How to bring back the default configuration on a Cisco device running IOS

1. Connect to the IOS device (Switch or Router) via telnet or Console.
2. Type  “enable“ and press enter to go to privileged mode.
3. Type  “write erase“ and press enter to clear the config from nvram.
4. Type “reload“ and press enter. When asked whether to save the configuration say “no“.
   The IOS device will reboot and come back up with the default configuration.
5. When the device comes back up. Go back into privileged mode (step 2 above).
6. Type “config-register 0×2102“ and press enter.
7. Type “write mem“ and press enter.

Now you can create a new configuration and it will be recognized if the IOS device is restarted.