Basic Cisco IOS Commands for Routers and Switches

In this article, I will give you a short introduction to the Cisco Internetwork Operating System, also known as the IOS. This is the operating system that controls Cisco routers and switches and provides the interface to configure Cisco devices as well.

We will take a look at the various router command modes, set router passwords and banners, configure router hostnames and interface descriptions, and much more.

For details on how to connect to a Cisco router or switch, see an article on 'Steps to Back Up and Restore Configuration on CISCO Devices'. This article will show you with the basic commands for smoother and more linear path into more specialized stuff.

Overview of Router Modes

Once you login into a router you instantly enter the so-called user mode – the prompt [Router name] > appears. From this mode you can only see some statistic information.

To view and change the configuration of the router you have to enter into theprivileged mode by entering the enable command. Here are the steps:

RouterA>
RouterA>enable
RouterA#

To be able to make global changes to the configuration of the router you have to enter the global configuration mode. Once you achieve that, you are able to access any possible subsequent configuration mode and make global changes:

RouterA#configure terminal or config t for short
RouterA(config)#

Now you have the control of your device. If you need to modify your router’s configuration then you enter the appropriate configuration mode. For example, if you want to make a change to your Fastethernet interface 0/0 then you have to enter interface configuration mode and make your modifications from there. Here are the steps:

RouterA(config)# interface fastethernet 0/0
RouterA(config-if)#

Prompt RouterA(config-if)# indicates you are in interface configuration mode. At any time, and any mode you are in, you can insert a question mark ? and the router will automatically show all the possible commands or option parameters you can apply at that specific time.

RouterA(config-if)# ?

Interface configuration commands:

access-expression    Build a bridge boolean access expression
arp                  Set arp type (arpa, probe, snap) or timeout
backup               Modify backup parameters
ip                   Interface Internet Protocol config commands .
.
.
.

Below there is a list of the most important configuration modes along with the commands used to enter these modes:

Name Your Router and Adjust its Clock

To apply an identity to your router, use the hostname command. To adjust the clock on the router use the clock set command. Always use the ? to help you correctly finish the command:

Setting Banners

It’s a good idea to configure security notices to be displayed every time someone accesses your router. These notices are considered to be valid legal notices. The most useful banner is the so-called Message Of The Day(MOTD) banner.

This banner is displayed to every user connecting to the router either via telnet, console port or auxiliary port. This is how you can set it on your router:

RouterA(config)# banner motd ?
Line c banner-text c, where c is a delimiting character
RouterA(config)# banner motd # ***AUTHORIZED USERS ONLY***#

Setting Passwords

The first thing you need to set on your router, before you even modify its configuration, is the secret password which is used to secure privileged mode access to the router. This password uses strong encryption algorithms making it impossible to break.

It should really be your number one priority to make sure you have set this password on your router. Use the enable secret commands to apply it to your router:

RouterA(config)# enable secret TEST

For added security you can apply user mode passwords. When users access the router either remotely via telnet or via the console terminal then this kind of passwords prompts the users for authentication.

To set this password for users accessing the router remotely via telnet, use the line vty [first line number] [last line number] command. By default, Cisco IOS enables 5 virtual terminal lines – lines 0 through 4. The necessary steps to configure this are the following:

RouterA(config)# line vty 0 4
RouterA(config-line)#login
RouterA(config-line)#password TESTING

To set user mode password for the console line, perform the following:

RouterA(config)# line console 0
RouterA(config-line)#login
RouterA(config-line)#password TESTING

A couple of important commands you can use at the line configuration mode are the exec-timeout [minutes][seconds] and logging synchronouscommands.

The first one sets the timeout for the specific line and the second one stops annoying pop-up messages from disrupting whatever you are trying to type. Here is an example of how to apply these commands:

RouterA(config)# line console 0
RouterA(config-line)#exec-timeout 10 0
RouterA(config-line)#logging synchronous

Managing Router Interfaces

At the interface configuration mode, you can apply an IP address and subnet mask to an interface using the ip address command. You can also shutdown or activate an interface by using the shutdown or no shutdown commands respectively. These two commands are very basic and you will definitely use them all the time. Here is an example:

RouterA(config)# interface fastethernet 0/0
RouterA(config-if)#ip address 192.168.10.1 255.255.255.0
RouterA(config-if)#no shutdown

It is a good idea to apply descriptions to your connected interfaces as well. This will be very helpful, especially when you’re troubleshooting things. Simply use the description command, like this:

RouterA(config)# interface fastethernet 0/0
RouterA(config)# description CONNECTION TO CENTRAL SWITCH

Displaying and Verifying Things

There is one privileged mode command that you will find yourself using extensively. I am talking about the show command which has a broad usage, including:

• Viewing routers configuration
• Verifying interfaces configuration
• Viewing interfaces status

Here are a few important show commands you will find helpful to use:

• RouterA# show running configuration
• RouterA# show clock
• RouterA# show interfaces
• RouterA# show ip interfaces brief
• RouterA# show interfaces description

5 Cisco IOS Commands Every Network Admin Should Know

There are 5 commands that every Cisco network administrator should know. These are core administration commands that will help you to really know what is going on.

If you are new to Cisco networking, these are good commands to memorize. On the other hand, even if you are a seasoned administrator, you may find a few commands in this article on which you could use a refresher. We will start the list at the bottom and move to #1.

IOS Command #5 – show interfaces

It is important to know what interfaces are on your router, important statistics about those interfaces, and whether they are up or down. The show interfaces command is a very verbose command that provides a lot of output. You may have to pick through that output to find what you are looking for; just about everything to do with interfaces is shown in the output from this command. For example: errors on an interface, bandwidth utilization, and interface speed.

This command can be abbreviated as sh int.

IOS Command #4 – copy running-config startup-config

Once you make your configuration changes, you need to save those changes. This command copies the configuration in RAM that you have been modifying to the configuration in flash. By copying it to flash, it will be saved when the router is powered off and restarted.

A shortcut for this command is wr (short for write memory).

IOS Command #3 – show ip route

Once you know that your interfaces are up and you have saved your configuration, you need to know if you have full network reach-ability. In other words, are your routing protocols working? What static routes have you entered? With the show ip route command, you can find out.

This command can be abbreviated sh ip ro.

IOS Command #2 – show ip interface brief

We saw the show interfaces command. This is the show ip interface brief command. So, you are only showing information about the interfaces as it pertains to the IP protocol. You are also asking for brief output. This command gives you the best summary of the status and IP addresses of your interfaces.

This command can be abbreviated sh ip int brie

And the #1 most important command that every network administrator should know is…

IOS Command #1 – show running-config

With the show running-config command, you see the entire router’s config. It’s that simple. You see the IP addresses, interfaces, passwords (that are in clear text), routing protocols, and other settings.

This command can be abbreviated sh ru or wr t.

Summary

Using the Cisco IOS is all about knowing what command to type in order to do what you need to do. The 5 commands shown in this post are just a few of the thousands of possible Cisco IOS commands. However, In my view these commands are some of the core commands that every network administrator should know.