Initial startup of Cisco routers:

When a Cisco router powers up, it performs a power-on self test (POST). During this self test, the router executes diagnostics from ROM on all hardware modules. After the POST, the following events occur.

1. ROM

1. POST
2. Bootstrap code executed
3. Check Configuration Register value (NVRAM)
0 = ROM Monitor mode
1 = ROM IOS
2 - 15 = startup-config in NVRAM

2. Check for IOS boot system commands in startup-config file (NVRAM). If boot system commands in startup-config

a. Run boot system commands in order they appear in startup-config to locate the IOS
b If boot system commands fail, use default fallback sequence to locate the IOS (Flash, TFTP, ROM)

3. Locate and load IOS, Default fallback sequence: No IOS boot system commands in startup-config

a. Flash (sequential)
b. TFTP server (netboot) - The router uses the configuration register value to form a filename from which to boot a default system image stored on a network server.
c. ROM (partial IOS) or keep retrying TFTP depending upon router model
- If no IOS located, get partial IOS version (mini IOS) from ROM

4. Locate and load startup-config

a. If startup-config found, copy to running-config
b. If startup-config not found, prompt for setup-mode
c. If setup-mode bypassed, create a “skeleton” default running-config (no startup-config)