Returning a Cisco Router to its original factory setting isn't a hard task at all. There are many methods that can be used, but I will show you the two most straight-forward approaches.

METHOD 1

Check the configuration register by issuing the 'show version' command or 'sh ver' for short. The configuration register will be displayed on the last line of the 'sh ver' output. The router should be set to 0x2102, if not issue the 'config-register 0x2102' once you are in global configuration mode. An example of this is below:

router# configure terminal
router(config)# config-register 0x2102
router(config)# end
router#

After entering this command, the 'sh ver' output will display (will be 0x2102 at next reload).

Now you can erase the current start-up configuration with the 'write erase' command or 'erase start'. This deletes all of the previous configuration information from NVRAM, so if you need to keep this info be sure and back up the data onto your terminal.

After you have erased the startup config, enter the 'reload' command. You will be asked to save the system configuration, reply with 'n' or 'no' and confirm.

router#reload
System configuration has been modified. Save? (yes/no): n
Proceed with reload? (confirm)

Once the router reloads, the user will be presented with a 'System Configuration Dialog'.

--- System Configuration Dialog ---
Would you like to enter the initial configuration dialog? (yes/no):

The router is now reset back to the original factory defaults.

METHOD 2

This method is similar to method 1, but uses a different config-register, specifically 0x2142. Enter the 'config-register 0x2142' command in global configuration mode, just as you did with 0x2102 in Method 1. This causes the router to ignore the contents of NVRAM which stores the startup-config.

router(config)# config-register 0x2142

Reload the router using the 'reload' command. Once again there is no reason to save the config when the router prompts before reload, simply say 'n' or 'no' and confirm.

router#reload
System configuration has been modified. Save? (yes/no): n
Proceed with reload? (confirm)

Once the router reloads, you will be presented with 'System Configuration Dialog', enter 'no' at the prompt.

--- System Configuration Dialog ---
Would you like to enter the initial configuration dialog? (yes/no): n

Change the configuration register back to 0x2102 by issuing the 'config-register 0x2102' in global configuration mode, just as you did in Method 1. Then you can issue the 'write erase' or 'erase start' commands to delete the startup configuration and reload. After reload you will see confirmation of your reset when the 'System Configuration Dialog' appears.