Cisco's Internetworking Operating System (IOS)

This chapter describes how to configure a Cisco IOS device using the Cisco IOS command‐line interface (CLI). The Cisco Internetwork Operating System (IOS) is the kernel of Cisco routers as well as all current Catalyst switches. In case you didn't know, a kernel is the elemental, indispensable part of an operating system that allocates resources and manages tasks like low‐level hardware interfaces and security. The Cisco IOS is a proprietary kernel that provides routing, switching, internetworking, and telecommunications features. The first IOS was written by William Yeager in 1985 and enabled networked applications. When you first bring up a Cisco IOS device, it will run a power‐on self‐test‐a POST. Upon passing that, the machine will look for and then load the Cisco IOS from flash memory if an IOS file is present, then expand it into RAM. As you probably know, flash memory is electronically erasable programmable read‐only memory‐an EEPROM. The next step is for the IOS to locate and load a valid configuration known as the startup‐config that will be stored in nonvolatile RAM (NVRAM). Once the IOS is loaded and up and running, the startup‐config will be copied from NVRAM into RAM and from then on referred to as the running‐config.