Cisco IOS Operating System

Lab Overview

This lab explores basic navigation of the Cisco IOS operating system CLI (Command Line Interface). Only a single device is required. This is a guided walkthrough of the IOS command line interface.

Step-by-Step Instructions

1- Initial Setup

1. Open Cisco Packet Tracer.
2. Drag and drop the Router 2911 as shown in the below screenshot:

2- Accessing the CLI

1. Click on Router0 and then the CLI tab to access the console.


2. If prompted to enter the initial configuration dialog after the device has booted up, enter ‘no’.


3. Press Enter to get started, then use the command “enable” to enter Privileged Exec mode.
   For rebooting the device, use the command “reload”.

3- Understanding Modes

• User Exec Mode: The symbol “>” in “Router>enable” means that we are in user Exec mode.
• Privileged Exec Mode: The symbol “#” in “Router#” means that we are in privileged Exec mode.

4- User Exec Mode and CLI Command Help

Enter a question mark “?” to explore the commands available in User Exec mode.

Attempt to run a command not available in User Exec mode (e.g., ‘show startup-config’).

5- Privileged Exec (Enable) Mode and Context Sensitive Help

Enter Privileged Exec mode. This mode is often commonly known as Enable mode. Notice that the prompt changes to ‘Router#’

Router>enable
Router#

Back to User Exec mode by using the command disable.

Router#disable
Router>

Go back to Privileged Exec mode by using shortened command abbreviation.

Router>en
Router#

6- Command Abbreviation and Context Sensitive Help

Command abbreviation only works when you enter letters which could only match one unique command. Attempt to return to User Exec mode by entering the command di

Router#di
% Ambiguous command: "di"

Check to see all the possible commands which begin with the letters di

Router#di?
dir disable disconnect

We can see that the shortest combination we could use for Disable would be disa

We can access detailed informational and debug output in Privileged Exec mode. Check to see all commands that begin with sh

Router#sh?
show

show is the only command that begins with sh so we can use that as the abbreviation.

Enter sh ? to see all available show commands. Notice that we have now included a space before the question mark. This enters context sensitive help for the show command.

7- Using Tab Completion and Error Handling

Press the Enter key when you see --Mor-- to cycle through the additional output one line at a time.

One line at a time is a very slow way to view additional output so press the Space Bar to cycle through it one page at a time instead.

Keep hitting the Space Bar until you reach the end of the output.

Explore available options for the show aaa command. The term aaa is used for illustration purposes only, and the specific commands under aaa are not relevant for this exercise.

Enter sh aaa us and then hit the Tab key to see Tab completion in action.

The Tab key will complete a partially entered command for you. Again this will only work if you’ve entered enough letters to be a unique match.

Router#sh aaa us
Router#sh aaa user

Enter the command sh aaa user all.

Router#sh aaa user all
Router#

Note the absence of output upon entering the command. This is expected behavior indicating that AAA configuration is not set up. The CLI returns to the Enable prompt as there is no information to display.

Upon entering the command sh aaa usor all

Router#sh aaa usor all
                ^
% Invalid input detected at '^' marker.

If you input an incorrect command, an error message will be displayed.

In this case, a typographical error was made. The CLI indicates that invalid input was detected and highlights the location of the error, specifically at the 'o' in 'usor', where 'user' should have been typed instead.

To correct this, enter the command sh aaa and press Enter.

Router#sh aaa
% Incomplete command.

The router alerts us that the command entered is incomplete and requires additional input. To view the available options, we can re-enter sh aaa ?.

Router#sh aaa ?
  local     Show AAA local method options
  sessions  Show AAA sessions as seen by AAA Session MIB
  user      Show users active in AAA subsystem

8- Global Configuration Mode

Enter Global Configuration mode. (The command can be abbreviated to conf t.)

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

Upon entering the command, observe the prompt change to Router(config)#.

Global Configuration mode allows configuration changes that affect the entire device, rather than specific interfaces or features.

Let's add a few host entries. (The specific function of this command is not critical for this demonstration; we will use it to illustrate command history shortly.)

Router(config)#ip host Server1 1.1.1.1
Router(config)#ip host Server2 2.2.2.2

Attempt to change the device's hostname to R1 by typing the command hostname R1.

Router(config)#hostname R1
R1(config)#

Observe how the command prompt updates to display the router’s hostname.

Use the Up Arrow key multiple times to navigate through your recent command history, and then use the Down Arrow to revisit previous commands. Note that command history is limited to your current command level – only commands entered in Global Configuration mode will be displayed.

Execute the command show ip interface brief to examine the available interfaces on the router.

R1(config)#show ip interface brief
            ^
% Invalid input detected at '^' marker.

When receiving the invalid input detected error message despite no typos, it's likely due to entering commands in the wrong mode. Commands like show should be executed in Privileged Exec mode, not Global Configuration mode. To override this, prepend do to the command do show regardless of your current mode to execute show commands correctly and view available interfaces.

Please enter the correct command to check available interfaces from Global Configuration mode.

R1(config)#do show ip interface brief 
Interface IP-Address OK? Method Status Protocol 
GigabitEthernet0/0 unassigned YES unset administratively down down 
GigabitEthernet0/1 unassigned YES unset administratively down down 
GigabitEthernet0/2 unassigned YES unset administratively down down 
Vlan1 unassigned YES unset administratively down down

9- Interface Configuration Mode

Enter Interface Configuration mode for one of your interfaces.

R1(config)#interface gigabitEthernet 0/0
R1(config-if)#

Upon entering Interface Configuration mode, you'll notice a prompt change indicating your current mode.

To return to Global Configuration mode, simply use the command exit.

R1(config-if)#exit
R1(config)#

Drop all the way back down to Privilege Exec mode with a single command.

R1(config-if)#end
R1#

The command end allows you to revert to Privilege Exec mode from any operational level. Alternatively, pressing Ctrl-C achieves the same effect.

To view the complete device configuration, execute the command show running-config.

R1#show running-config 
Building configuration...
Current configuration : 687 bytes
!
version 15.1
no service timestamps log datetime msec
no service timestamps debug datetime msec
no service password-encryption
!
hostname R1
!
!
!
!
!
!
!
!
no ip cef
no ipv6 cef
!
!
!
!
license udi pid CISCO2911/K9 sn FTX152457FX
!
!
!
!
!
!
!
!
!
!
!
spanning-tree mode pvst
!
!
!
!
!
interface GigabitEthernet0/0
no ip address
duplex auto
speed auto
shutdown
!
interface GigabitEthernet0/1
no ip address
duplex auto
speed auto
shutdown
!
interface GigabitEthernet0/2
no ip address
duplex auto
speed auto
shutdown
!
interface Vlan1
no ip address
shutdown
!
ip classless
!
ip flow-export version 9
!
!
!
!
!
!
!
line con 0
!
line aux 0
!
line vty 0 4
login
!
!
!
end

10- Viewing and Filtering Configuration

note that the iOS command line is generally case-insensitive, except when commands are piped.

R1#sh run | begin Hostname 
R1#

Here, we entered 'Hostname' with an initial capital letter, but in the configuration, it appears in lowercase. Consequently, the router did not locate any instances of 'Hostname' and returned no results.

To view configuration lines containing the keyword 'interface'

R1#sh run | include interface
interface GigabitEthernet0/0
interface GigabitEthernet0/1
interface GigabitEthernet0/2
interface Vlan1

View all configuration lines which do not include the word ‘interface’.

R1#sh run | exclude interface
Building configuration...

Current configuration : 737 bytes
!
version 15.1
no service timestamps log datetime msec
no service timestamps debug datetime msec
no service password-encryption
!
hostname R1
Output truncated –

11- IOS Configuration Management

Copy the running configuration to the startup configuration.

R1#copy run start
Destination filename [startup-config]? 
Building configuration...
[OK]

Change the hostname of the router to RouterX

R1#config t
R1(config)#hostname RouterX
RouterX(config)#

Note that commands executed in IOS take immediate effect, evidenced by the prompt changing to reflect the new hostname.

To determine the hostname that will be applied upon system reboot

RouterX(config)#do show startup-config
Using 690 bytes
!
version 15.1
no service timestamps log datetime msec
no service timestamps debug datetime msec
no service password-encryption
!
hostname RouterX
!
!
!

Commands take effect immediately but are not persistent across a reboot until we save them.

Backup the current running configuration to flash memory in the router using the command copy run flash:.

RouterX#copy run flash:
Destination filename [running-config]? 
Building configuration...
[OK]

It's not advisable to back up a device to itself. Instead, execute the command to backup the running configuration to an external TFTP server.

RouterX#copy run tftp
Address or name of remote host []? 10.10.10.10
Destination filename [RouterX-confg]?

Writing running-config........
%Error opening tftp://10.10.10.10/RouterX-confg (Timed out)

The command attempted to execute for a period but eventually timed out and failed due to the absence of configured connectivity to a TFTP server in the lab environment

After reloading the device, verify it restarts with the anticipated configuration displaying the hostname "RouterX".

RouterX#reload
Proceed with reload? [confirm]

Conclusion

This lab provided a comprehensive introduction to navigating and using the Cisco IOS operating system. From basic command execution to configuration management, these steps lay the foundation for more advanced networking tasks.

By following this guide, you will gain a solid understanding of the Cisco IOS CLI, enabling you to effectively manage and troubleshoot Cisco devices in your network.