Introducing Cisco IOS

Cisco Router IOS
The Cisco IOS is a proprietary kernel that provides routing, switching, internetworking, and telecommunications features. The first IOS was written by William Yeager in 1986,

Cisco router IOS software is responsible for:
1- Carrying network protocols and functions
2- Connecting high-speed traffic between devices
3- Adding security to control access and stop unauthorized network use
4- Supplying network reliability for connecting to network resources

Connecting to a Cisco Router
You can access the Cisco IOS through the console port of a router, from a modem into the auxiliary (or Aux) port, or even through Telnet. You can connect to a Cisco router to configure it, verify its configuration, and check statistics. Most often, the first method you would connect to router is the console port. The console port is usually an RJ-45 (8-pin ) connection located at the back of the router.

Booting Process of Router
1-When you first start a Cisco router, it will run a power-on self-test (POST).
2-If it passes, it will then look for and load the Cisco IOS from flash memory.
3-If an IOS file is present then it would expands it into RAM.
4-After that, the IOS loads and looks for a valid configuration the startup-config that’s stored in (NVRAM).

Router Configuration Modes
One key to navigating the CLI is to always be aware of which router configuration mode you are currently in (see Table 1.1). You can tell which configuration mode you are in by watching the CLI prompt.

Router Configuration Modes
Mode             Definition                                                                             Example
-------------------------------------------------------------------------------------------------------------------------
User                 EXEC mode Limited to basic monitoring commands            Router>
-------------------------------------------------------------------------------------------------------------------------
Privileged       EXEC mode Provides access to all other router commandsRouter#
-------------------------------------------------------------------------------------------------------------------------
Global mode   Commands that affect the entire system                             Router(config)#
-------------------------------------------------------------------------------------------------------------------------
Specific mode Commands that affect interfaces, routing processes,         Router(config-subif)#
or lines only
-------------------------------------------------------------------------------------------------------------------------         
Once you understand the different modes, you will need to be able to move from one mode to another within the CLI.

Command Meaning
Changes from user EXEC to privileged EXEC mode
Router>enable

Changes to user EXEC from privileged EXEC mode
Router#disable

Changes to global configuration mode from privileged mode
Router#config term

Exits from any configuration mode to privileged
mode (Ctrl+Z is also commonly used)
Router(config)#exit

Enters interface configuration mode from global configuration mode
Router(config)#interface <int>
example: Router(config)#interface fa0/0

Enters subinterface configuration mode from global configuration mode
Router(config)# interface <subint>
example: Router(config)#interface fa0/0.1

Enters line configuration mode from global configuration mode
Router(config)#line <line>
example: Router(config)#line console 0

Enters router configuration mode from global configuration mode
Router(config)# router eigrp 1 

Setting a hostname on a router:
Router#config t
Router(config)#hostname dixit
dixit(config)#