Understanding Configuration-register

The configuration register tells the router whether to use a full-featured IOS, ROMMON, or the limited-feature IOS, which is also called RXBOOT mode. The configuration register is a 16-bit software register in the router, and its value is set using the config-register global configuration command.

Binary version of Configuration register (0x2102) is shown here:

15 14 13 12	11 10 9 8	7 6 5 4	3 2 1 0
0 0 1 0	0 0 0 1	0 0 0 0	0 0 1 0

The boot field is the name of the low-order 4 bits of the configuration register. This field can be considered a 4-bit value, represented as a single hexadecimal digit. (Here it is set as 0x2102, it can be written as 0x2)

If the boot field is hex 0 (0x0), ROMMON is loaded.

If the boot field is hex 1 (0x1), RXBOOT mode is used.

For anything else (0x2-0xF), it loads a full-featured IOS.

Note: To ignore the NVRAM content during booting (for password recovery) set the bit 6 in config-register as 1, this will change the config-register value to 0x2142.