The show diag command displays information about which of the following?

The show diag command is used to provide detailed diagnostic information about the installed hardware on a Cisco device and its configuration. This includes specifics about various hardware components such as memory, processors, and physical interfaces. By running this command, network technicians can view the status and attributes of the hardware components, which is essential for troubleshooting hardware issues or verifying the configuration of the device.

Understanding hardware configurations is critical in network management, as it allows technicians to check for compatibility issues, ensure that components are functioning correctly, and assess whether upgrades or replacements are needed. The depth of information presented can also help in fine-tuning performance or resolving problems related to specific hardware components.

The other options pertain to more specialized command outputs that serve different purposes. For instance, ISDN interface status and identifiers is covered under a different command focused specifically on ISDN links. Similarly, router interface speed and status are revealed through the interface-specific show commands, while CDP (Cisco Discovery Protocol) neighbors and their capabilities are shown through a distinct command dedicated to discovering other devices on the network. Therefore, the show diag command remains uniquely focused on hardware diagnostics and configuration details.