Cisco Router is similar to a Computer. Cisco devices use an IOS operating system which is in CLI and GUI based.
IOS stands for Internetwork operating system. IOS is stored in a memory called Flash. Flash is integrated memory in Cisco Devices.
Apart from the IOS, Cisco devices need Startup-configuration and running configuration. Startup-configuration is stored in NVRAM and loads along with IOS when Router boots up. NVRAM is a volatile memory which gets power from lithium battery so when we reboot the Router we don’t lose the previously saved configuration.
Running-Configuration is the current setting of the router which is loaded in RAM until the system is up. This configuration is lost when you reboot the Router.
Cisco Router also goes through the following boot sequence order:
1. POST – Power-on Self-test:-
It is an initial and quick low-level diagnostic process which performs various tests. POST checks all the hardware components of Router to verify that everything is functioning properly. The POST is the function of Read Only Memory.
2. Loading the Bootstrap Program:-
Bootstrap is loaded once the post is done. Bootstrap is a tiny program which helps to find the location of the IOS based on the configuration register value. Default configuration register value is 0x2102 in hexadecimal. 0x2102 value lets the router know that which IOS should be loaded from Flash. Once the IOS is found then Router loads the IOS into RAM from Flash, It’s called extracting the IOS from Flash to RAM. if IOS is not found in Flash the other possible way to get IOS is from TFTP server.
Note:- In the worst case if Router IOS is crashed or Router doesn’t have IOS then System will lend up in ROMMON MODE. ROMMON MODE is the mode of Bootstrap program which is used for password recovery, IOS installation and etc.
3. Loading the Startup-Configuration:-
The router now looks for the previously saved configuration in NVRAM which is called startup-configuration. If Router finds the previous startup-configuration stored in NVRAM then Router loads the startup-configuration in RAM along with IOS.
If Router doesn’t find any previous startup-configuration then it boots in default mode along with IOS and Router will jump into setup-mode.
4. User-Exec Mode:–
This is the first mode to enter into Router’s CLI. This mode is loaded when router finds Startup configuration in NVRAM. Prompt looks like below example:
5. Setup Mode:–
This mode appears when the router doesn’t load the startup configuration or didn’t find startup configuration in NVRAM.
The user can skip the setup mode and directly jump into user exec mode by typing “no”
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