At startup, the Cisco device performs three basic functions:
- Verifies hardware -- the router will perform Power on Self Test (POST) for its hardware such as CPU, RAM and motherboard, which are most common components to routers, switches, and computers.
- Loads the IOS image -- the router operating system (IOS) is loaded from flash memory into the Random Memory (RAM), where the IOS runs. There are three locations the router can load the IOS image from. The default location is Flash memory. If router didn't find IOS image there, it then try to load IOS image from Trival File Transfer Protocol (TFTP) server. If failed agin, the router will look for the trouble-shooting version IOS image called ROMMON from the Read-Only Memory (ROM).
- Applies the device configuration -- the router will try to load the startup-config file, a file customizes the router configuration, from the Non-Volatile RAM (NVRAM). If there is no startup configuration file stored in NVRAM, the router will enter the setup mode, which guides you through the process of configuring the router.