An Introduction to Cisco Router Configuration

When the router is started for the first time, it will provide an option ‘would you like to enter the initial configuration dialog? [yes/no]’. The initial configuration dialog is used to provide basic configuration to the router. If you type ‘yes’ to enter the configuration, it will ask a series of questions, such as the hostname for the router, enable password, etc.

If you do not go through initial configuration, there will still be a very simple default configuration, which you can see with show run. If you are going to be using another router’s config as a template for this router, you will first need to clear the config with erase startup-config. You can then restart the router (with reload), enter Global Configuration mode, and paste in your template config. Note that with some terminal emulation packages, you will need to adjust how fast the script is pasted in. You can do this with character and line delay in the configuration options of the terminal emulation software. If you don’t do this, you may find that parts of the configuration get missed, as it is pasted faster that the router can accept the input.

You will see the hostname for the router as the CLI prompt (eg, if the router prompt is Router1# then the router’s hostname is Router1). If you’re working on multiple routers at a time, this is particularly useful, as it can get confusing. To configure the host name, enter Global Configuration mode and type hostname name.

The router can have an enable password or an enable secret. Although slightly misleading, they are both passwords. When you first log on to the router and enter ‘enable’ mode, this is the password you are asked for. The difference between the two is that the enable secret is encrypted, but the enable password is not. Enable secret should always be used in preference to the enable password, but you will sometimes still see enable password in use. To set the password, use enable password password or enable secret password. You can have both set if you want.

Usernames can optionally be set up. If configured, you will be asked for a username and password when you login to the router. To add a user, type username user privilege level secret 5 password. User is the username, level is the privilege level (1 – 15) that the user has (1 is user exec mode, 15 is privilege exec mode), ‘secret 5’ tells the IOS to encrypt the password. Many admin accounts are created with level 15 privileges.

The easiest way to make a configuration change is look at the current configuration, then copy and modifies the commands to suit what you’re doing. For example, if you need to forward a port from the router to a server, you could do a show run to see the commands that are already in the config. You can then type out the command you need based on the command you can see in the running config. If you need to remove a command, just add no to the beginning of the command. For example, ip address 192.168.0.254 255.255.255.0 adds the IP address 192.168.0.254 to the interface you’re configuring. Typing no ip address 192.168.0.254 255.255.255.0 will remove that IP address.

IOS Basics

The IOS is the operating system that runs on Cisco devices. The feature set of the IOS will depend on the version that’s running, and the hardware it’s running on. The IOS generally addresses four types of memory, which are ROM, Flash, RAM and NVRAM.

The ROM is read only, and is where the bootstrap software is usually stored. The bootstrap software is the first thing to boot when the router is turned on. Flash memory is located on the system board, and can sometimes be upgraded with PCMCIA cards. The flash memory is used to store the IOS images, configuration files and the SDM (if used). The RAM is used in a similar way to a PC. It will store the running configuration, buffers, tables, etc. NVRAM is used to store the start-up configuration, which is loaded by the bootstrap software on start-up.

The router configuration comes in two forms; the start-up configuration, and the running configuration. The start-up configuration is stored in NVRAM, and is loaded into RAM on startup. When the configuration is in the RAM, it becomes the running configuration.

Either configuration can be edited independently of the other. That means that you can change the running configuration of the router to test it, and if there’s a problem, reboot the router, and the startup config will be applied again. This means that after changes are made, they must be written to NVRAM to be made permanent. To write the configuration to NVRAM use copy running-config startup-config, or write memory. Either configuration can be displayed with show running-config or show startup-config.

When using the CLI, you can get a list of supported commands by typing help. If you know the first part of a command, but are not sure what parameters to use, use ? to display a list of options. For example, if you can remember that you need to use the copy command, but can’t remember what comes next, type copy ?. As soon as you press ? a list of possible options is displayed. Also, if you can’t remember the whole command, you can use the ? to get a list. For example, typing co? will display ‘configure’, ‘connect’ and ‘copy’, as they all start with ‘co’.

CLI commands can be shortened for brevity, as long as it’s obvious to the IOS which command you are using. For example, show running-config can be shortened to sh run. If you shorten a command too far, you will get an ‘ambiguous command’ error, and you may need to be more specific. If you fail to enter enough parameters (eg, copy run instead of copy run start), you will get an ‘incomplete command’ error, meaning that you have to enter more information.

Occasionally, you may see some debug messages pop up. These will only appear in a session that is run over the console by default. These messages will sometimes get in the way if you happen to be typing at the time, which can lead to forgetting where you are up to. If this happens, simply press the Tab key to bring up the incomplete command.

Cisco routers use the config register to store information on how the router behaves. The config register is a hexadecimal number that represents specific configurations. 0x2102 is the default register. The register can be changed to specify the baud rate for the console cable, whether to ignore ‘break’ and whether the router boots up normally or into ROMMON (which is similar to ‘safe mode’ for windows). You can find the value of the config register by typing show version.