Lab Answers

Figure 1: Topology for this Lab

router ospf 1
 auto-cost reference-bandwidth 100000
!
interface GigabitEthernet0/0
 ip ospf cost 80
!
interface GigabitEthernet0/0/0
 ip ospf cost 50
!
interface GigabitEthernet0/2/0
 ip ospf cost 120

Example 1: R1 Config

Lab Commentary

First, all routers should use the same OSPF reference bandwidth setting, no matter what value is used. The lab asks you to configure a value of 100,000, so each router needs to be configured with the auto-cost reference-bandwidth 100000 command in OSPF config mode.

Once configured, note that the GigE interfaces now have a calculated metric of 100. The reference bandwidth uses a unit of Megabits per second (Mbps). Unless set explicitly, a router calculates cost as:

reference_bandwidth / interface_bandwidth.

Finally, router gigabit interfaces default to a bandwidth setting of 1,000,000 Kilobits per second (Kbps). To use the same Mbps unit in the calculation, 1,000,000 Kbps is the equivalent of 1,000 Mbps. Given those facts, each GigabitEthernet interface’s cost is calculated as:

100,000 / 1000 = 100

Next, consider router R1’s route to subnet 172.16.2.0/24, the subnet connected to both R2 and R3. After changing the reference bandwidth, but before changing the individual interface costs, that route would have a metric of 200, based on the detail in Figure 2:

Figure 2: Cost 200 Route from R1 to 172.16.2.0/24

Of note, the route’s cost is the sum of the costs shown in a box because those are outgoing interfaces for the route from R1 to subnet 172.16.2.0/24. 

The lab also requires you to solve a small puzzle. The lab does not tell you to change the cost of specific interfaces to specific values. Instead, it tells you:

• Configure R1 only.
• Make the cost of a route be some total.

To solve the puzzle, you have to think about each route to decide what interface costs will be added to find the cost of the route, and then change the cost of the interface on R1 to change the route’s total cost. Given the detail and analysis with Figure 2, you should configure R1’s G0/0/0 OSPF cost. The lab also asked you to cause that route to cost 150, and R2’s G0/0 cost of 100 cannot change. By configuring the ip ospf cost 50 command under R1’s G0/0/0 interface, you change the cost of that interface and the cost of R1’s route to subnet 172.16.2.0/24 to be 150.

Table 1 repeats the requirements from lab, now with a notation of the router, interface, and new cost value needed to achieve the lab goals.

Table 1: OSPF Route Costs to Achieve Through Configuration

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 owning 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. Check the OSPF metrics per the routers from the show ip route command, and compare those values to the values requested in the lab.
2. Use the show ip ospf interface command to confirm the OSPF cost of each interface on each router.