IOS Architecture Overview

IOS was originally designed to be a small, embedded system for early Cisco routers. At that time, routers themselves were viewed mostly as hardware appliances—little distinction was made between the hardware and software. In fact, initially IOS wasn't even called “IOS”; it was just referred to as “the OS” that ran a Cisco router.

As routed networks gained popularity, demand rose for routers to support an increasing number of protocols and to provide other functionality, such as bridging. Cisco responded to this enormous demand by adding new features into the router software, resulting in the multi-functional routing and bridging software IOS is today. Interestingly, although the functionality of IOS has grown considerably, ...

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