This document contains frequently asked questions about the NTP chimer.
Q. How do I enable the NTP chimer on my Cisco router?
A. You can use one of these basic commands:
The command you use depends on whether you want a client/server or peer relationship. There are other features available for MD5 authentication, broadcast, and access control. You can also use the context-sensitive help feature to puzzle it out; type ntp? in the config mode.
You also need to look at the show ntp router commands. Here are two examples:
router# show ntp assoc
address ref clock st when poll reach delay offset disp
220.127.116.11 .WWVB. 1 109 512 377 97.8 -2.69 26.7
18.104.22.168 .GOES. 1 309 512 357 55.4 -1.34 27.5
master (synced), # master (unsynced), + selected, - candidate, ~ configured
router# show ntp stat
Clock is synchronized, stratum 2, reference is 22.214.171.124 nominal freq is
250.0000 Hz, actual freq is 249.9981 Hz, precision is 2**19 reference time
is B1A8852D.B69201EE (12:36:13.713 PDT Tue Jun 14 1994) clock offset is
-1.34 msec, root delay is 55.40 msec root dispersion is 41.29 msec, peer
dispersion is 28.96 msec
Q. Are there sample Cisco NTP configurations available?
A. You need to substitute your own NTP peers, timezones, and GMT offsets into the examples below, of course. Example 1 is in US Central Time Zone, while Example 3 is in US Pacific Time Zone. Both account for normal US Daylight Savings Time practices.
Example 1—US Central Time Zone:
clock timezone CST -6
clock summer-time CDT recurring
ntp source eth 0
ntp peer (host1)
ntp peer (host2)
ntp peer (host3)
ntp source Ethernet0/0
ntp peer (host1)
ntp peer (host2) prefer
Example 3—US Pacific Time Zone:
!--- Granular timestamping of debug and syslog messages.
service timestamps debug datetime localtime
service timestamps log datetime localtime
!--- Pacific Standard Time clock timezone PST -8.
!-- U.S. standard daylight saving time is in effect.
clock summer-time PDT recurring
ip address 10.1.1.1 255.255.255.0
!--- NTP broadcast packets out onto the local Ethernet.
ntp clock-period 17180319
!--- Ethernet0 is the source for NTP packets.
ntp source Ethernet0
ntp server (host1)
ntp server (host2)
ntp server (host3)
Note: The ntp clock-period command is added automatically to jump-start the NTP frequency compensation when the box is rebooted. (Do not configure this command manually.) This is essentially a representation of the frequency of the crystal used as the local timebase, and may take several days to calculate otherwise. Use the write mem command after a week or so to save a good value.
Q. How can I set NTP to update the clock chip in my router?
A. You can use the ntp update-calendar command. This causes NTP to periodically (every hour) update the clock chip in high end routers.
Q. Where can I get more information on NTP?
A. Refer to the University of Delaware EE/CIS FTP server for more NTP information. The clock.txt file in that directory has information about various public NTP servers. There is also information on radio time receivers that can be connected to an NTP server.