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Cisco IOS Internetwork Operating System

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Cisco IOS Internetwork Operating System

IOS : Cisco internetworking operating system runs on  cisco router and switches. From Cisco IOS we can configure the router and switch. It was created to deliver network service and enable network application. IOS was found in flash.It is command line interface.

Basic responsibility of Cisco IOS (internetworking operating system)
Carrying network protocols and functions.
Connecting high-speed traffic between devices.
Adding security to control access and stop unauthorized network use.
Providing scalability for ease of network growth and redundancy.
Supplying network reliability for connecting to network resources.

Cisco IOS Command Modes

1)  User mode (User EXEC mode)
2) Privileged mode (Privileged EXEC Mode)
3) Global Configuration mode

1)  User EXEC mode :  Limited command set. No change of system parameters
Example : ping, telnet, traceroute

Router1 >

2) Privileged EXEC mode :  Manage configuration files examine state of router . Access control with password  (enable secret).


3) Global configuration mode : Change system wide configuration parameters


Following are the important Global Configuration submodes.

• Interface mode


• Subinterface mode


• Line mode


• Router configuration mode


Cisco Router configuration files used to configure the router.

There are two types,

i) running-config
ii) startup-config

i) running-config : The running configuration file stores the configuration changes made while the router is up and running. The “running-config” file is stored in RAM

ii) startup-config : The “startup-config” file is kept in NVRAM and the contents of the “startup-config” file are retained after a reboot.

To save the changes of “running-config” file to “startup-config”, run the following IOS command.

RouterA# copy running-config startup-config

Cisco Router Configuration Commands

How to Configure a Router Hostname

Router#configure terminal
Enter configuration commands, one per line. End with CNTL/Z.
Router(config)#hostname Router1

How to Configure a MOTD Banner for Router

MOTD (Message of the DAY) will display message when connecting router.

Router1#configure terminal
Router1(config)#banner motd #Welcome to

How to enable DNS lookup

Router1#configure terminal
Router1(config)#ip name-server

How to assign a Local Name to an IP address

Router1#configure terminal
Enter configuration commands, one per line. End with CNTL/Z.
Router1(config)#ip host PC001

How to assign an IP address to a router interface

Router1(config-if)#ip address

Cisco Router Show Commands

Displays statistics for all interfaces

Router1#show interfaces

Displays statistics of fa0/0 interface.

Router1#show interface fa0/0

Displays a summary of all IPv4 interfaces

Router1#show ip interface brief

Displays a summary of all IPv6 interfaces

Router1#show ipv6 interface brief

Displays statistics for interface hardware serial 1/0.

Router1#show controllers serial 1/0

Displays the system clock of the router

Router1#show clock

Displays the configured hostnames

Router1#show hosts

Displays all users connected to the router

Router1#show users

Displays history of Cisco IOS commands used

Router1#show history

Displays info about Flash memory

Router1#show flash

Displays info about loaded Cisco IOS software

Router1#show version

Displays the ARP table of the router

Router1#show arp

Displays status of configured Layer 3 protocols

Router1#show protocols

Displays configuration saved in NVRAM

Router1#show startup-config

Displays configuration currently running in RAM

Router1#show running-config

Displays the IPv4 routing table of the router

Router1#show ip route

Displays the IPv6 routing table of the router

Router1#show ipv6 route

Cisco IOS Shortcut Keys are

CTRL-A     Move to  “Beginning” of the Line.
CTRL-E     Moves to the “End” of the line
CTRL-B     Moves back “Back One Character”. (or Left Arrow)
CTRL-F     Moves forward “Back One Character”. (or Right Arrow)
Esc-B  Moves “Backward to the Beginning of the Next Word”.
Esc-F  Moves “Forward to the Beginning of the Next Word”.
CTRL-U     Delete the line completely
CTRL-W     Delete the word the cursor is under
CTRL-Z    Move from Configuration mode back to Privilege EXEC mode
CTRL-D     Delete

IOS provides four internetwork services :

Routing services Manage optimal paths using  routing techniques that reduce cost by using network bandwidth efficiently.

WAN Optimization services Supports all WAN services.

Management and security services IOS provides network management and security features, Such as  monitoring and diagnostics.

Login into Router router user mode from outer enable router privileged from router disable Router router log out


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