Router Config

Cisco routers run a POST when powered on and then look for and load the Cisco IOS. It then looks for a file called startup-config that is stored in NVRAM. If there is no configuration in NVRAM you will go into setup mode. This is not the only way you can get into setup up mode, also by typing setup you can get into it. When going through setup answers in brackets are the default answers. Command line interface (CLI) gives you the most flexibility in configuring a router.

RAM–Stores the running configuration and the routing tables. It is also used for the caching and packet buffers. Therefore, RAM is important to a busy router. The more buffers that are used the more RAM you will need. Remember, if the router loses power, RAM will be emptied and all will be lost.

Cisco Router Startup


ROM-Stores a bootable IOS image.

Four areas of ROM microcode are:

bootstrap code POST code ROM monitor



Flash Memory-Default location of the IOS. Information is retained even when the router is powered down. To get the IOS you need to copy it from TFTP the command for that is copy tftp flash, this command will copy the IOS on the TFTP server to your flash memory. If you want to back up your IOS on the TFTP server you use the command copy flash tftp, notice the difference in the commands. You must be in privileged mode to perform these tasks. To complete this command you will be asked to give additional information about the TFTP server. Such as the IP address or the host name of the server, the name of the file, and whether or not there is enough memory in flash to download the program into it.

If you do not want to boot from flash you must use the boot system command. The command is boot system tftp ios_filename tftp_ip_address. First part of the command is the command itself, followed by the location of the IOS, The IOS filename, and finally the IP address of the TFTP server. If you want to use the IOS in ROM you type the boot system ROM command. If you want to revert back to the original in flash you type the boot system flash command. The squeeze command will delete the IOS where it is loaded.

NVRAM-Stores the initial startup configuration files and the configuration register. Contains information even when the router is powered down. The configuration register controls how the router boots up and where to locate the IOS image.

When a router boots up, the first thing that happens is that it performs POST (power on self-test). Then it loads the bootstrap from the ROM. After that it locates and loads the operating system from Flash, finally it locates the config file in NVRAM.

Command Line Logging on to a Router

Enable Command allows a user to move into the privileged mode. Privileged mode is the only way to configure the router. User is the default mode used for checking router status and viewing the basic system. Access is very limited.

Router>enable Router#

??To go back just type disable. ??To exit type exit

Router#disable Router>

Router#exit Router>

Help Commands

? -Can be typed at anytime to help on all commands ?Help- Text of help commands ?Com?- Commands that start with COM Command?- Help on all command options


Router> User Mode

Router>enable Router# Privileged Mode

Router#config t Router(config)# Global Config Mode

Router(config)#int e0 Router(config-if)# Interface Config Mode

User Mode

User mode will provide display only. You will be able to see information but you will not be able to make any changes.

Privileged Mode

This mode will enable you to do an extensive review of the router. You will be able to do testing, debugging and use commands to manage the configuration files

Setup Mode When you are going through setup mode on a router it will ask you if you would like to save the configuration files if you say yes it copies the files from DRAM to NVRAM and names the file startup-config. It continues to use this configuration every time the router is rebooted.


Maintenance can be performed on the router using RXBOOT Mode. This mode will enable recovery functions if the router password is lost to the IOS file that is stored in Flash memory is accidentally erased. You can enter this mode by pressing the Break key within the first 60 seconds of startup.


Up-arrow previous command ?Down-arrow next command ?CTRL+A moves cursor to the first character of command CTRL+E moves cursor to the end of the command ESC+B moves backward one word ESC+F moves forward one word

To enter a banner type banner motd#

This will allow you to display a message of the day. However, typically it is used to display legal messages. Example:

router(config)#banner motd * Authorized Access Only, All Violations Will Be Prosecuted *

Notice the asterisk (*) before and after the message. This outlines the message that will be visible to users.

Startup Config There are two types of configuration files, startup and running. Previously mentioned DRAM configurations are copied into NVRAM during boot. The files are “_running”_when there are in NVRAM and “_tartup”_files when they are in DRAM. To show configuration type, show startup-config. When changes are made by typing config t in global configuration mode, they need to be copied to startup by typing copy running-config startup-config.

The command copy run start will copy the running config to the startup up config. The command copy start run will copy the startup config to the running config. Here are a few other commands and their functions.

show startup-config shows config that router will be loaded when router boots

show running-config config that is currently loaded into RAM

Erase startup-config erases config and enters setup

copy running-config copies current config to startup config

startup-config configuration file

copy startup-config copies startup-config to running-config