Lab Commentary

Although you configure a pair of switches for this lab, the configuration uses commands associated more with routing than switching because the lab enables routing on the switches.

First, most Cisco switches capable of routing – that is, layer 3 switching – require the ip routing global command. Note that in some switch models, you may also need to use another command to enable some low-level hardware functions in the switch ASIC so that the ASIC will perform routing. Because those commands vary from device to device, this lab does not require adding any such command.

Next, the lab specifically asks you not to use Switched Virtual Interfaces (SVIs) but instead use the physical ports as routed ports instead of as switch ports. Using the no switchport interface subcommand, you tell the switch to stop using layer 2 switching logic on the port but instead use layer 3 routing logic, making that port a routed port.

Once the switch treats a port as a routed port, it allows the direct configuration of an IP address and mask on the port. Note that before you issue the no switchport command, the ip address interface subcommand is rejected.

Finally, the two commands suggested to enable OSPF should enable OSPF on all three ports on each distribution switch, placing those in OSPF area 0. Both R1 and R2 use identical OSPF configuration. Once OSPF has time to form neighbor relationships and converge, all four devices should learn routes to all six subnets and be able to ping all other devices’ interface IP addresses.

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. The distribution switches act as layer 3 switches, routing IP packets. The links between the routers and layer 3 switches include six subnets. Confirm each switch has routes to all six of those subnets using the show ip route command.
2. For each distribution switch, ping the IP address of the device on the other end of each link.