|  | 2.2 | WANs and Routers
| 2.2.1 | Router basics
Computers have four basic components: a CPU, memory, interfaces, and a bus. A router also has these components; therefore, it can be called a computer. However, it is a special purpose computer. Instead of having components that are dedicated to video and audio output devices, keyboard and mouse inputs, and all of the typical easy-to-use GUI software of a modern multimedia computer, the router is dedicated to routing. Just as computers need operating systems to run software applications, routers need the Internetworking Operating Software (IOS) to run configuration files. These configuration files control the flow of traffic to the routers. Specifically, by using routing protocols to direct routed protocols and routing tables, they make decisions regarding best path for packets. To control these protocols and these decisions, the router must be configured. You will spend most of this semester learning how to build configuration files from IOS commands in order to get the router to perform the network functions that you desire. While at first glance the router configuration file may look complex, by the end of the semester you will be able to read and completely understand them, as well as write your own configurations. The router is a computer that selects the best paths and manages the switching of packets between two different networks. Internal configuration components of a router are as follows: RAM/DRAM -- Stores routing tables, ARP cache, fast-switching cache, packet buffering (shared RAM), and packet hold queues. RAM also provides temporary and/or running memory for the router’s configuration file while the router is powered on. RAM content is lost when you power down or restart. NVRAM -- nonvolatile RAM; stores a router’s backup/startup configuration file; content remains when you power down or restart. Flash -- erasable, reprogrammable ROM; holds the operating system image and microcode; allows you to update software without removing and replacing chips on the processor; content remains when you power down or restart; multiple versions of IOS software can be stored in Flash memory ROM -- contains power-on diagnostics, a bootstrap program, and operating system software; software upgrades in ROM require replacing pluggable chips on the CPU interface -- network connection through which packets enter and exit a router; it can be on the motherboard or on a separate interface module