Introduction to Routing and packet fowarding

Like any operating system on any other computer, Cisco IOS is responsible for managing the hardware and software resources of the router. Although the Cisco IOS might appear to be the same on many routers, there are many different IOS images: a file that contains the entire IOS for that router.
Although some routers provide a GUI (graphical user interface), the CLI (command-line interface) is a much more common method of configuring Cisco routers.
Upon bootup, the startup-config file in NVRAM is copied into RAM and stored as the running-config file. Any changes entered by the network administrator are stored in the running-config file and immediately implemented by the IOS.
Router Bootup Process
Like all computers, a router uses a systematic process to boot up. The four phases are
1. POST: Testing the router hardware
2. Loading the bootstrap program
3. Locating and loading the IOS
4. Locating and loading the startup configuration file or entering setup mode
Power-on self test (POST) is a common process that occurs on most every computer during bootup. The POST process is used to test the router hardware.