Router Bootup Process – How Router Boots up





In this article we will discuss Router Bootup Process – How Router Boots up , will make brief discussion on Router Bootup Process. In last article we discuss about LAN and WAN Interfaces In router.




How Router Boots up

There are three major phases to the bootup process, they are:

1. Perform the POST and load the bootstrap program.

2. Locate and load the Cisco IOS software.

3. Locate and load the startup configuration file or enter setup mode.



Router Bootup Process - How Router Boots up
Router Bootup Process – How Router Boots up

During the Power-On Self-Test (POST), the router executes diagnostics from ROM on several hardware components, including the CPU, RAM, and NVRAM. After the POST, the bootstrap program is copied from ROM into RAM. The main task of the bootstrap program is to locate the Cisco IOS and load it into RAM.

Router booting process





Router goes through the three steps when booting up

  • POST (Power on self test)
  • Locate and load IOS
  • Locate and run device configuration file
  • Router is powered on.
  • Power on self test (POST) is performed to check hardware components including memory and interfaces.
  • Bootstrap program is loaded and executed.
  • Bootstrap reads configuration register value to determine how the router will boot up.
  • Depending on the value of configuration register, bootstrap program finds and loads the IOS image.
  • If bootstrap fails to load IOS from all possible locations it will drop boot sequence in ROMMON mode for troubleshooting.
  • If IOS is loaded, the IOS will try to find and load the configuration.
  • If configuration is not presented, system configuration dialog would be launched.
  • If configuration is loaded, you would be presented in CLI interface.




The IOS is typically stored in flash memory and is copied into RAM for execution by the CPU. If the IOS image is not located in flash, then the router may look for it using a Trivial File Transfer Protocol (TFTP) server. If a full IOS image cannot be located, a limited IOS is copied into RAM, which can be used to diagnose problems and transfer a full IOS into Flash memory.

The bootstrap program then copies the startup configuration file from NVRAM into RAM. This becomes the running configuration. If the startup configuration file does not exist in NVRAM, the router may be configured to search for a TFTP server. If a TFTP server is not found, then the router displays the setup mode prompt.

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