Choose Which Cisco OS to Boot

Let’s have a look at what Cisco OS gets booted at startup and how we can influence that deciscion using the configuration register.

The configuration register is a 2-byte field, represented as 4 hex digits, that you can set with the config-register global configuration command.

The last nibble of the register is called the boot field. This is the logic it uses:

1. If boot field = 0, use the ROMMON OS.
2. If boot field = 1, load the first IOS file found in flash memory.
3. If boot field = 2-F:
   • Try each boot system command in the startup-config file, in order, until one works.
   • If none of the boot system commands work, load the first IOS file found in flash memory.
4. If all other attempts fail, load ROMMON, from which you can perform further steps to recover by copying a new IOS image into flash.

– ICND1 Chapter 35

The process visually (image taken from the same source as the quote):