Answers

Figure 1: NTP Topology

clock timezone EST -5 0
clock summer-time EDT recurring
!
ntp master 3
ntp server pool.ntp.org
ntp source Loopback0

Example 1: R1 Config

clock timezone EST -5 0
clock summer-time EDT recurring
!
ntp server 1.1.1.1

Example 2: R2 Config

clock timezone EST -5 0
clock summer-time EDT recurring
!
ntp server 1.1.1.1

Example 3: R3 Config

Commentary

The configuration of NTP is vital on production networking equipment as it ensures that the timestamps that are placed on all logs and events will be referenced to the same time source. Without NTP, any troubleshooting effort that expands past a single device can be complicated to track down. In a typical network, there will be a master source (or sources) that are referenced by all of the devices in the network; each of the network devices will then reference their clocks back to these sources.

For this lab you were tasked with configuring the routers to use NTP. R1 needs to be configured as the main clock in the network while R2 and R3 use NTP as clients, referencing R1. All of the routers also need to be configured to use Eastern Standard Time AND Eastern Daylight Time.

To configure R1 in NTP client/server mode, R1 requires a few extra NTP commands. The first step is to configure it as the master clock source in the network (stratum 3 as suggested in the lab) using the ntp master 3 global command. The second step is to configure R1 to use pool.ntp.org using the ntp server pool.ntp.org command. The third step is to configure it to use its Loopback0 as the NTP source; to configure this, use the ntp source Loopback0 command.

All three routers happen to be in the same time zone and all use day light savings time, so all three routers:

• Set the use of Eastern Standard Time with the clock timeone EST -5 command.
• Set the router to switch to Eastern Daylight Time with the clock summer-time EDT recurring command.

R2 and R3 act as NTP clients, referencing R1 as the server, so their NTP configuration is identical. The first step is to configure so that they use R1 as their main clock source (1.1.1.1) with the ntp server 1.1.1.1 command. (Note that because R1 used the ntp source loopback0 command, R2 and R3 must reference the IP address used on R1’s loopback0 interface.)

Known Issues in this Lab

This section of each Config Lab Answers post hopes to help with those issues by listing any known issues with Packet Tracer related to this lab. In this case, the issues are:

In fact, you cannot perform the configuration requested in this lab using Cisco Packet Tracer. As an alternative, consider trying this alternative limited set of lab requirements if you want to try something that could work in CPT 8.0. Also, we expect this lab to be available over many versions of CPT, so you can always try this lab with CPT at versions after 8.0 and you might have better results with NTP.

The alternative more basic requirements:

Configure NTP for this network, as follows:

• Configure R1 to be the NTP server:
  • R1 should use its internal clock as its only time source, with Stratum 3.
  • Do not use a loopback interface address as the clock source.
• Configure R2 and R3 to be NTP clients, using R1 as their NTP server. Refer to R1 by IP address.
• Configure all routers to use Eastern Standard Time
• Assumptions:
  • All device interfaces shown in the lab are up, working, and correctly configured with IP addresses
  • Routing is configured and working correctly
  • Name resolution is configured and working on all devices

Figure 1: NTP Topology

Why Would Cisco Packet Tracer Have Issues?

(Note: The below text is the same in every Config Lab.)

Cisco Packet Tracer (CPT) simulates Cisco routers and switches. However, CPT does not run the same software that runs in real Cisco routers and switches. Instead, developers wrote CPT to predict the output a real router or switch would display given the same topology and configuration – but without performing all the same tasks, an actual device has to do. On a positive note, CPT requires far less CPU and RAM than a lab full of devices so that you can run CPT on your computer as an app. In addition, simulators like CPT help you learn about the Cisco router/switch user interface – the Command Line Interface (CLI) – without having to own real devices.

CPT can have issues compared to real devices because CPT does not run the same software as Cisco devices. CPT does not support all commands or parameters of a command. CPT may supply output from a command that differs in some ways from what an actual device would give. Those differences can be a problem for anyone learning networking technology because you may not have experience with that technology on real gear – so you may not notice the differences. So this section lists differences and issues that we have seen when using CPT to do this lab.

Beyond comparing your answers to this lab’s Answers post, you can test in Cisco Packet Tracer (CPT) or Cisco Modeling Labs (CML). In fact, you can and should explore the lab once configured. For this lab, once you have completed the configuration, try these verification steps. 

1. Use the show ntp status and show ntp associations command to verify the status of NTP and its associations.
2. Particularly when using CPT, note that NTP synchronization on Cisco devices takes time. For instance, it may take at least 10 minutes (or more) for all clocks to become synchronized (assuming your configuration was correct).