Cisco Router Boot Process

A router initially loads the following two files into RAM:

IOS image file: IOS simplifies the basic operation of the device’s hardware components. The IOS image file is stored in flash memory of router.

Initial configuration file: The initial configuration file contains commands that are 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 in the IOS.

The running configuration is changed when the network administrator performs the device configuration. When changes are made to the running-config file, it 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, loads it, or enters setup mode.

1. Power-on Self-Test (POST) is a common 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: If there is a console connection to the router, the results appear on the screen.

2. Typically, IOS is stored in flash memory and copied to RAM for CPU operation. During the self-decompression of the IOS image file, a symbol sequence is displayed.

If the IOS image is not in flash memory, the router can search with the TFTP server. 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 “running-config”.

The Running-config file contains interface addresses, initiates routing, configures router passwords, and defines other device properties.

If the start-config file is not present in the NVRAM, the router can search for a trivial file transfer protocol (TFTP) server. If the router detects an active connection to another configured router, it sends a broadcast to search for a configuration file over the active connection.

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. Setup mode is not designed to enter complex router configurations, and network administrators normally do not use it.

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