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Basic Router Configuration

When configuring a router, certain basic tasks are performed including:
Naming the router
Setting passwords
Configuring interfaces
Configuring a banner
Saving changes on a router
Verifying basic configuration and router operations

You should already be familiar with these configuration commands; however, we will do a brief review. We begin our review with the assumption that the router does not have a current startup-config file.

The first prompt appears at user mode. User mode allows you to view the state of the router, but does not allow you to modify its configuration. Do not confuse the term "user" as used in user mode with users of the network. User mode is intended for the network technicians, operators, and engineers who have the responsibility to configure network devices.


The enable command is used to enter the privileged EXEC mode. This mode allows the user to make configuration changes on the router. The router prompt will change from a ">" to a "#" in this mode.


Hostnames and Passwords

The figure shows the basic router configuration command syntax used to configure R1 in the following example. You can open Packet Tracer Activity 1.2.2 and follow along or wait until the end of this section to open it.

First, enter the global configuration mode.

Router#config t

Next, apply a unique hostname to the router.

Router(config)#hostname R1

Now, configure a password that is to be used to enter privileged EXEC mode. In our lab environment, we will use the password class. However, in production environments, routers should have strong passwords. See the links at the end of this section for more information on creating and using strong passwords.

Router(config)#enable secret class

Next, configure the console and Telnet lines with the password cisco. Once again, the password cisco is used only in our lab environment. The command login enables password checking on the line. If you do not enter the command login on the console line, the user will be granted access to the line without entering a password.

R1(config)#line console 0
R1(config-line)#password cisco
R1(config)#line vty 0 4
R1(config-line)#password cisco

Configuring a Banner

From the global configuration mode, configure the message-of-the-day (motd) banner. A delimiting character, such as a "#" is used at the beginning and at the end of the message. The delimiter allows you to configure a multiline banner, as shown here.

R1(config)#banner motd #
Enter TEXT message. End with the character '#'.
WARNING!! Unauthorized Access Prohibited!!

Configuring an appropriate banner is part of a good security plan. At a very minimum, a banner should warn against unauthorized access. Never configure a banner that "welcomes" an unauthorized user.


For discussions about using strong passwords, see:

"Cisco Response to Dictionary Attacks on Cisco LEAP," at

"Strong passwords: How to create and use them," at

Router Interface Configuration

You will now configure the individual router interfaces with IP addresses and other information. First, enter the interface configuration mode by specifying the interface type and number. Next, configure the IP address and subnet mask:

R1(config)#interface Serial0/0
R1(config-if)#ip address

It is good practice to configure a description on each interface to help document the network information. The description text is limited to 240 characters. On production networks a description can be helpful in troubleshooting by providing information about the type of network that the interface is connected to and if there are any other routers on that network. If the interface connects to an ISP or service carrier, it is helpful to enter the third party connection and contact information; for example:

Router(config-if)#description Ciruit#VBN32696-123 (help desk:1-800-555-1234)

In lab environments, enter a simple description that will help in troubleshooting situations; for example:

R1(config-if)#description Link to R2

After configuring the IP address and description, the interface must be activated with the no shutdown command. This is similar to powering on the interface. The interface must also be connected to another device (a hub, a switch, another router, etc.) for the Physical layer to be active.

Router(config-if)#no shutdown

Note: When cabling a point-to-point serial link in our lab environment, one end of the cable is marked DTE and the other end is marked DCE. The router that has the DCE end of the cable connected to its serial interface will need the additional clock rate command configured on that serial interface. This step is only necessary in a lab environment and will be explained in more detail in Chapter 2, "Static Routing."

R1(config-if)#clock rate 64000

Repeat the interface configuration commands on all other interfaces that need to be configured. In our topology example, the FastEthernet interface needs to be configured.

R1(config)#interface FastEthernet0/0
R1(config-if)#ip address
R1(config-if)#description R1 LAN
R1(config-if)#no shutdown

Each Interface Belongs to a Different Network

At this point, note that each interface must belong to a different network. Although the IOS allows you to configure an IP address from the same network on two different interfaces, the router will not activate the second interface.

For example, what if you attempt to configure the FastEthernet 0/1 interface on R1 with an IP address on the network? FastEthernet 0/0 has already been assigned an address on that same network. If you attempt to configure another interface, FastEthernet 0/1, with an IP address that belongs to the same network, you will get the following message:

R1(config)#interface FastEthernet0/1
R1(config-if)#ip address overlaps with FastEthernet0/0

If there is an attempt to enable the interface with the no shutdown command, the following message will appear:

R1(config-if)#no shutdown overlaps with FastEthernet0/0
FastEthernet0/1: incorrect IP address assignment

Notice that the output from the show ip interface brief command shows that the second interface configured for the network, FastEthernet 0/1, is still down.

R1#show ip interface brief

FastEthernet0/1 YES manual administratively down down
Verifying Basic Router Configuration

Currently in the example, all of the previous basic router configuration commands have been entered and were immediately stored in the running configuration file of R1. The running-config file is stored in RAM and is the configuration file used by IOS. The next step is to verify the commands entered by displaying the running configuration with the following command:

R1#show running-config

Now that the basic configuration commands have been entered, it is important to save the running-config to the nonvolatile memory, the NVRAM of the router. That way, in case of a power outage or an accidental reload, the router will be able to boot with the current configuration. After the router's configuration has been completed and tested, it is important to save the running-config to the startup-config as the permanent configuration file.

R1#copy running-config startup-config

After applying and saving the basic configuration, you can use several commands to verify that you have correctly configured the router. Click the appropriate button in the figure to see a listing of each command's output. All of these commands are discussed in detail in later chapters. For now, begin to become familiar with the output.

R1#show running-config

This command displays the current running configuration that is stored in RAM. With a few exceptions, all configuration commands that were used will be entered into the running-config and implemented immediately by the IOS.

R1#show startup-config

This command displays the startup configuration file stored in NVRAM. This is the configuration that the router will use on the next reboot. This configuration does not change unless the current running configuration is saved to NVRAM with the copy running-config startup-config command. Notice in the figure that the startup configuration and the running configuration are identical. They are identical because the running configuration has not changed since the last time it was saved. Also notice that the show startup-config command also displays how many bytes of NVRAM the saved configuration is using.

R1#show ip route

This command displays the routing table that the IOS is currently using to choose the best path to its destination networks. At this point, R1 only has routes for its directly connected networks via its own interfaces.

R1#show interfaces

This command displays all of the interface configuration parameters and statistics. Some of this information is discussed later in the curriculum and in CCNP.

R1#show ip interface brief

This command displays abbreviated interface configuration information, including IP address and interface status. This command is a useful tool for troubleshooting and a quick way to determine the status of all router interfaces.
Related Topic Router

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Cindy Dy said...

It's enjoyable to learn more and more from your blog. Thanks for sharing.


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