When a Cisco router boots up, it performs a series of steps, called the router boot sequence, to test the hardware and load the necessary software.
Cisco router boot sequence consists of the following steps:
1. The router performs a POST. The POST tests the hardware to verify that all components of the device are operational and present. For example, the POST checks for the different interfaces on the router. The POST is stored in and run from ROM (read-only memory).
2. The bootstrap looks for and loads the Cisco IOS software. The bootstrap is a program in ROM that is used to execute programs. The bootstrap program is responsible for finding where each IOS program is located and then loading the file. By default, the IOS software is loaded from flash memory in all Cisco routers. Other possible locations of the IOS image are a TFTP (Trivial File Transfer Protocol) server, or the Mini-IOS (the mini IOS is a stripped-down version of IOS in ROM).
3. The IOS software looks for a valid configuration file stored in NVRAM. This file is called startup-config.
4. If a startup-config file is in NVRAM, the router will load and run this file. If a valid startup-config file is not in NVRAM, IOS will display System Configuration setup.
5. Once the startup-config configuration is loaded, IOS will present CLI interface in User mode.