Cisco Router Bootup Process Explained

By | 9th November 2015

The Cisco router bootup process has three stages.

  1. 1. Perform Power-on self test (POST) and load the bootstrap program.

cisco router boot process

The POST is a process is used to test the router hardware. After POST, the bootstrap program is loaded.

  1. Locate and load the Cisco IOS software.

cisco router boot process2

The bootstrap program locates the Cisco IOS software and loads it into RAM. Cisco IOS files can be located in one of three places: flash memorya TFTP server, or another location indicated in the startup configuration file. The Cisco IOS software loads from flash memory by default. The configuration settings must be changed to load from one of the other locations.




  1. 3. Locate and execute the startup configuration file orenter setup mode.

cisco router boot process3

After the Cisco IOS software is loaded, the bootstrap program searches for the startup configuration file in NVRAM. This file contains the previously saved configuration commands and parameters, including interface addresses, routing information, passwords, and other configuration parameters.

If a configuration file is not found, the router prompts the user to enter setup mode to begin the configuration process.

If a startup configuration file is found, it is copied into RAM and a prompt containing the host name is displayed. The prompt indicates that the router has successfully loaded the Cisco IOS software and configuration file.

To avoid the loss of data, it is important to have a clear understanding of the difference between the startup configuration file and the running configuration file.




Cisco Router Startup Configuration File.

The startup configuration file is the saved configuration file that sets the properties of the device each time the device is powered up.This file is stored in non-volatile RAM(NVRAM), meaning that it is saved even when power to the device is turned off.

When a Cisco router is first powered up, it loads the Cisco IOS software to working memory, or RAM. Next, the startup configuration file is copied from NVRAM to RAM. When the startup configuration file is loaded into RAM, the file becomes the initial running configuration.

Running Configuration File

The term running configuration refers to the current configuration running in RAM on the device. This file contains the commands used to determine how the device operates on the network.

The running configuration file is stored in the working memory of the device. Changes to the configuration and various device parameters can be made when the file is in working memory. However, the running configuration is lost each time the device is shut down, unless the running configuration is saved to the startup configuration file.

Changes to the running configuration are not automatically saved to the startup configuration file. It is necessary to manually copy the running configuration to the startup configuration file.

When configuring a device via the Cisco command line interface (CLI) the command copy running-config startup-config, or the abbreviated version copy run start, saves the running configuration to the startup configuration file. When configuring a device via the Cisco SDM GUI,there is an option to save the router running configuration to the startup configuration file each time a command is completed.

Copyright Cisco.com




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