Answers

Figure 1: Multi-area OSPF Topology

router ospf 50
 router-id 1.1.1.1
!
interface GigabitEthernet0/1
 ip ospf 50 area 0
!
interface GigabitEthernet0/2
 ip ospf 50 area 10

Example 1: Core1 Config

router ospf 50
 router-id 2.2.2.2
!
interface GigabitEthernet0/1
 ip ospf 50 area 0
!
interface GigabitEthernet0/2
 ip ospf 50 area 20

Example 2: Core2 Config

interface loopback 0
 ip address 172.30.99.1 255.255.255.255
!
router ospf 50
!
interface GigabitEthernet0/1
 ip ospf 50 area 10
!
interface GigabitEthernet0/2
 ip ospf 50 area 10

Example 3: Branch1 Config

interface loopback 0
 ip address 172.30.99.2 255.255.255.255
!
router ospf 50
!
interface GigabitEthernet0/1
 ip ospf 50 area 20
!
interface GigabitEthernet0/2
 ip ospf 50 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 ip ospf interface subcommands. This style of OSPF configuration can be simple to configure, especially if you have good documentation about router interfaces and areas. This lab supplies a figure with the routers, interface numbers, and areas. For about half of the requirements of this lab, all you have to do is issue a command like ip ospf 50 area 0 or similar under each interface.

Work through examples 1 through 4, focusing on only the ip ospf commands under each interface. Compare those commands to Figure 1. You should see the ip ospf 50 area 0 command on both Core1 and Core 2, under the interfaces that connect Core1 and Core2. This command references each router’s local process ID (50), and the area number into which to put that link. Continuing through the various examples:

• Router Branch1 uses the same ip ospf 50 area 10 command on both interfaces, enabling OSPF process 50 on both, and placing both in area 10.
• Router Branch2 uses the same ip ospf 50 area 20 command on both interfaces, enabling OSPF process 50 on both, and placing both in area 20.
• Router Core1 uses the ip ospf 50 area 10 command on its G0/2 interface (which connects to router Branch1), placing that link into area 10.
• Router Core2 uses the ip ospf 50 area 20 command on its G0/2 interface (which connects to router Branch1), placing that link into area 20.

As for the routers’ process IDs, the lab asks that all routers use process ID 50. Note that all show the router ospf 50 command as a result.

Finally, on the topic of OSPF router IDs (RIDs), the requirements asked you to explicitly configure the RID on Core1 and Core2, so those routers list the router-id 1.1.1.1 and router-id 2.2.2.2 commands, respectively. For the two branch routers, just for variety, the lab asked you to let these routers set their RID based on the IP addresses of their loopback0 interfaces. To make that work, you have to first create the loopback interface (interface loopback 0), and then configure the IP address. (Note that if you tried this on your own lab, you need to create the loopback interface first, before bringing up the OSPF process, so that the loopback interface is up when OSPF comes up.)

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 172.30.1.65 source 172.30.1.1 command to issue an extended ping from Branch1’s G0/2 interface IP address (172.30.1.1) to router Branch2’s G0/2 interface address (172.30.1.65).