Cisco Networking: Boot Process Overview
The process of booting up Cisco network devices is pretty straightforward but the details are important to understand. To get started, here is an overview of the Cisco boot process.
The Boot ROM initializes all hardware and performs a Power-On Self Test (POST) to ensure that the hardware is functional.
Any errors are reported to the console as part of the POST process.
The Boot ROM locates and loads a valid Cisco IOS image and loads the first valid image it finds unless a boot value has been set.
The Boot ROM looks for an image first in Flash and then if a Boot-Helper image is available, attempts to load an IOS from a TFTP server (tftpdnld). If none is found, the Boot ROM loads the rxboot IOS image, or the ROM Monitor mode (ROMmon).
When a valid image is found, the IOS is loaded into system memory, or RAM.
The IOS checks the configuration register value stored in NVRAM.
This is normally set to boot the startup-config file. If the startup-config is missing or if the configuration register value dictates, the device enters Setup mode rather than loading the configuration.