Cisco IOS (Internetwork Operating System) is the system that drives Cisco switches and routers. You use it from the command line (CLI), organised into hierarchical modes that you can recognise from the prompt symbol.

The three main modes

• User mode — prompt >, limited read-only access.
• Privileged mode (EXEC) — prompt #, access to the full show commands and to configuration.
• Global configuration mode — prompt (config)#, where you change the device settings.

You move up through the modes with specific commands, and step back down with exit or end.

Switch> enable
Switch# configure terminal
Switch(config)# interface vlan 1
Switch(config-if)# exit
Switch(config)# end
Switch#

From global mode you enter sub-modes (interface, line, router), each identifiable by its own prompt, such as (config-if)# for an interface. Reading the prompt carefully stops you from typing a command in the wrong place.

Key point: enable takes you to privileged mode, configure terminal to global configuration. The prompt always tells you where you are. See Lab 4 – Navigating the IOS modes.