Answers

Figure 1: Device Discovery Topology

lldp run
!
interface GigabitEthernet0/1
 no cdp enable
!
interface GigabitEthernet0/2
 no lldp transmit
 no lldp receive

Example 1: R1 Config

lldp run
!
interface GigabitEthernet0/1
 no cdp enable
!
interface GigabitEthernet0/2
 no lldp transmit
 no lldp receive

Example 2: R2 Config

no cdp run
lldp run

Example 3: R3 Config

no cdp run
lldp run

Example 4: R4 Config

no cdp run
lldp run

Example 5: SW2 Config

Commentary

The default behavior of all Cisco devices is to have the Cisco Discovery Protocol (CDP) enabled on all interfaces. This behavior works well as long as the other deployed equipment in the network is either from Cisco or from a vendor that supports CDP. However, in cases where the connected device happens to use the standards-based Link Layer Discovery Protocol (LLDP), you need to disable CDP and enable LLDP.

For this lab, you were tasked with several configuration tasks that test your ability to configure different variations of CDP and LLDP; the specific order of the configuration is subjective.

On R1 and R2, the first task to configure is to enable LLDP globally with the lldp run global command. Then to run CDP only on some interfaces and LLDP only on other interfaces, you need to use some interface subcommands. Specifically, to disable CDP on their G0/1 interfaces, use the no cdp enable interface subcommand. LLDP uses a slightly different command; to disable LLDP on the R1 and R2 G0/2 interfaces, use both the no lldp transmit and no lldp receive interface subcommands.

For R3 and R4, because both use LLDP only, you can disable CDP completely and enable LLDP on all interfaces. First, to disable CDP globally, configure the no cdp run global command. Then, to enable LLDP globally, which automatically enables LLDP for both transmit and receive on all interfaces, configure the lldp run global command.

Next, consider the switches. Switch SW1 needs only default configuration, with CDP enabled globally, and LLDP disabled globally, so no new configuration is needed. Example 11 shows the configuration needed on switch SW2 to reverse those settings so that SW2 runs LLDP but not CDP.

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. Connect to any of the routers and display the LLDP and CDP neighbors. Use commands like show cdp neighbors, show lldp neighbors, show cdp interfaces, and show lldp interfaces. For instance, R1 should see SW1 as a neighbor with CDP and SW2 as a neighbor with LLDP. SW2 should see R1, R2, R3, and R4 as a neighbor with LLDP.