1. Preforming a power-on self test (POST)
2. Loading a bootstrap program
3. Loading an IOS
4. Loading a configuration file
The last three of these steps require the router to copy the files into RAM. Normally, most routers load an IOS image that is stored in flash memory and an initial configuration stored in NVRAM.
2. Routers use the following logic, in order, to attempt to load an IOS: 1. Load a limited-function IOS if the configuration register’s last hex digit is set to 0 or 1. 2. Load an IOS based on the configuration of boot system commands in the startup-configuration file. 3. Load the first file in flash as the IOS.
4. Use TFTP broadcasts to find a TFTP server and download an IOS from that server. 5. Load a limited-function IOS from ROM.
In Step 1, the configuration register is a16-bit number that Cisco routers store in a hidden area of NVRAM. It can be set in a couple of ways and can be seen using the show version command. The most commonly used way to set its value is to use the config-register global configuration command. Routers use the 4 low-order bits of the configuration register as the boot field, which tells the router what to do in the first of the preceding five decision steps. If the value of the field is set to 0, the router loads the ROMMON (ROM Monitor), which is used for password recovery. If the value is set to1, the router loads the Boot ROM or RX Boot, which can be used to copy a new IOS into flash. If the value is set to anything between 2 and F, the router proceeds to Step 2.
In Step 2, the router looks in the startup-config file for any boot system commands and loads the IOS file listed. If multiple commands are configured, the router tries each of them sequentially. If no boot system commands exist or all fail, the router proceeds to Step 3.
In Step 3, the router loads the first IOS listed in flash memory.
In Step 4,...