Cisco IOS Fundamentals

Cisco IOS is the operating software behind the command-line experience on many Cisco routers and switches. Cisco IOS XE documentation uses the same core administrative model. Exact features vary by product and release.

You use the command-line interface, or CLI, to monitor a device, change its configuration, and investigate problems. The CLI is organized into command modes. Each mode limits which commands you can run and shows a distinct prompt. That structure helps you see both your current privilege and the part of the configuration you are editing.

The central mental model

Think of an IOS device as three connected views:

1. Operational state tells you what the device is doing now. Interfaces, routes, counters, and protocol neighbors belong here.
2. Running configuration contains the commands currently controlling the device. Most configuration changes take effect here as you enter them.
3. Startup configuration supplies configuration commands during startup. It can differ from the running configuration until you save.

This distinction prevents a common mistake. A working change in the running configuration is not necessarily durable. If the device restarts before you copy the running configuration to the startup configuration, that change may disappear.