Config Lab: Telnet Config

Wendell Odom
By Wendell Odom August 31, 2021 11:05

Today’s post takes us from a cleanly wiped router config (except hostnames), to one router being ready for Telnet support. As usual, you can do the lab on paper or in a text editor in just a few minutes, or use Cisco Packet Tracer or Cisco Modeling Labs.

All about Config Labs

The blog has a series of lab exercises called “Config Labs.” Each lab presents a topology with the relevant initial configuration for each device. The lab also lists new requirements, after which you should create the additional configuration to meet those requirements. You can do the lab on paper, in a text editor, or use software tools like Cisco Packet Tracer or Cisco Modeling Labs.

Once you have created your answer, you can click various tabs at the bottom of this post to see the lab answers, comments about the lab, and other helpful information.

The Lab Exercise

Cisco routers support inbound Telnet connections. However, to allow Telnet, the router needs some non-default configuration added. For this lab, you need to identify the bare minimum of non-default commands that must be configured to allow Telnet into the router.

Configure router R1 in the figure below. The initial config has all default configuration other than the hostname R1 command. (The switch has all default configuration, notably with both ports in the same VLAN.) Your job is to list the configuration on R1 such that:

  1. A user at PC1, in the same VLAN as R1’s G0/0 interface, can Telnet successfully into R1.
  2. The user has to supply only a password; no username is required.
  3. Configure no unnecessary commands (use defaults).
  4. Assume PC1, as well as all LAN devices, have already been configured correctly.
  5. Use the numerically highest IP address in the subnet for any router IP address configuration.

Figure 1 – Lab Topology for this Lab

Answer Options - Click Tabs to Reveal

You can learn a lot and strengthen real learning of the topics by creating the configuration – even without a router or switch CLI. In fact, these labs were originally built to be used solely as a paper exercise!

To answer, just think about the lab. Refer to your primary learning material for CCNA, your notes, and create the configuration on paper or in a text editor. Then check your answer versus the answer post, which is linked at the bottom of the lab, just above the comments section.

You can also implement the lab using the Cisco Packet Tracer network simulator. With this option, you use Cisco’s free Packet Tracer simulator. You open a file that begins with the initial configuration already loaded. Then you implement your configuration and test to determine if it met the requirements of the lab.

(Use this link for more information about Cisco Packet Tracer.)

Use this workflow to do the labs in Cisco Packet Tracer:

  1. Download the .pkt file linked below.
  2. Open the .pkt file, creating a working lab with the same topology and interfaces as the lab exercise.
  3. Add your planned configuration to the lab.
  4. Test the configuration using some of the suggestions below.

Download this lab’s Packet Tracer File

You can also implement the lab using Cisco Modeling Labs – Personal (CML-P). CML-P (or simply CML) replaced Cisco Virtual Internet Routing Lab (VIRL) software in 2020, in effect serving as VIRL Version 2.

If you prefer to use CML, use a similar workflow as you would use if using Cisco Packet Tracer, as follows:

  1. Download the CML file (filetype .yaml) linked below.
  2. Import the lab’s CML file into CML and then start the lab.
  3. Compare the lab topology and interface IDs to this lab, as they may differ (more detail below).
  4. Add your planned configuration to the lab.
  5. Test the configuration using some of the suggestions below.

Download this lab’s CML file!

 

Network Device Info:

This table lists the interfaces listed in the lab exercise documentation versus those used in the sample CML file.

Device Lab Port  CML Port
SW1 F0/1 G0/0

Lab Answers Below: Spoiler Alert

Lab Answers: Configuration (Click Tab to Reveal)

Answers: Configuring Telnet

This lab asks for the required, non-default configuration to enabled Telnet on router R1, with other conditions. Check out the original lab problem statement for the details. Figure 1 repeats the network diagram, and Example 1 lists the answer.

Figure 1: Lab Topology with IP Subnet

This lab does require that you do a little subnetting math as well. The requirements state for the router to use the highest IP addresses in the subnet, but the lab did not list the specific IP address to be used for each router interface. R1 needs an IP address on its G0/0 interface, which is shown in subnet 172.18.1.0/26. That subnet has a range of usable addresses from 172.18.1.1 – 172.18.1.62, so the configuration uses the .62 address for R1’s F0/0.

Example 1: R1 Telnet Config

Commentary, Issues, and Verification Tips (Click Tabs to Reveal)

Cisco routers and switches allow a simple security method for both console and Telnet access. Basically, all you have to do is tell the device to use that method by supplying a prompt for the password (the login subcommand) and defining the password’s value (the password subcommand).

Note that the answer shows the configuration of the VTY password (to support Telnet) with VTYs 0 through 4 as separate from the configuration of VTYs 5 through 15. This quirk of Cisco output has to do with the fact that older IOS versions support only VTYs 0 through 4. You could have used the commands literally shown in the answer example, or you could have used the command line vty 0 15, followed by those same password and login commands.

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:

# Summary Detail
1 None No known issues related to this lab.

 

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. Open a command prompt on the PC and test connectivity to R1’s G0/0 IP address using the ping command.
  2. If successful, test Telnet using the telnet command to that same IP address. If successful, you should be prompted for a password, and if you supply the password you just configured, you should reach the user mode prompt of the router.

More Labs with Related Content!

Config Lab: CLI Miscellany 1
Config Lab: SSH Config
Wendell Odom
By Wendell Odom August 31, 2021 11:05
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2 Comments

  1. Alfonso February 21, 14:43

    third lab with no packet tracer file. I guess all the lab files moved? Please advise as I really want to do all of your labs. thank you

    Reply to this comment
    • certskills February 28, 16:12

      Alfonso,
      Thanks for the note. The site had a problem that was introduced by some Linux maintenance applied by the hosting company. Should all be good as of later that same day.

      Great to hear of your enthusiasm for doing the labs!

      Reply to this comment
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