Password Recovery on a Cisco 2500 Series Router
If you have built a home lab (or are running really ancient hardware in production), you may find yourself in need of resetting the password configured on a 2500 series router. Depending on the router platform, the syntax is different. Here's how to do it on a 2500:
1. Reboot the router and send a break sequence to enter ROMMON mode. Break sequences can be sent using a Ctrl+Break key combination, or if you are on a system that doesn't have a break sequence, try using your terminal software to send a break sequence.
2. (optional) Type the letter O and hit enter. Record the existing setting, it's most likely 0x2102.
3. To reset the configuration register and have the router bypass the startup-config, type the following:
To boot the router, type the letter i and hit enter.
>i System Bootstrap, Version 11.0(10c), SOFTWARE Copyright (c) 1986-1996 by cisco Systems 2500 processor with 14336 Kbytes of main memory
The router should load and ask you to enter the initial configuration bla bla bla. Skip it and go into config terminal and change the configuration register back to 0x2102 (or whatever it was in step 2 if different). The command to change the configuration register in global config mode is as follows:
You can also copy startup-config to running-config to load the config file, or you can just reboot.
Reference: Password Recovery Procedure (cisco.com)