Configuration Register Fields

Understanding the boot parameters is pretty important for support tasks such as enabling or disabling the Break key, setting the baud rate, booting an alternative image, password recovery, and upgrading the image software. Take a few minutes to review the configuration register bits in Figure B-2. Use show version to see what your config-register is set to and lookup the value in Figure B-2.

The config-register Boot field determines how the router boots. ROMMON and RxBoot (bootflash) are two different implementations of the IOS subset. Be aware, however, that not all models have RxBoot to act as a boot helper. ROMMON performs a diagnostic check and either keeps the device in ROMMON (> or rommon>) or loads RxBoot (router(boot)>). ROMMON only provides you with one-letter commands. Boot helper mode keeps the device in rxboot or bootflash, where it has host-mode functionality and checks the startup-config or loads the fully functional IOS (router>).