[Day 6] 8. Register Values and Password Recovery

0x2142:
- Bypasses the NVRAM startup configuration during the boot process.
- This is typically used for password recovery or when you want to boot the device without applying the saved configuration.
0x2100:
- Boots the device into ROMMON mode (ROM Monitor mode).
- This is used for troubleshooting, firmware recovery, or manual boot configuration.
0x2101:
- Boots the device into Mini IOS (also known as RXBOOT mode or Bootstrap mode).
- This mode provides a minimal IOS image, often used for diagnostics or basic operations.
0x2102 to 0x210F:
- Boots the device normally using the NVRAM startup configuration.
- 0x2102 is the default configuration register value for most devices, ensuring a normal boot sequence.

Configuration register values are stored in the router's NVRAM and can be modified using the
```
config-register
```
command in global configuration mode.
These values directly influence how the router behaves during boot-up, particularly in terms of where it looks for the IOS image and configuration files.

Power on the router and connect via the console.
Press Ctrl + Break to interrupt the boot sequence, entering ROMMON mode (or mini IOS).
Enter commands to change the configuration register to bypass NVRAM:
- 800 series:
```
set iso-conf-0x2142
```
- 2500 series:
```
o/r 0x2142
```
- 2600 series:
```
confreg 0x2142
```
Restart the router.
After reboot, when prompted for setup, select No.
Enter Router> mode, and type
```
enable
```
to enter Router# mode.
Run
```
A#copy start run
```
to copy the startup configuration to the running configuration.
Enter configuration mode with
```
A#conf t
```
.
Change the password with
```
A(config)#enable secret <newpassword>
```
.
Reset the configuration register with
```
A(config)#config-reg 0x2102
```
.
Exit configuration mode with
```
A#exit
```
.
Finally, save the running configuration with
```
A#copy run start
```
.