Router booting process

The points below are the step by step process which runs in a router when you switch it on.

• The first thing it does is a POST, Power On Self Test, which checks whether all the components like the fan, interfaces, memory and CPU are working.
• The router then loads the IOS. There are three possible locations to find an IOS,

- Flash Memory, which is the default location

- TFTP server

- ROM, the router will boot in the ROM Monitor Mode

• 
  Now the router has booted up and it looks for a startup configuration file in the NVRAM. Whenever we configure a router, the changes are done in the RAM memory and it has to copied to the NVRAM (Non-Volatile RAM) using the ‘write’ command, so that when you restart the router the startup configuration file is still there.
• If there is no valid startup configuration file, the router prompts you to go to a set up mode. This is a tedious method, so type No and then the router prompts whether you would like to have an auto install. Press Enter key. The router auto configures and takes you to the User EXEC mode.

Now you have a fresh Cisco router with an IOS and a startup configuration, the next thing we configure is Host names and password for access control.