Router boots with a blank configuration after saving: You executed 'copy running-config startup-config' and rebooted, but the router came up with an empty configuration. What is the most likely cause?

Introduction / Context:
A Cisco router loading blank after you saved the configuration commonly indicates a configuration register problem. The register can instruct IOS to ignore startup-config, which is frequently used during password recovery and sometimes left unchanged by mistake.

Given Data / Assumptions:

• 'copy running-config startup-config' completed successfully.
• After reboot, the router shows the initial configuration dialog or no saved settings.
• No intentional factory reset was performed.

Concept / Approach:

If the configuration register is 0x2142, the router ignores NVRAM on boot. This exactly produces a blank configuration despite valid contents in startup-config. Restoring the default 0x2102 re-enables reading startup-config at boot time.

Step-by-Step Solution:

Verification / Alternative check:

After reload, the router should load the saved startup-config automatically. Confirm by checking interface IPs, routing processes, and banners.

Why Other Options Are Wrong:

NVRAM corruption (B) is possible but less common; most cases trace to the register.

Incorrect boot command or IOS compatibility (A, D) typically affect image loading, not reading startup-config specifically.

Common Pitfalls:

Leaving 0x2142 set after password recovery; confusing boot variable (boot system) with the configuration register; not saving the corrected setting before reload.

Final Answer:

The configuration register setting is incorrect.