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Cisco® Router Basics

The Router

Router Components (internal)

Router Components (external)

Router's Startup Procedure

Configuration Register

Cisco® CLI Command Modes

User Exec Mode

Privileged Exec Mode

Setup Mode

ROM Monitor Mode


The Router

A Router is a layer 3 network device that moves data between different network segments and can look into a packet header to determine the best path for the packet to travel. Routers can connect network segments that use different protocols. They also allow all users in a network to share a single connection to the Internet or a WAN. It is used to improve network performance by:-

• segmenting the network and creating separate collision & broadcast domains.

• reducing competition for bandwidth.

• Broadcasts are not forwarded to other network segments.

• Increases security by using Access Lists.

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Router Components (internal)

ROM is used to store the router's bootstrap startup program, operating system software, and power-on diagnostic tests programs. In order to perform ROM upgrades you remove and replace pluggable chips on the motherboard.

Flash Memory

It holds operating system image(s). Flash memory is erasable, reprogrammable ROM. You can perform Cisco® IOS software upgrades without having to remove and replace chips. Flash content is retained when you switch off or restart the router.


RAM is used to store operational information such as routing tables, router's running configuration file. RAM also provides caching and packet buffering capabilities. Its contents are lost when you switch off or restart the router.
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NVRAM (nonvolatile RAM), is used to store the router's startup configuration file. It does not lose data when power is switched off. So the contents of the startup configuration file are maintained even when you switch off or restart the router.

Network Interfaces

The router's network interfaces are located on the motherboard or on separate interface modules. You configure Ethernet or Token Ring interfaces to allow connection to a LAN. The synchronous serial interfaces are configured to allow connection to WANs. You can also configure ISDN BRI interfaces to allow connection to an ISDN WAN..


Router Components (External)

A router can be configured over any of its network interfaces. You can supply configuration information to a router using:-
  TFTP servers : Trivial File Transfer Protocol; A simplified version of FTP that allows files to be transferred from one computer to another over a network.
  virtual terminals
  network management stations
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Router's Startup Procedure

Each time you switch on the router, it goes through power-on self-test diagnostics to verify basic operation of the CPU, memory and network interfaces.
The system bootstrap software in ROM (boot image) executes and searches for valid router operating system software (Cisco® IOS image). IOS is acronym for Internetwork Operating System.

There are three places to find the Cisco® IOS image to load:
• Flash memory
• A TFTP server on the network

The source of the Cisco® IOS image is determined from the boot field setting of the router's configuration register.
Configuration Registration: A 16-bit register used to control how the router boots up, where the IOS image, how to deal with the NVRAM configuration, setting the console baud rate and enabling or disabling the break function.
The default setting for the configuration register indicates that the router should attempt to load a Cisco® IOS image from flash memory. 
If the router finds a valid IOS image, it searches for a valid configuration file. If your router does not find a valid system image, or if its configuration file is corrupted at startup, and the configuration register (bit 13) is set to enter ROM monitor mode, the system will bypass the NVRAM setting and enters ROM monitor mode. This also allow access to the router in the event a password is lost.

The configuration file, saved in NVRAM, is loaded into main memory and executed one line at a time. These configuration commands start routing processes, supply addresses for interfaces, and set media characteristics.

If no configuration file exists in NVRAM, the operating system executes a question-driven initial configuration routine called the system configuration dialog.

This special mode is also called the Setup mode.
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Cisco® CLI Command Modes

The Cisco® IOS software provides you with access to several different command modes. Each command mode provides a different group of related commands.
The Cisco® Command Line Interface (CLI) is called EXEC. EXEC has two modes:-
• User mode
• Privileged mode
For security purposes the two EXEC modes serve as two levels of access to Cisco® IOS commands.

EXEC user commands allow you to
• connect to remote devices
• make temporary changes to terminal settings
• perform basic tests
• list system information

If you want to access privileged mode you have to enter a password. The commands available in Privileged mode also include all those available in User mode. You can use Privileged EXEC commands to:-
• set operating parameters
• perform a detailed examination of the router's status
• test and debug router operation
• access global and other included configuration modes
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From Privileged mode you can enter global configuration mode. This gives you access to configuration commands that affect the system as a whole, and to other configuration modes.
You can specify the source of the configuration commands as being from :-
• a terminal
• memory
• the network
You can access many other specific configuration modes from Global Configuration mode that allow complex configurations to be performed.

Setup Mode: If the router does not have a configuration file it will automatically enter Setup mode when you switch it on. Setup mode presents you with a prompted dialog, called the system configuration dialog, in which you establish an initial configuration.

Rom Monitor Mode: If the router does not find a valid operating system image, or if you interrupt the boot sequence, the system may enter ROM monitor mode. From ROM monitor mode you can boot the device or perform diagnostic tests.

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