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A router is a special type of computer. It has the same basic components as a standard desktop PC. It has a CPU, memory, a system bus, and various input/output interfaces. However, routers are designed to perform some very specific functions that are not typically performed by desktop computers. For example, routers connect and allow communication between two networks and determine the best path for data to travel through the connected networks.

Just as computers need operating systems to run software applications, routers need the Internetwork Operating System (IOS) software to run configuration files. These configuration files contain the instructions and parameters that control the flow of traffic in and out of the routers. Routers use routing protocols to determine the best path for packets. The configuration file specifies all the information for the correct setup and use of the selected, or enabled, routing and routed protocols on a router.

This course will demonstrate how to build configuration files from the IOS commands in order to get the router to perform many essential network functions. The router configuration file may seem complex at first, but it will be easier to understand by the end of the course.

The main internal components of the router are random-access memory (RAM), nonvolatile random-access memory (NVRAM), flash memory, read-only memory (ROM), and interfaces. 

RAM has the following characteristics and functions:

• Stores routing tables
• Holds ARP cache
• Holds fast-switching cache
• Performs packet buffering as shared RAM
• Maintains packet-hold queues
• Provides temporary memory for the configuration file of a router while the router is powered on
• Loses content when a router is powered down or restarted

NVRAM has the following characteristics and functions:

• Provides storage for the startup configuration file
• Retains content when a router is powered down or restarted

Flash memory has the following characteristics and functions:

• Holds the IOS image
• Allows software to be updated without removing and replacing chips on the processor
• Retains content when a router is powered down or restarted
• Can store multiple versions of IOS software
• Is a type of electrically erasable programmable read-only memory (EEPROM)

ROM has the following characteristics and functions:

• Maintains instructions for power-on self test (POST) diagnostics
• Stores bootstrap program and basic operating system software
• Requires replacing pluggable chips on the motherboard for software upgrades

Interfaces have the following characteristics and functions:

• Connect routers to a network for packet entry and exit
• Can be on the motherboard or on a separate module