Cisco Device Boot-Up Sequence

Cisco routers can boot Cisco IOS software from these locations:

1. Flash memory
2. TFTP server
3. ROM (not full Cisco IOS)

Default Boot Sequence for Cisco IOS software:

1. NVRAM
2. Flash (sequential)
3. TFTP server (network boot)
4. ROM (partial IOS)

Booting up the router and locating the Cisco IOS

1. POST (power on self test)
2. Bootstrap code executed
3. Check Configuration Register value (NVRAM) which can be modified using the config-register command
   1. 0 = ROM Monitor mode
   2. 1 = ROM IOS
   3. 2 – 15 = startup-config in NVRAM
4. Startup-config file: Check for boot system commands (NVRAM)
5. If boot system commands in startup-config
   1. Run boot system commands in order they appear in startup-config to locate the IOS
      1. [If boot system commands fail, use default fallback sequence to locate the IOS (Flash, TFTP, ROM)?]
   2. If no boot system commands in startup-config use the default fallback sequence in locating the IOS:
      1. Flash (sequential)
      2. TFTP server (netboot)
      3. ROM (partial IOS) or keep retrying TFTP depending upon router model
6. If IOS is loaded, but there is no startup-config file, the router will use the default fallback sequence for locating the IOS and then it will enter setup mode or the setup dialogue.
7. If no IOS can be loaded, the router will get the partial IOS version from ROM