Router boot process steps

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There are three major steps to the boot-up process. As illustrate in Figure 1, which are:

Step-1. Perform the POST and load the bootstrap program.

Step-2. Locate and load the Cisco IOS software.

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

Step-1- Performing POST and Load Bootstrap Program

  1. When the router is powered on it first runs Power-On Self Test (POST)
  2. During to POST, the router executes diagnostics from ROM on several hardware components, including the CPU, RAM, and NVRAM.
  3. After the Post, the bootstrap program is copied from ROM into RAM. The bootstrap function is to locate the Cisco IOS and load it into RAM. Its checks the Configuration Register value to specify where to load the IOS. The bootstrap default value is 2102, in hexadecimal.

Step-2. Locating and Loading Cisco IOS

  1. The IOS typically store location is flash memory. The router first looks for “boot system” commands in the startup-config file. If it finds these commands, it will run boot system commands in the order they appear in startup-config to locate the IOS. If not, the IOS image is loaded from Flash.
  2. And If the IOS is not found in Flash, the bootstrap can try to load the IOS from Trivial File Transfer Protocol (TFTP) server
  3. If full image is not located in Flash or TFTP then router copied limited IOS from ROM into RAM. This limited IOS can be used to diagnose problems and transfer a full IOS into Flash memory.
  4. After the IOS is found, it is loaded into RAM.

Step-3. Locating and Loading the Configuration File

  1. The IOS attempts to load the configuration file (startup-config) from NVRAM to RAM. This becomes the running configuration.
  2. If the startup-config is not found in NVRAM, the IOS attempts to load a configuration file from TFTP.
  3. If no TFTP server responds, the router enters Setup Mode (Initial Configuration Mode).
  4. At this point, if we have a console connection to the router, we can check the output on the screen and can configure the router from here.
  5. When router prompted to enter setup mode, give the answer no. If you answer yes and enter setup mode, press Ctrl+C at any time to terminate the setup process. Setup is the wizard for router configuration. Which we will discuss later.
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