NTP Deep Dive

You already know the basics of NTP.
This lesson goes deeper into how the protocol actually works.

What NTP Does

Without NTP, every device keeps its own time and they drift apart:

Figure 1 – Without NTP, clocks drift apart

With NTP, all devices point to one NTP server, allowing them to converge on the same time.

Figure 2 – With NTP, devices share one time

NTP is a standardized protocol in RFC 5905, and NTPv4 is the current version.

NTP Stratums

NTP establishes a hierarchy of clock sources.
It uses the term stratum to measure distance from the reference.

Stratum 0 is the reference clock itself, a GPS or atomic source.

Figure 3 – The NTP stratum hierarchy

The server directly connected to it is a Stratum 1 server.
Each step down the chain adds one.

• The usable values range from 1 to 15.

• Stratum 16 means that the device is unsynchronized and has no valid source yet.

NTP uses UDP port 123.

Stratum does not count the number of routers between two devices.
Instead, it counts the number of NTP client-server steps.

Each NTP step down the chain adds one stratum.

Figure 4 – Each NTP step adds one stratum

If you put a whole IP network between two of them, only the NTP step will count.

Figure 5 – Network hops don't raise the stratum

R2 queries its Stratum 1 server directly.
Several routers sit on the path between them.

It is still Stratum 2, because it is one NTP step away.

Let's configure a simple NTP topology now.
If you look at the topology below:

Figure 6 – R2 syncs to the server, R3 syncs to R2

• The NTP server is synchronized to a GPS reference, so it sits at stratum 1

• R2 becomes stratum 2

• R3 becomes stratum 3

Configure the NTP server

You can configure R2 to point to the NTP server:

R2# configure terminal
Enter configuration commands, one per line.  End with CNTL/Z.
R2(config)# ntp server 209.165.200.225
R2(config)# end

• R2 now uses the stratum 1 server at 209.165.200.225 as an NTP source

On R3 we can tell the router to synchronize to R2:

R3# configure terminal
Enter configuration commands, one per line.  End with CNTL/Z.
R3(config)# ntp server 192.168.1.1
R3(config)# end

• The server feeds R2, and R2 feeds R3, the chain is complete.

Now, verify that the devices are synchronized.

Before Synchronization

You just pointed R2 at the server.
Does it lock onto the time straight away?

Run show ntp status right now and read the first line.

R2# show ntp status
Clock is unsynchronized, stratum 16, no reference clock
nominal freq is 250.0000 Hz, actual freq is 249.9990 Hz, precision is 2**24
reference time is 00000000.00000000 (00:00:00.000 UTC Mon Jan 1 1990)
clock offset is 0.00 msec, root delay is 0.00 msec
root dispersion is 0.00 msec, peer dispersion is 0.00 msec
system poll interval is 4, never updated.

• The clock is not synchronized

• Stratum 16 message

This tells you R2 isn't synchronized yet.
NTP needs a moment, so this is normal right after you configure it.

After Synchronization

We can try the command again after leaving some time:

R2# show ntp status
Clock is synchronized, stratum 2, reference is 209.165.200.225
nominal freq is 250.0000 Hz, actual freq is 250.0000 Hz, precision is 2**10
ntp uptime is 1893700 (1/100 of seconds), resolution is 4000
reference time is E0E2D211.E353FA40 (22:50:58.888 UTC Thu Aug 14 2026)
clock offset is 0.5120 msec, root delay is 9.45 msec
root dispersion is 8.21 msec, peer dispersion is 3.40 msec
loopfilter state is 'CTRL' (Normal Controlled Loop), drift is 0.000000000 s/s
system poll interval is 64, last update was 9 sec ago.

• Clock is synchronized,

• Stratum 2,

• Reference is 209.165.200.225

This tells you R2 is now synchronized and the reference is the IP address of the stratum 1.

Reading show ntp associations

Check the NTP associations to see every server your device talks to:

R2# show ntp associations
address             ref clock     st  when  poll  reach  delay    offset   disp
*~209.165.200.225   .GPS.         1   115   1024    1    1.914    0.613    191.13
* sys.peer, # selected, + candidate, - outlyer, x falseticker, ~ configured

Here we can see one line:

• * means that R2 chose this server.

• ~ means that you configured it manually.

• Its stratum is 1.

• .GPS. is what the server itself syncs to.

R2 has successfully selected the stratum 1 server as its time source.
Let's also verify the local clock to confirm the source has changed:

R2# show clock detail
22:51:7.176 UTC Thu Aug 14 2026
Time source is NTP

The time is now synchronized with the NTP server.
In this lab, assume the date is 2026.

Now check whether R3 synced correctly to R2.