IOS (Internetwork Operating System) is a text-based operating system (Command Line Interface) that runs on Cisco devices that provide routing and switching functions. IOS on the router at first glance looks much the same, but each IOS on devices are different, depend on the router models. IOS image is a file that contains the IOS with the extension .bin. IOS image file used during the installation or upgrade IOS on the router.

Devices running IOS has storage components, among others:

• Flash memory is used to store the IOS image.
• RAM is used to store the running config which are configurations currently running on the router. Content in RAM will be lost if router turned off or reboot.
• NVRAM is used to store the startup config which is the backup configuration file on the router. If the running config is not copied to NVRAM, when the router turned off or reboot, the router will back to the condition where the router has not been configured.
• ROM is used to store bootstrap program called ROM Monitor (ROMMON), is in charge of running the POST (Power On Self Test).

At boot time, the router will perform the following sequence:

1. Routers turned on, then bootstrap program in ROM (ROMMON) will run the POST (Power-On Self Test).
   
   
   
   
   
   
2. Router load the IOS image from Flash memory. If no IOS image in Flash memory, then the router will load the IOS from the TFTP server. If to the TFTP server failed, the router will look at the ROM.
   
   
   
   
   
3. After the IOS is found, IOS will be loaded into RAM.
   
4. IOS load configuration file (startup config) from NVRAM. If the startup config is not found in NVRAM, the router will load the configuration from the TFTP server. If the startup config absolutely not found the router will enter the Setup mode (Initial Configuration Mode) is a condition in which the router is not configured.
   

Cisco IOS command line.
Once finished booting, you will be taken to this following modes :

1. User EXEC mode.
Once the device is finished booting, you will go to User EXEC Mode, marked with a ">" symbol.

router>

In this mode, you can perform basic operations, terminal settings, and view system information on the device. Type the command "enable" to enter Privileged Mode.

router>enable

2. Privileged Mode.

In this mode, you can display any informations that related to the system, protocols, network, interface and etc. In this mode you can also perform the connection test (ping, trace route), remote management to other devices, and file management / content on the device for example, read / write / delete configurations. Priveleged mode marked with a "#" symbol.

router#

Use the command "configure terminal" to enter Global Configuration Mode.

router#configure terminal

You can return to the user EXEC mode by using command "disable".

router#disable

router>

3. Global Configuration Mode.

This is where the network configuration is done such as IP address, VLAN, protocol, etc. Global configuration mode is indicated by "(config) #".

router (config)#

From this mode you alos can access priviledged mode commandss by adding "do" before the command you want.

for example, you want to display VLAN information on the switch, in priveleged mode you enter the command "show vlan brief".

Switch (config)#do show vlan brief

is equal to :

Switch#show vlan brief

In this mode, each time you configure any network related thing for example, interfaces, VLAN, protocol and so on, you will taken to configuration submode which is a more specific level of the Global Configuration Mode.

4. Configuration submode.

For example, you want to configure IP address on the interface :

Router (config)#interface fa0/1

Router (config-if)#

This is a sub-mode configuration for the interface level for Fa0/1. In this mode you can config anything that related to the interfaces such as IP address, speed, bandwidth, switchport, etc.

NOTES:

- Configuration on Cisco IOS is a line-by-line, that mean if you type the configuration command and press "Enter", the configuration changes will be applied immediately.

- You can delete the configuration that you created with the command "no".

for example, you want to disable the IP address that you have configured : 

Router(config)#interface fa0/1                                               

Router(config-if)#ip address 10.10.10.10 255.255.255.0  

Router(config-if)#no ip address              << how to delete IP address on interface

- To exit from certain mode or return to the previous mode, you can use the command "exit" from any mode.

Router(config-if)#exit
Router(config)#exit
Router#
%SYS-5-CONFIG_I: Configured from console by console
Router#disable
Router>

- You can use symbol "?" after any command to see a next possible command you can use.

- You do not have to type full text for configuration command, you can even execute the incomplete command as long as the pieces of the command is not ambiguous.

Router#confi ter

Router(config)#

Router(config)#interf fa0/1
Router(config-if)#

- You can automatically complete the incomplete command by pressing "Tab" key.

Router#confi           << Press "Tab"

Router#configure  

Router(config)#inte          << Press "Tab"
Router(config)#interface