To reset a Cisco router to factory default (removing the startup configuration file), perform these steps:
1. To erase the configuration file, issue the erase nvram: command.
Reload the router by issuing the reload command.2.If this does not solve the problem, attempt to break into ROM Monitor (ROMmon) by issuing the break sequence (usually Ctrl and break from the Hyperterminal) from a console connection.
You should see this ROMmon prompt:
3.Change the configure register value to ignore the startup configuration by issuing the confreg command, as shown in this example:
rommon 2> confreg 0x2142
4.To reload the router, issue the reset command, as shown in this example:
rommon 3> reset
5.After the router boots, issue the enable command at the Router > prompt.
The prompt changes to Router#, indicating that the router is now in privileged mode.
6.To enter config mode, issue the config terminal command.
You should now see a Router(config)# prompt.
7.To change the configure register to recognize the startup configuration, issue the config-register command, as shown in this example:
Router (config)# config-register 0x2102
To break out of configuration mode, press Cntl and Z.
To save the blank configuration, issue the copy-running config-startup config command.