A router initially loads the following two files into RAM:
IOS image file: IOS simplifies the essential operation of the device’s hardware components. The IOS image file is stored in the flash memory of the router.
Initial configuration file: The initial configuration file contains commands used to perform the initial configuration of the router and to create the running configuration file stored in RAM. The initial configuration file is stored in NVRAM. All configuration changes are stored in the running configuration file and the IOS.
The running configuration is changed when the network administrator performs the device configuration. Changes to the running-config file must be saved to the NVRAM as the initial configuration file if the router restarts or shuts down.
Router Boot Process
The boot process consists of three main steps:
1. Performs POST and loads the boot program.
2. Find and install the Cisco IOS software.
3. Locates the initial configuration file, load it, or enter setup mode.
1. Power-on Self-Test (POST) is a standard process that occurs on almost any computer at startup. POST is used to test the router hardware. When the router is turned on, the software in the ROM chip runs POST. During this self-diagnostics, the router works with the ROM diagnosis of various hardware components, including CPU, RAM, and NVRAM. When POST is finished, the router runs the boot program.
After POST, the boot program is copied from ROM to RAM. After entering the RAM, the CPU performs the instructions of the boot program. The main task of the boot program is to find the Cisco IOS and install it in RAM.
Note: The results appear on the screen if there is a console connection to the router.
2. Typically, IOS is stored in flash memory and copied to RAM for CPU operation. A symbol sequence is displayed During the IOS image file’s self-decompression.
The router can search with the TFTP server if the IOS image is not in flash memory. If a full IOS image is not found, a reduced version of the IOS from the ROM is copied to RAM. This version of IOS is used to help diagnose any problem and can be used to install the full version of IOS into RAM.
3. The bootstrap program then looks for the initial configuration file (also known as “startup-config da) in NVRAM. The file contains previously saved parameters and configuration commands. If so, it is copied to RAM as a running configuration file or the “running-config” command.
The Running-config file contains interface addresses, initiates routing, configures router passwords, and defines other device properties.
The router can search for a trivial file transfer protocol (TFTP) server if the NVRAM does not present the start-config file. If the router detects an active connection to another configured router, it sends a broadcast to search for a configuration file over the active link.
If a TFTP server is not found, the router displays the request to enter setup mode. The setup mode consists of a series of questions that ask the user for basic configuration information. The setup mode is not designed to enter complex router configurations; network administrators usually do not use it.