Config Lab: OSPF Interface Config 1

Wendell Odom
By Wendell Odom September 21, 2021 09:05

From the first days of OSPF in Cisco routers, OSPF configuration has included a network command. The network command enables OSPF on that router’s interfaces, but it uses indirect logic. In fact, the network command does not list interface numbers. Cisco added an alternative OSPF configuration method a while back – a method that does directly enable OSPF on the interface, and avoids using the OSPF network command. I’m sure you guessed by now: today’s lab gives you a chance to practice configuring OSPF with this alternative method.

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

Requirements

Configure OSPF for the lab network shown in the figure. However, do not use the traditional configuration with network commands in OSPF configuration mode. Instead, use OSPF interface configuration. (This lab does not tell you the name of the command, in the spirit of letting you exercise your knowledge.)

The specific rules for this lab are:

  • Configure so each router uses a router-id of x.x.x.x where x equals the router number.
  • Do not rely on interface IP addresses for the setting of the router IDs.
  • Use OSPF area 0 for all interfaces
  • Enable OSPF directly on each interface, rather than using the indirect method and the OSPF network command
  • Assume all interfaces shown in the lab are up and working

 

Figure 1: Three Router Triangle with IP Subnets

 

Initial Configuration

Example 1, 2 and 3 show the beginning configuration state of R1, R2 and R3.

Example 1: R1 Config

 

Example 2: R2 Config

 

Example 3: R3 Config

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:

The interfaces in the Lab topology match the interfaces in the CML topology.

Lab Answers Below: Spoiler Alert

Lab Answers: Configuration (Click Tab to Reveal)

Answers

Figure 1: Three Router Triangle with IP Subnets

 

Example 4: R1 Config

 

Example 5: R2 Config

 

Example 6: R3 Config

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

Commentary

The legacy method of configuring OSPF uses network statements inside of OSPF router configuration mode. The network command causes the OSPF process to match the interfaces that will be included into the OSPF area specified. The alternative is to use commands inside interface configuration mode to specify that it will be included inside the OSPF area specified. This newer interface method is considered by many to be easier and a more readable configuration method.

First, for the OSPF process itself, the configuration sits at the bottom of each of the three examples (Examples 4, 5, and 6). The lab asked that we not rely on interface IP addresses to find an OSPF router ID, so R1 is configured with a router-id of 1.1.1.1, R2 with a router-id of 2.2.2.2 and R3 with a router-id of 3.3.3.3, using the router-id command. This command is still located under OSPF router configuration mode even when using interface configuration commands to specify the included OSPF interfaces.

Note the absence of network commands under the router ospf commands.

Individual ip ospf process-id area area-id interface subcommands replace the network command’s purpose. Each of these commands enables the OSPF process listed by number – process number 11 on router R1, 22 on R2, and 33 on R3. Note that as always, OSPF process IDs are local settings and do not need to match between neighboring routers. However, the Individual ip ospf process-id area area-id interface subcommands must refer to that local router’s own process ID.

Note that the Individual ip ospf process-id area area-id interface subcommands also refer to the OSPF area, which in this lab is area 0 in all cases.

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. Each router should have two OSPF neighbors; verify that fact with the show ip ospf neighbor command.
  2. Each router should list three OSPF-learned routes, which you can verify with the show ip route command. (If you view the links between routers as WAN links, the three OSPF routes will be for: the two remote LANs, plus the WAN link between the other two routers.)
  3. Verify which interfaces are enabled for OSPF with the show ip ospf interface brief command.
  4. Verify the OSPF router ID with the show ip ospf database command.

More Labs with Related Content!

Config Lab: L3 EtherChannel 1
Config Lab: OSPF Interface Config 2
Wendell Odom
By Wendell Odom September 21, 2021 09:05
Write a comment

4 Comments

  1. DaApprentice January 28, 08:26

    Hi Wendell,
    Love all you’ve done with your site. It has been very helpful to my studying for Cisco certs.

    Small thing I noticed, but thought you may want to know–your topology map shows interfaces on each router as g0/0, g0/1, and g0/2. However, your answers don’t list g0/0 but instead g0/3.

    Thanks again for all the work you and your team have done with Certskills.com

    Reply to this comment
    • certskills February 1, 13:19

      Thanks! Glad you like the site.
      I fixed the interface IDs as well – appreciate the help cleaning up the errors.
      Wendell

      Reply to this comment
  2. Kwadell February 8, 18:10

    Why for the process-id did you use 11,22,33? I used 1,2, and 3. Does it matter? I know each router is suppose to have their own unique process-id.

    Reply to this comment
    • certskills February 8, 18:29

      No reason, really. 1, 2, 3 work fine as well.
      FYI, note that all three routers could have used the same process ID number – they do not have to be different.

      Reply to this comment
View comments

Write a comment

Comment; Identify w/ Social Media or Email

Subscribe

Subscribe to our mailing list and get interesting stuff and updates to your email inbox.

Thank you for subscribing.

Something went wrong.

Search

Categories