Config Lab: Multi-area OSPF 1

Wendell Odom
By Wendell Odom September 21, 2021 17:05

When using the OSPF network command, whether single area of multi-area, you have to be careful with the wildcard mask used and which interfaces each network command matches. With multiple areas, that concern increases, because incorrect matching can place a router interface into the wrong area. This latest OSPF lab gives you a small multi-area OSPF design and asks you to enable OSPFv2 with network commands, for some practice with those network commands in particular.

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 multi-area OSPFv2 (that is, OSPF for IPv4) on the four routers shown in the figure. Use the area design shown in the figure. The configuration should use OSPF network commands, and not use interface subcommand ip ospf. The specific rules for this lab are:

  • Use an OSPF process-ID of 10 on all routers
  • Use only the network command to enable OSPF on an interface.
  • Use one network command for each router interface shown in the figure. That network command should use a wildcard mask that matches all IP addresses in the subnet connected to the interface. (Note: I added this rule in lab just so that there is one correct way to answer this lab; in real life many different wildcard masks could be used.)
  • Configure router-ID’s explicitly in OSPF configuration mode, with router ID’s as follows:
      • Core1: 1.1.1.1
      • Core2: 2.2.2.2
      • Branch1: 10.10.10.10
      • Branch2: 20.20.20.20
  • Use all default OSPF parameters unless otherwise requested.
  • Assume all device interfaces shown in the lab are up, working and with correct IP addresses assigned.

Figure 1: Multi-area Topology

 

Initial Configuration

Example 1, 2, 3 and 4 show the beginning configuration state of Core1, Core2, Branch1 and Branch2.

Example 1: Core1 Config

 

Example 2: Core2 Config

 

Example 3: Branch1 Config

 

Example 4: Branch2 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 CML topology matches this lab topology.

Lab Answers Below: Spoiler Alert

Lab Answers: Configuration (Click Tab to Reveal)

Answers

Figure 1: Multi-area OSPF Topology

 

Example 1: Core1 Config

 

Example 2: Core2 Config

 

Example 3: Branch1 Config

 

Example 4: Branch2 Config

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

Commentary

For this lab you are tasked with configuring the four different routers with OSPF and placing each of the interfaces into their respective areas by using network commands. If you have mastered how to configure OSPF network commands, the lab is relatively basic, and all you have to do is work through the details. If you are still unsure about the OSPF network command, this lab gives you several chances to practice. When creating your configuration, keep these key points in mind:

  • The OSPF network command’s wildcard mask works like an ACL wildcard mask
  • If more than one network command exists, the OSPF process places an interface into a particular OSPF area based on the first network command to match the interface

So, a multi-area OSPF configuration is not that much more complex than a single area configuration, but it may cause more problems if you get the configuration incorrect.

For this review, begin with router Core1 (found in Example 1). Per the requirements, Core1 should use process ID 10 with the router ID 1.1.1.1 explicitly configured, so the config begins with the router ospf 10 command followed by the router-id 1.1.1.1 command. Core1 then needs two network statements, because the requirements ask for wildcard masks that cause each network command to match all the addresses in one subnet. In the case of Core1, it connects to two subnets, with this result:

  • Core1 G0/1, uses subnet 100.100.100.0 with mask 255.255.255.252, so the OSPF configuration should use network 100.100.100.0 0.0.0.3 area 0
  • Core1 G0/2 uses subnet 100.100.100.128 with mask 255.255.255.192, so the OSPF configuration should use network 100.100.100.128 0.0.0.63 area 10.

Note that Core1’s two network commands place those interfaces in two different areas, making Core1’s configuration be multi-area.

Core2’s configuration follows the exact same concepts, but with different details. In particular, for the network commands:

  • Core2 G0/1, uses subnet 100.100.100.0 with mask 255.255.255.252, so the OSPF configuration should use network 100.100.100.0 0.0.0.3 area 0
  • Core2 G0/2 uses subnet 100.100.100.192 with mask 255.255.255.192, so the OSPF configuration should use network 100.100.100.192 0.0.0.63 area 20.

Like Core1, Core2 also serves as an ABR because it connects to two different areas.

For the two branch routers, both have two interfaces, but each router is an internal router: Branch1 internal to area 10, and Branch2 internal to area 20. Working through the logic for the various network commands on routers Branch1 and Branch2:

  • Branch1 G0/1 uses subnet 100.100.100.128 with mask 255.255.255.192, so the OSPF configuration should use network 100.100.100.128 0.0.0.63 area 10
  • Branch1 G0/2 uses subnet 100.100.101.0 with mask 255.255.255.128, so the OSPF configuration should use network 100.100.101.0 0.0.0.127 area 10
  • Branch2 G0/1 uses subnet 100.100.100.192 with mask 255.255.255.192, so the OSPF configuration should use network 100.100.100.192 0.0.0.63 area 20
  • Branch2 G0/2 uses subnet 100.100.101.128 with mask 255.255.255.128, so the OSPF configuration should use network 100.100.101.128 0.0.0.127 area 10.

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. Verify the OSPF neighbor relationships with the show ip ospf neighbor command. The two core routers should list two neighbors each, and the two branch routers should list one neighbor each.
  2. Check the routing tables, using the show ip route command, to ensure that all known networks have been learned on each device. Each router should list five subnets, with 3 subnets learned by OSPF.
  3. From the branch routers, also test connectivity with the ping command. For instance, on router Branch1, use the ping 100.100.101.254 source 100.100.101.126 command to issue an extended ping from Branch1’s G0/2 interface IP address (100.100.101.126) to router Branch2’s G0/2 interface address (100.100.101.254).

More Labs with Related Content!

Config Lab: OSPF Network Config 2
Config Lab: Multi-area OSPF 2
Wendell Odom
By Wendell Odom September 21, 2021 17:05
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