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Configuring NTP on a Cisco Router (NTP Client and NTP Server)

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Network Time Protocol (NTP) is a vital service not only for Cisco devices but almost every network device. Any computer-based device needs to be accurately synchronised with a reliable time source such as an NTP server.

This article will show you how to configure your Cisco router to synchronise its software clock from external sources such as NTP servers. We will also show you how to configure your router to act as an NTP server for your internal network devices, ensuring all devices are synchronised.

When it comes to Cisco routers, obtaining the correct time is extremely important because a variety of services depend on it. The logging service shows each log entry with the date and time - very critical if you're trying to track a specific incident or troubleshoot a problem.

Generally, most Cisco routers have two clocks (most people are unaware of this!): a battery-powered hardware clock, referenced as the 'calendar' in the IOS CLI, and a software clock, referenced as the 'clock' in the IOS CLI.

The software clock is the primary source for time data and runs from the moment the system is up and running. The software clock can be updated from a number of sources:

  • NTP Server
  • SNTP (Simple NTP)
  • VINES Time Source
  • Hardware clock (built into the router)

Because the software clock can be configured to be updated from an external source, it is considered more accurate in comparison to the hardware clock. The hardware clock can be configured to be updated from the software clock.

Example Scenario

First example involves setting up the router to request NTP updates and synchronise itself from a public NTP server. This will ensure the router's time is constantly synchronised, however it will not act as an NTP server for internal hosts:


We'll need to configure the router to resolve FQDN using our ISP's name server:

R1(config)# ip nameserver

Now we instruct our Cisco router to obtain its updates from the public NTP server.

R1(config)# ntp server

As soon we issue the command, the router will resolve the FQDN into an ip address and begin its synchronisation. Right after issuing the command, we can verify the router is correctly configured and awaiting synchronisation:
R1# show ntp associations
   address       ref clock      st   when   poll  reach   delay    offset      disp
~   2   30     64     1      0.000    -1539.9     7937.5
* sys.peer,  # selected, + candidate, - outlyer, x falseticker, ~ configured
R1# show ntp status
Clock is unsynchronised, stratum 16, no reference clock
nominal freq is 250.0000 Hz, actual freq is 250.0006 Hz, precision is 2**24
reference time is 00000000.00000000 (02:00:00.000 Greece Mon Jan 1 1900)
clock offset is 0.0000 msec, root delay is 0.00 msec
root dispersion is 0.00 msec, peer dispersion is 0.00 msec
loopfilter state is 'FSET' (Drift set from file), drift is -0.000002405 s/s
system poll interval is 64, never updated.
The 'show ntp associations' command shows that the system is configured (~) to synchronise with our selected NTP server, however, it is not yet synchronised. When it is, expect to see the star (*) symbol in front of the tilde (~). The 'ref. clock' column shows the IP address of the NTP server from which our public server ( is synchronising.

It is also worth noting the column named 'st' which is equal to two (2). This represents the stratum level. The higher the stratum, the closer to the Atomic clock source we are. As a general rule, always try to synchronise with a server that has a low stratum.

The 'show ntp status' command confirms that we are yet to be synchronised with the NTP server as it clearly states that the 'clock is unsynchronised' and also shows us the current system time: 1st of Jan. 1900.

After a couple of minutes, we re-visit the CLI prompt and re-issue the commands with the following results:

R1# show ntp associations
  address         ref clock       st   when  poll   reach     delay    offset    disp
*~    2    19    64      377      0.000    -59.272   5.317
* sys.peer, # selected, + candidate, - outlyer, x falseticker, ~ configured
R1# show ntp status
Clock is synchronised, stratum 3, reference is
nominal freq is 250.0000 Hz, actual freq is 250.0007 Hz, precision is 2**24
reference time is CD94CFEB.901B3679 (01:17:15.562 Athens Sun Apr 23 2023)
clock offset is -0.0592 msec, root delay is 0.09 msec
root dispersion is 0.12 msec, peer dispersion is 0.00 msec
loopfilter state is 'CTRL' (Normal Controlled Loop), drift is -0.000003099 s/s
system poll interval is 64, last update was 476 sec ago.

Looking at the new output, we can see that our Cisco router is now synchronising with the configured peer (*) - public NTP server. Polling of the public NTP server will occur every 64 seconds, as shown in the command output.

The 'show ntp status' command also confirms the synchronisation, however, notice that the router has set its stratum level to 3. This is expected as the reference is stratum 2. The time is now correctly shown (01:17:15.562 Athens Sun Apr 23 2023).

Synchronising Software Clock and Hardware Clock

Here we'll see how in fact the software and hardware clocks on a Cisco router can have different times and how we can synchronise them between each other.

The following two commands show the difference in time between the two clocks on our Cisco router:

R1# show clock
01:51:31.532 Athens Sun Apr 23 2023

R1# show calendar
01:51:42    Athens Sun Apr 23 2023

While the difference is minor, we want to keep everything in our network synchronised as precisely as possible.

Keep in mind that 'show clock' refers to the software clock and 'show calendar' refers to the hardware clock of your router.

To synchronise the two clocks all we need to do is issue the following command:

R1# configure terminal
R1(config)# ntp update-calendar

The 'ntp update-calendar' forces the hardware clock to synchronise with the system's software clock. After a couple of minutes, we check to see if the two clocks have synchronised:

R1# show clock
02:07:05.011 Athens Sun Apr 23 2023

R1# show calendar
02:07:05 Athens Sun Apr 23 2023

We can see now that both clocks are accurately synchronised.

Configuring The System as an Authoritative NTP Server

If you want your system to become an authoritative NTP server from which other internal routers or machines can synchronise, you can achieve this with the following command:

R1(config)# ntp master

The router now acts as an NTP server and is able to respond to internal clients NTP requests. Executing the 'ntp association' command reveals our router is obtaining its time synchronisation from itself:

R1# show ntp associations
address        ref clock   st.   when   poll   reach   delay    offset   disp
*~      LOCL.   7      4     16      377    0.000    0.000    0.253
~   2      64       64       1     0.000  -179.09    7937.5
* sys.peer, # selected, + candidate, - outlyer, x falseticker, ~ configured

Troubleshooting and Monitoring NTP Status

Troubleshooting NTP messages and events is important when you are trying to verify everything is working correctly. You might notice that your Cisco router is not able to create a peer connection with a configured NTP server or your internal LAN clients might not be able to synchronise with your Cisco router; In any case, knowing how to troubleshoot NTPs is something every engineer must be aware of.

Thankfully Cisco provides a number of options that allow you to troubleshoot many aspects of your NTP service.

R1# debug ntp ?
adjust NTP clock adjustments
all NTP all debugging on
core NTP core messages
events NTP events
packet NTP packet debugging
refclock NTP refclock messages

The most useful debug commands are the 'debug ntp events', 'debug ntp adjust' and 'debug ntp core'. These three commands provide enough debugging to help you troubleshoot problems you might encounter.

Closing, if you would like more information on the ntp associations created by your router you can try the following command:

R1# show ntp associations detail configured, our_master, sane, valid, stratum 2
ref ID , time CD94E194.58EA9A78 (02:32:36.347 Athens Sun Apr 19 2009)
our mode client, peer mode server, our poll intvl 64, peer poll intvl 64
root delay 0.05 msec, root disp 41.32, reach 377, sync dist 0.16
delay 0.00 msec, offset -292.8879 msec, dispersion 3.02
precision 2**20, version 4
org time CD94E561.D6FE7162 (02:48:49.839 Athens Sun Apr 19 2009)
rec time CD94E562.287A2A0D (02:48:50.158 Athens Sun Apr 19 2009)
xmt time CD94E562.22E5E145 (02:48:50.136 Athens Sun Apr 19 2009)
filtdelay = 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02
filtoffset = -0.30 -0.30 -0.29 -0.28 -0.27 -0.27 -0.26 -0.25
filterror = 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
minpoll = 6, maxpoll = 10

The 'show ntp associations detail' command will provide much information on the association created with the NTP servers. This is most helpful when you see you are unable to create an association with an NTP server.


This article provided an insight to NTP configuration on Cisco routers. We analysed why the NTP service is important and how it can be used to keep every node in a network synchronised. We examined different methods of NTP synchronisation and provided a fairly in-depth analysis.

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