Cisco Router Boot Sequence

1. The router performs Power-On Self Test (POST) to discover the hardware components and verify that all components work properly.

2. The router copies a bootstrap program from ROM into RAM and run the bootstrap program.

3. The bootstrap program decides which IOS image to load into RAM based on the low-order four bits in configuration register:
            0 = use ROMmon OS
            1 = load first IOS file in flash
            2-F = a) try each boot system command in startup config in order until one works
                     b) if none of the boot system command works load first IOS file in flash

4. If the bootstrap program has loaded IOS, IOS finds configuration file (typically startup config in NVRAM) and loads it into RAM as running-config. If startup config cannot be located in NVRAM, enter setup mode.

5. If no IOS file is found in flash or tftp, simply load ROMmon (an IOS file be copied into flash from a tftp using ROMmon).

Configuration Register:
0x2102 –  default
0x2142 – ignore NVRAM contents

Router(config)#config-register value

boot system commands:
The first file from flash is loaded

Router(config)#boot system flash

IOS with the filename is loaded from flash

Router(config)#boot system flash filename

IOS with the filename is loaded from tftp server

Router(config)#boot system tftp filename address

Reference: CCNA ICND1 Official Cert Guide (3rd Ed) by Wendell Odom