How to Reset a Cisco 3900 Router?

Resetting a Cisco 3900 Series router can be done in two main ways: a soft reset (to factory defaults through the CLI) and a hard reset (using the router’s mode button). Here's how to do both:

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✅ Method 1: Factory Reset via CLI (Command-Line Interface)

Use this if you have access to the router's console.

Step-by-Step:

1. Connect to the Router

   • Use a console cable and connect via terminal emulator (e.g., PuTTY, Tera Term).

2. Enter Privileged EXEC Mode

   shell
   Router> enable
   

3. Erase Startup Configuration

   arduino
   Router# write erase
   

   or

   arduino
   Router# erase startup-config
   

4. Confirm Erasure

   • The router will prompt to confirm. Type y or yes.

5. Reload the Router

   arduino
   Router# reload
   

6. Do Not Save When Prompted

   • When asked to save the configuration, type no.

After reboot, the router will return to its default (out-of-box) settings.

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🔁 Method 2: Physical Reset (Using the Mode Button)

Use this if you've lost CLI access or password.

Steps:

1. Power Off the Router

   • Unplug the power cable.

2. Press and Hold the Mode Button

   • It’s usually on the front panel of the device.

3. Power On While Holding the Mode Button

   • Keep holding the button for 10–15 seconds until the router enters ROMMON mode (you'll see the prompt change in the console: rommon>).

4. Initialize Flash File System

   shell
   rommon> confreg 0x2142
   rommon> reset
   

   • This bypasses the startup configuration, letting you enter the router without a password.

5. Access Privileged Mode

   shell
   Router> enable
   

6. Erase Existing Config

   arduino
   Router# write erase
   Router# reload
   

7. Reset Config Register
   After reload, run:

   arduino
   Router> enable
   Router# conf t
   Router(config)# config-register 0x2102
   Router(config)# end
   Router# write memory
   

This restores the router to factory defaults and ensures it boots normally next time.

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⚠️ Important Notes

• Always backup configurations before resetting if possible.

• After reset, the router will lose all custom settings (IP addresses, passwords, VLANs, etc.).

• If you’re working in a production environment, notify stakeholders before reset.

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Need help with a specific configuration after reset (e.g., setting up interfaces or routing)? Let me know!