Router configuration information can come from many sources.
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Broadly speaking, configuration components may be divided into external and internal components.
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All Cisco router models include an asynchronous serial port.
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This console port provides local access to the router using a console terminal (an ASCII terminal or a PC running terminal emulation software).
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Routers also have an auxiliary port, allowing you to configure them remotely using a modem.
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A router can be configured over any of its network interfaces.
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You can supply configuration information to a router using

• TFTP servers
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• virtual terminals
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• network management stations
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The internal configuration components of a router include

• ROM and flash memory
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• RAM and NVRAM
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• Interfaces
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Routers use ROM to store the bootstrap startup program, operating system software, and power-on diagnostics.
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So to perform ROM upgrades you remove and replace pluggable chips on the motherboard.
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Flash memory is erasable, reprogrammable ROM.
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It holds an operating system image and microcode.
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You can perform Cisco IOS software upgrades without having to remove and replace chips.
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Flash content is retained when you switch off or restart the router.
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RAM is used to store operational information such as routing tables.
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RAM also provides caching and packet buffering capabilities.
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RAM also holds the router's running configuration file while the router is switched on.
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Of course RAM 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.
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Nonvolatile memory does not lose data when power is removed from it.
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So the contents of the startup configuration file are maintained even when you switch off or restart the router.
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The router's network interfaces are located on the motherboard or on separate interface modules.
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You configure Ethernet or Token Ring interfaces to allow connection to a LAN.
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The synchronous serial interfaces are configured to allow connection to WANs.
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You can also configure ISDN BRI interfaces to allow connection to an ISDN WAN.
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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.
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The system bootstrap software in ROM (boot image) executes and searches for valid router operating system software (Cisco IOS image).
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There are three places to find the Cisco IOS image to load:

• from flash memory
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• from a tftp server on 
  the network
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• from ROM
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The source of the Cisco IOS image is determined from the boot field setting of the router's configuration register.
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The default setting for the configuration register indicates that the router should attempt to load a Cisco IOS image from flash memory.
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If the router finds a valid IOS image, it searches for a valid configuration file.
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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 enters ROM monitor mode.
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The configuration file, saved in NVRAM, is loaded into main memory and executed one line at a time.
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These configuration commands start routing processes, supply addresses for interfaces, and set media characteristics.
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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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The Cisco IOS software provides you with access to several different command modes.
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Each command mode provides a different group of related commands.
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The Cisco command line user interface is called EXEC.
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EXEC has two modes:

• User mode
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• Privileged mode
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For security purposes the two EXEC modes serve as two levels of access to Cisco IOS commands.
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EXEC user commands allow you to

• connect to remote devices
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• make temporary changes 
  to terminal settings
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• perform basic tests
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• list system information
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If you want to access privileged mode you have to enter a password.
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The commands available in Privileged mode also include all those available in User mode.
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You can use Privileged EXEC commands to

• set operating parameters
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• perform a detailed examination 
  of the router's status
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• test and debug router operation
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• access global and other included 
  configuration modes
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From Privileged mode you can enter global configuration mode.
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This gives you access to configuration commands that affect the system as a whole, and to other configuration modes.
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You can specify the source of the configuration commands as being from

• a terminal
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• memory
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• the network
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You can access many other specific configuration modes from Global Configuration mode that allow complex configurations to be performed.
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If the router does not have a configuration file it will automatically enter Setup mode when you switch it on.
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Setup mode presents you with a prompted dialog, called the system configuration dialog, in which you establish an initial configuration.
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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.
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From ROM monitor mode you can boot the device or perform diagnostic tests.
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