NTP Questions

Quick review of NTP

– NTP is designed to synchronize the time on a network. NTP runs over the User Datagram Protocol (UDP), using port 123 as both the source and destination.
– An Authoritative NTP Server can distribute time even when it is not synchronized to an existing time server. To configure a Cisco device as an Authoritative NTP Server, use the ntp master [stratum] command.
– To configure the local device to use a remote NTP clock source, use the command ntp server <IP address>. For example: Router(config)#ntp server 192.168.1.1
– The ntp authenticate command is used to enable the NTP authentication feature (NTP authentication is disabled by default).
– The ntp trusted-key command specifies one or more keys that a time source must provide in its NTP packets in order for the device to synchronize to it. This command provides protection against accidentally synchronizing the device to a time source that is not trusted.
– The ntp authentication-key defines the authentication keys. The device does not synchronize to a time source unless the source has one of these authentication keys and the key number is specified by the ntp trusted-key number command.
– Two most popular commands to display time sources statistics: show ntp status and show ntp associations

Question 1

Explanation

To configure a Cisco device as an Authoritative NTP Server, use the ntp master [stratum] command.

To configure a Cisco device as a NTP client, use the command ntp server <IP address>. For example: Router(config)#ntp server 192.168.1.1. This command will instruct the router to query 192.168.1.1 for the time.

Question 2

Explanation

Below is the output of the “show ntp status” command. From this output we learn that R1 has a stratum of 10 and it is getting clock from 10.1.2.1.

R1#show ntp status
Clock is synchronized, stratum 10, reference is 10.1.2.1
nominal freq is 250.0000 Hz, actual freq is 249.9987 Hz, precision is 2**18
reference time is D5E492E9.98ACB4CF (13:00:25.596 CST Wed Sep 18 2013)
clock offset is 15.4356 msec, root delay is 52.17 msec
root dispersion is 67.61 msec, peer dispersion is 28.12 msec

Question 3

Explanation

To configure authentication, perform this task in privileged mode:

Step 1: Configure an authentication key pair for NTP and specify whether the key will be trusted or untrusted.
Step 2: Set the IP address of the NTP server and the public key.
Step 3: Enable NTP client mode.
Step 4: Enable NTP authentication.
Step 5: Verify the NTP configuration.

Reference: https://www.cisco.com/c/en/us/td/docs/switches/lan/catalyst4000/8-2glx/configuration/guide/ntp.html

Note: A trusted NTP server may or may not require a secret key so it is not a “must” in this question.