Answers

Figure 1: Two Routers with IP Subnets

Scenario 1

First, here are the answers for Scenario 1:

Example 3: R1 Config

Example 4: R2 Config

Scenario 2

And here are the answers for Scenario 2:

Example 5: R1 Config, Scenario 2

Example 6: R2 Config, Scenario 2

Commentary

The legacy method of configuring OSPF uses network statements inside of OSPF router configuration mode. This statement is used by 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.

The most problematic area that people have with the legacy configuration of OSPF is with the network command, specifically the wildcard mask. The wildcard mask (as used with IP Access Control Lists, or ACLs) limits the parts of the interface IP addresses that is compared to the number in a network command.

Deeper Background

While this lab does not get into all the theory of wildcard masks, but I felt the need for a little more detail than normal in this case. To match all addresses in a classful network, you need to use one of three easily-understood wildcard masks. In particular:

• Class A: Wildcard mask 0.255.255.255 means “compare the first 1 octet, ignore the last 3”, which is useful for matching all addresses in a class A network
• Class B: Wildcard mask 0.0.255.255 means “compare the first 2 octets, ignore the last 2 octets”, which is useful for matching all addresses in a class B network
• Class C: Wildcard mask 0.0.0.255 means “compare the first 3 octets, ignore the last 1 octet”, which is useful for matching all addresses in a class C network

To match all addresses in the subnet connected to an interface, you have two calculate two values. First, calculate the subnet ID as usual. Then, calculate the wildcard mask by subtracting the subnet mask from 255.255.255.255. That is, if you subtract a subnet mask (in dotted decimal form) from 255.255.255.255, the resulting wildcard mask can be used when matching all packets in a subnet that uses that mask. For instance:

• To match subnet 10.1.1.0 255.255.255.0, use wildcard mask 0.0.0.255.
• To match subnet 10.1.1.0 255.255.255.192, use wildcard mask 0.0.0.63.
• To match subnet 10.1.1.0 255.255.255.224, use wildcard mask 0.0.0.31.

This Lab’s Answers: First Scenario

The first set of answers (Examples 3 and 4, for routers R1 and R3, respectively) show the first requirement: a configuration with each network command matching the IP addresses in each classful network. As it turns out, R1 connects to one classful network (192.168.4.0), so only the network 192.168.4.0 0.0.0.255 area 0 command is needed. Note that this command also sets the area number.

R2’s two subnets happen to sit in two different classful networks (192.168.4.0 and 192.168.2.0), requiring two network commands.

This Lab’s Answers: Second Scenario

The second set of examples (Examples 5 and 6, again for routers R1 and R2) show the solution with the requirement that each network command match only the addresses in a single subnet. To create this configuration, each network command uses the DDN subnet mask, subtracted from 255.255.255.255. In this case, that value is 0.0.0.15. So, in Example 5, for R1:

Network 192.168.4.0 0.0.0.15 area 0 – matches R1’s G0/1 interface (and all addresses in that subnet)

Network 192.168.4.128 0.0.0.15 area 0 – matches R1’s G0/2 interface (and all addresses in that subnet)

And on R2 (Example 6):

Network 192.168.4.0 0.0.0.15 area 0 – matches R2’s G0/1 interface (and all addresses in that subnet)

Network 192.168.2.192 0.0.0.15 area 0 – matches R2’s G0/2 interface (and all addresses in that subnet)

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. Each router should have one OSPF neighbor, so verify that fact with the show ip ospf neighbor command.
2. Each router should list one OSPF-learned route, which you can verify with the show ip route command. The route should list the other router’s LAN subnet as the destination subnet.
3. Verify which interfaces are enabled for OSPF with the show ip ospf interface brief command.