Which command would you use to diagnose WAN configuration issues?

The command used to diagnose WAN configuration issues is "show controllers." This command provides critical insights into the operational status of the physical and logical connections of WAN links. Specifically, it displays information about the hardware controllers associated with the WAN interfaces, revealing details about the state of the link, any detected errors, and the configuration details of the corresponding physical layer.

This information is essential for diagnosing complex WAN issues such as frame relay or T1/E1 configurations where the health of the link can be impacted by various factors such as line coding, synchronization problems, or hardware malfunctions. By using this command, a technician can identify if the issues lie at the controller level, facilitating effective troubleshooting and ensuring the WAN connection can be restored.

In contrast, the other commands provide information that is less specific to WAN configurations. For example, "show interfaces" displays statistics about all interfaces on the device, but might not give detailed insights into the controllers themselves. "show diag" typically provides a diagnostic overview of the hardware installed in the device, such as module types and their statuses, rather than focusing specifically on WAN connectivity. Meanwhile, "show cdp neighbors" offers details about directly connected Cisco devices but does not touch on specific WAN configuration diagnostics.