Answers

Figure 1: Multi-area OSPF Topology

router ospf 10
 router-id 1.1.1.1
 network 100.100.100.0 0.0.0.3 area 0
 network 100.100.100.128 0.0.0.63 area 10

Example 1: Core1 Config

router ospf 10
 router-id 2.2.2.2
 network 100.100.100.0 0.0.0.3 area 0
 network 100.100.100.192 0.0.0.63 area 20

Example 2: Core2 Config

router ospf 10
 router-id 10.10.10.10
 network 100.100.100.128 0.0.0.63 area 10
 network 100.100.101.0 0.0.0.127 area 10

Example 3: Branch1 Config

router ospf 10
 router-id 20.20.20.20
 network 100.100.100.192 0.0.0.63 area 20
 network 100.100.101.128 0.0.0.127 area 20

Example 4: Branch2 Config

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 20.

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:

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).