Cisco Router start-up description
How does a Cisco router startup?
Once a Cisco router is switched on it performs what is called a (Post) Power On Self Test which executes "ROM" which performs diagnostics on all all hardware modules. Things like checking CPU, Memory, and Network interfaces. After post the generic bootstrap loader in ROM executes which initialises the the (IOS)
The boot field of the configuration register determines the location that is used to load the IOS. If the bootfield indicates a flash or network load then boot system commands in the config file indicate the name and location of the image. Once the image (IOS) is loaded and operational a list of available hardware is sent to the console screen. The config file saved in NVRAM is loaded in to the main memory 1 line at a time. If no config file is found in NVRAM an available TFTP server is searched for. Sometimes there is no TFTP server so the setup dialog is started, setup mode is for basic config. You can press Ctrl and C to terminate setup.