coordi nated (UT C), also
known as Green wich M e an Time (G M T). Yo u ca n configu re in for mat ion a bout t he lo cal tim e 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 o r not (that is, whether it has been set
by a time source considered to be authoritative). If it is n ot authoritative, the time is available only for
display purposes an d is not redistrib uted . For config uration infor mation, see th e “Configuring Time and
Date Manually ” secti on on pa ge 7-43.

Understanding Network Time Protocol

The NTP is designed to time-synchronize a network of devices. NTP runs over User Datagram Protocol
(UDP), which run s over IP. N TP is docume nted in RFC 1305.
An NT P n etwork u sua lly g ets its tim e fr om an au th orita tive time source , such as a r adio cl ock o r an
atomic clock attached to a time server. NTP then distribu tes this time across the network. NTP is
extremely efficien t; no more th an one pa cket per minu te is nec essary to syn chroniz e two devices to
within a m i llise co nd of on e an othe r.