The Cisco IOS command line

Why learn the CLI?

Most enterprise network devices are still configured by typing commands. Cisco IOS (Internetwork Operating System) is the most common, and the CCNA exam and most network jobs expect you to use it comfortably. Other vendors have similar command lines, so the skill carries over.

Getting in: console and SSH

A brand-new device has no IP address, so the first time you connect with a console cable from your laptop to the console port (RJ45 or USB), using a terminal program such as PuTTY or Tera Term (9600 baud, 8 data bits, no parity, 1 stop bit). Once the device has an IP address and SSH is set up (covered in the security module), you manage it over the network with SSH. Telnet also works but sends passwords in plain text, so avoid it.

The modes and their prompts

exit goes back one level. end (or Ctrl+Z) jumps straight back to privileged EXEC. disable goes from # back to >.

Help, completion and abbreviations

• Type ? to list the commands allowed in this mode. Type sh? to see commands starting with sh. Type show ? to see what can follow show.
• Press Tab to complete a word: conf Tab becomes configure.
• You can shorten any word as long as it is unique: conf t for configure terminal, sh ip int br for show ip interface brief, wr for write.
• The up arrow recalls earlier commands.
• In configuration mode, show commands do not work directly. Put do in front: do show ip interface brief.

Running-config and startup-config

Every command you type in configuration mode takes effect at once and is stored in the running-config, which lives in RAM. RAM is cleared when the device reboots or loses power. The startup-config is stored in NVRAM (non-volatile RAM), which survives a reboot; the device loads it when it starts. So after a good change, save it.

Switch# show running-config        ! what the device is doing now (RAM)
Switch# show startup-config        ! what it will load at boot (NVRAM)
Switch# copy running-config startup-config
Destination filename [startup-config]?    ! press Enter
Switch# write memory               ! older shortcut that does the same

A first configuration

Switch> enable
Switch# configure terminal
Switch(config)# hostname SW1-Floor2
SW1-Floor2(config)# enable secret Str0ng!Lab
SW1-Floor2(config)# no ip domain-lookup
SW1-Floor2(config)# line console 0
SW1-Floor2(config-line)# password C0nsole!Lab
SW1-Floor2(config-line)# login
SW1-Floor2(config-line)# exit
SW1-Floor2(config)# interface vlan 1
SW1-Floor2(config-if)# ip address 192.168.1.2 255.255.255.0
SW1-Floor2(config-if)# no shutdown
SW1-Floor2(config-if)# exit
SW1-Floor2(config)# ip default-gateway 192.168.1.1
SW1-Floor2(config)# end
SW1-Floor2# copy running-config startup-config

• hostname changes the name in the prompt, so you always know which device you are on.
• enable secret sets the password for the enable command, stored as a hash (a one-way scrambled form). The older enable password is stored in plain text or weak encryption, so use enable secret.
• line console 0 with password and login asks for a password on the console port.
• no ip domain-lookup stops the device trying to look up a mistyped command as a host name, which otherwise freezes the prompt for several seconds.
• A Layer 2 switch gets its management IP address on a virtual interface, interface vlan 1. A router gets IP addresses on its physical interfaces, for example interface g0/1 then ip address 10.0.0.1 255.255.255.0.
• no shutdown turns an interface on. Router interfaces are shut down by default; switch ports are on by default.

The command you will use most

R1# show ip interface brief
Interface              IP-Address      OK? Method Status                Protocol
GigabitEthernet0/0     192.168.1.1     YES manual up                    up
GigabitEthernet0/1     10.0.0.1        YES manual administratively down down
GigabitEthernet0/2     unassigned      YES unset  down                  down