Information Technology Reference
In-Depth Information
time. These advantages make NTP the preferred clock-setting method. The accuracy of
the clock on your device depends on the stratum number of the NTP server. A stratum 1
time server is one that has a radio or atomic clock directly attached. The device that re-
ceives its time from this server via NTP is considered a stratum 2 device. The device that
receives its time from this stratum 2 device via NTP is considered a stratum 3 device, and
so on. There are many publicly accessible stratum 2 and 3 (and even some stratum 1) de-
vices on the Internet.
Note:
You can obtain a list of publicly accessible NTP servers at
www.ntp.org.
After you obtain one or more NTP servers to use, you can configure NTP support on your
Cisco devices by using the syntax in Example 3-4.
Example 3-4
Configuring a Cisco Router to Receive Time via NTP
WAN_RTR#configure terminal
WAN_RTR(config)#ntp server 64.209.210.20
WAN_RTR(config)#clock timezone ARIZONA -7
The first command,
ntp server <ip address>
, configures your Cisco device to use the
specified NTP server; 64.209.210.20 is one of many publicly acces
s
ible NTP servers. If
this is the only command you enter, your clock on your device will set itself to the Univer-
sal Time Coordinated (UTC) time zone. To accurately adjust the time zone for your device,
use the
clock timezone <name> <hours>
command. The previous syntax example set the
time zone for Arizona to -7 hours from UTC.
Now that we configured the router to synchronize with an NTP server, we can verify the
NTP associations and the current time and date using the commands shown in Example 3-5.
Example 3-5
Verifying NTP Configurations
WAN_RTR#show ntp associations
address ref clock st when poll reach delay offset disp
*~64.209.210.20 138.23.180.126 3 14 64 377 65.5 2.84 7.6
* master (synced), # master (unsynced), + selected, - candidate, ~ configured
WAN_RTR#show clock
11:25:48.542 CA1_DST Mon Dec 13 2010
The key information from the
show ntp associations
command is just to the left of the
configured NTP server address. The asterisk indicates that your Cisco device has synchro-
nized with this server. You can configure multiple NTP sources for redundancy, but the
Cisco device will only choose one master NTP server to use at a time.
After you configure the Cisco router to synchronize with an NTP server, you can config-
ure it to provide date and time information to a CUCM server, which can then provide
that date and time information to the Cisco IP Phones in your network. To allow other
