This chapter introduced the fundamentals of Cisco IOS software, the operating system that runs on Cisco routers and switches. Cisco IOS software runs on a CPU and requires several types of memory to store the image, configuration, running applications, and data. The CLI is the most common method used to configure, monitor, and troubleshoot a system running Cisco IOS software. The software provides comprehensive debugging and logging capabilities. You can upgrade the software image by downloading it using one of many options and then following a series of simple steps.