During startup, Cisco devices follow a sequence of actions, including loading the Cisco IOS software and device configuration. To properly set up a router and efficiently troubleshoot issues, getting familiar with the involved actions is crucial.
As you can see in the image above, the power-on boot sequence consists of seven steps divided into three categories:
- Power-On-Self-Test (POST): A series of hardware tests used to confirm router functionality and identify hardware components.
- Load and Execute the Bootstrap Code: The bootstrap code performs events such as locating Cisco IOS software, loading it into RAM, and running it. This option is not used after the Cisco IOS software is running unless the router is rebooted.
- Locate Cisco IOS Software: Bootstrap code determines where the Cisco IOS software is stored, which typically is in flash memory, although it can also be on a TFTP server. The image location and version are determined by the configuration register and file in NVRAM.
- Load Cisco IOS Software: The bootstrap code loads the identified image into RAM, resulting in the start of the Cisco IOS software. Some older routers execute the image directly from flash memory.
- Locate the Startup Configuration: The bootstrap program searches NVRAM for the startup configuration file after loading the Cisco IOS software.
- Load the Startup Configuration: If the startup configuration is found, it is loaded into RAM as an active running configuration and its commands are executed. Interface information, routing processes, passwords, and router configurations are all included. In the absence of a startup configuration, the router enters the setup wizard (Setup Configuration Dialog) or attempts autoinstall from the TFTP.
- Start the Configured Cisco IOS Software: The router runs the Cisco IOS software with the existing configuration and allows the use of CLI commands.