IOS CLI access, mode navigation

Ways to reach the CLI

• Console: direct cabled connection, must be physically at the device. Called out-of-band (OOB) access since it uses no network bandwidth. In PuTTY: connection type Serial, COM port, speed 9600.
• SSH / Telnet: network-based, called in-band access. Requires an active network service on the device and consumes bandwidth. SSH is encrypted (port 22) and the production standard; Telnet is cleartext (port 23), lab only.
• AUX port: legacy option on some routers for remote CLI over a modem. OOB like console, no network services needed.

Any CLI session, however you got there, is an EXEC session.

Modal operating system

IOS is modal: distinct modes, each with its own command set and command history, arranged in a hierarchy from least to most specific. You enter a more specific mode from the one above it.

Interface commands only exist in interface config mode, so the full path there is: user EXEC > privileged EXEC > global config > interface config. Commands entered in interface config mode apply only to the interface you selected.

Mode navigation

• User EXEC, Switch>: where a session starts. Terminal settings, basic connectivity tests (ping, traceroute), limited show commands. Leave with logout, exit, or quit.
• Privileged EXEC, Switch#: enter with enable. Full show and verify commands, copy for backups and TFTP transfers, and the gateway to config modes. Password-protect this. disable drops back to user EXEC.
• Global config, Switch(config)#: enter with configure terminal (conf t). Device-wide settings: hostname, spanning tree mode, SVIs, HSRP. Leave with exit, end, or Ctrl+Z.
• Interface config, Switch(config-if)#: enter with interface <label>, like interface Ethernet 0/0. Per-interface settings: description, shutdown, VLAN assignment. exit goes one level up; end or Ctrl+Z go all the way to privileged EXEC.

Rules of thumb: exit is back one level, while end and Ctrl+Z are back to privileged EXEC from any config mode. Help (?) only shows commands valid at your current prompt.

You do not have to exit back to global config to switch interfaces. From (config-if) just type interface Ethernet 0/1 and you’re configuring the new one. The prompt won’t change (it doesn’t show which interface), so keep track of where you are.

Context-sensitive help

• ? lists available commands in the current mode.
• show v? lists all completions starting with v; show vlan ? lists the next options (brief, id, or <cr> meaning you can just hit Enter).
• Tab autocompletes a partial command.
• Paging: spacebar for the next page, q to quit the output.

Odds and ends from the lab demo

Default VLANs on a switch: VLAN 1 and 1002-1005 exist out of the box, reserved, and can’t be deleted.

SSH’d in? You won’t see syslog messages unless you run terminal monitor.

User EXEC is junior admin and server admin territory; privileged EXEC is the network engineer’s mode. Remote logins often drop you straight into privileged.

Configuration files

A config file is just the IOS commands that tell the device what to do. startup-config is the dedicated file read and parsed at boot; running-config is the live config in memory, where commands you type at the CLI execute immediately and land.

Reading a config: sections are separated by ! lines. Global commands sit at the left margin (hostname, spanning-tree mode, ip default-gateway, ntp server); indented lines belong to the block above them (an interface’s switchport mode, description, ip address).