The boot process of a Cisco Router is like a hardware test (POST) on computers.
How Cisco Router Works?
The Cisco Router’s operation or power-on mode takes place in 3 stages. These steps occur quickly and the device is switched on. If there is any problem at startup, it will give an error and the Router will not turn on. Or, when it cannot find the initial configuration file, it will enter Setup Mode.
Step 1: Router POST Test and Starting Bootstrap
When the router is powered on, the first step is to initialize POST and Bootstrap. First, POST will start and test the device hardware. If it does not find any errors, it will load Bootstrap.
Step 2: Locating and Installing IOS
In the second step, the Cisco Router runs Bootstrap and searches for the location of the IOS, finds it, and installs it on RAM.
The router first searches the Flash memory for IOS software. If it does not find it here, it searches for a TFTP server in the network environment and tries to install the IOS if it finds it. If it connects to TFTP and still can’t find IOS, it loads the initial configuration file.
By default, the router installs the IOS software from its own Flash Disk. You can configure another location instead of Flash Disk by default.
Step 3: Finding and Running the Startup Configuration File
After the second step, after the IOS software is installed, the Router looks for an initial configuration file on Bootstrap, NVRAM. If it cannot find such a file, it enters setup mode to create a new startup file.
If the boot loader finds the initial configuration file, it copies it directly to RAM.
Initial configuration file; The Cisco router has all the settings previously made. These settings cover all settings such as Interface configurations, routing information, and Console password.
What is a Startup-Config file?
The Startup-Config file is a config file that stores on RAM (NVRAM). Even if the Cisco router turned off, the initial configuration file is not deleted from NVRAM.
What is a Running-Config File?
When the router is started for the first time, the startup-config file is loaded because there is no configuration. However, when you save all the configurations you have made after configuring the router, these settings are saved in the running-config file.
When you turn the router off and on, the running-config file will now be loaded to NVRAM.
For example, if you forget the password of a user account that you created on the device, you can change the configuration file to startup-config and reset your device with the initial configuration settings to reset the password.
If you do not save any changes you have made to your Router to running-config, all your settings will be lost when you restart your device.
Saving Settings/Configurations on the Router
After you configure your Cisco Router for your network, you must save all settings. You can do this briefly in the privileged mode with the copy running-config startup-config command.
Router Working Logic
The operating time of a Cisco Router consists of 4 stages;
Starting POST Test and Bootstrap
Locating and Installing IOS
Finding and Running the Initial Configuration File
Router Initial Setup
What Information Does the Show Version Command Show?
CPU and Model
Interface Number and Types
How to Boot a Cisco Router ⇒ Video
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