Cisco Routers use the Internetwork Operating System(IOS)

by:Rayoptek     2020-12-09

Cisco Routers use the Internetwork Operating System(IOS) to control and manage the hardware it is running on.IOS provides the interface between the user and the hardware, enabling the user to execute the command to configure and manage the device using Command Line Interface (CLI.)

Cisco IOS Features:

1. Command Line Interface (CLI)

a. User EXEC Mode

b. Privileged EXEC Mode

c. Global Configuration Mode

d. Sub-Configuration Mode

2. Router Interface Configuration

3. RIP Configuration Using Cisco IOS

1. Command Line Interface (CLI)

The CLI is a character or text based interface used to configure or monitor the deviceit supports a command parser, whenever enter is pressed after typing the command IOS parses the command and parameters that you enterd and checks for correct syntax..There are many of accessing the IOS CLI like using console,auxiliary,telnet,web browser and an SNMP mnagment station, but most common way of accessing is through console.

IOS supports three different access modes for interaction with CLI:

1) User Exec---provides basic access to the IOS which includes monitoring and troubleshooting commands

2) Privilege Exec---provides a high-level management access to the IOS,User EXEC mode command can also be used at this mode

3) Configuration-----provides commands for configuring device

1.a User EXEC Mode

The initial access to the CLI is done through this mode. Depending upon the configuration of the router user may be prompted for password. This mode is typically used for monitoring and troubleshooting the device, it has a very limited set of commands.

This mode is identified by the prompt as '>'.


Here left to the prompt is the device name.

1.b Privilege EXEC Mode:

Giving 'enable' command at the above mode lets you enter to this mode. If the privilege EXEC password has been set, then you will be prompted for the password.

This mode is identified be the prompt as '#'.


In this mode, we can also use the User EXEC mode commands, this mode has more advanced management and troubleshooting commands. We cannot use the commands that modify the device configuration.

1.c Configuration Mode:

Use the 'configure terminal' command at Privilege EXEC Mode to enter to this mode, as shown in the below example.

R1#configure terminal


Prompt at this mode is '(config)#'.

Use the command in this mode to change the configuration of the device.

We cannot view the configuration changes at this mode for this we have to go back to Privilege EXEC Mode by either typing 'end' or pressing the control sequence CTRL-Z,

1.d Sub-configuration mode:

Configuration mode has two different levels:

a. Global Configuration mode and

b. Sub-configuration. mode

At the Sub-configuration mode, the commands that are entered affect only specific component of the device.

The following are the different sub-configuration modes and their corresponding prompts

Mode prompt

Interface Subconfiguration mode (config-if)#

Subinterface Subconfiguration mode (config-subif)#

Line Subconfiguration mode (config-line)#

Controller Subconfiguration mode (config-controller)#

IP Routing Protocol Subconfiguration mode (config-router)#

To leave a Sub-configuration mode and return to Global Configuration mode, use the exit command

2.0 Router Interface Configuration:

Interface Naming

Depending on the device interfaces can be either fixed, modular or both.

For Routers that have only fixed interfaces, the interface nomenclature is

type port_#.

If the router as two fixed serial interfaces they are name as serial 0 and serial 1

If the router has modular slots, the interface nomenclature is like type slot_#/port_#.

Each slot begins with 0 and has a unique slot number.within each slot port numbers begin at 0 and move their way up.

For example a modular router with two slots has slot 0 as first slot and slot 1 as second slot. If the first slot has 4 serial interfaces they are numbered from 0 to 3 and the if the second slot has 2 serial interfaces they are numbered as 0 and 1 as shown in the below representation.

First slotiSerial 0/0, Serial 0/1, Serial 0/2, Serial 0/3

Second slotiserial 1/0,serial 1/1


Router's physical interface can be broken down into multiple logical interfaces called sub-interfaces. Once the sub-interface is created router will treat this logical interface just like a physical interface. Below is the sub-interface nomenclature type port_#.subinterface_#


Sub-interface number 0 on a physical interface 'serial 0' is represented as

serial 0.0

Example 1: Here we configure the hostname on a router.


R1#configure terminal

R1(config)#hostname bangalore




Example 2: Here we configure the IP address on an interface of a router.


R1#configure terminal

R1(config)#interface serial 0

R1(config-if)#ip address




3. RIP Configuration Using Cisco IOS

RIP is a dynamic routing protocol. Configuring RIP on a Cisco router is given below. The network is enabled for RIP.


R1#configure terminal

R1(config)#router rip




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