Internetwork Operating System is the OS Cisco uses on their brand of switches and routers. It's basically all from from a Command Line Interface, and requires knowledge of hundreds of commands to configure. Well, actually it's pretty helpful if you forget some specifics... at any time typing '?' will get you a list of available commands and there is plenty of '?' information about each command. It has nice little things like it's *nix-like behaviour of completing lines for you when you hit Tab, but the massive amount of context sensitive help is great.

The IOS has a few different modes:

  • User Exec Mode This is the first mode you are dumped into after logging into a router. You can issue a few informational commands but you can't do any real damage because all the good commands are disabled. So you type enable (actually just typing 'en' will suffice)
  • Enabled / Privileged Mode Now we're talking, this is where you can enter real commands, and by that I mean: wreck shit. From here you can do a lot of show commands and pretty much get any information you need and there's a lot of other commands you can use here that are outside the scope of this doc. You can reach configuration mode by typing 'config t'
  • Configuration Mode From here you can edit the configuration of the router, routing protocols, interfaces and the console.
To back out of a current mode to go back to the last mode you can type 'exit' or hit ctrl + z. The default prompt consists of the hostname of the device followed by your location in the system, much like a *nix system works:

The current release at the time of this writing is 12.2. You can read about the various features assosiated with each release on Cisco's website ( ).

Some fun commands to explore:

This command is crazyass. I can't count the number of times I've been warned against this command. It basically logs all these debug messages to the console for debugging (duh) purposes. Sometimes this can generate a lot of extra overhead and crash a router.
delete nvram
NVRAM stands for Non-Volatile RAM, which is where the config is saved.
This leads you through setting up some basics on the router to get it up and running. It will lead you through configuring what protocols will be running on it as well as setting up any interfaces that are present. It's good for newbies, but 'router gods' will look down on you.
'show ?'
will get you a bunch of options. Basically show is your window into everything that's going on in the router. Some good options are: show interfaces, show version, show ip route, show running config. These will get you started, and these are commands you end up using all the time.
IOS runs telnet clients and servers on all the devices so you can hop around from router to router using this. It can get a bit confusing because you can keep hopping from router to router until you're going though a half dozen devices, and on top of that you can run multiple concurrent sessions on each router. You gotta keep an eye on your prompt and remember where you are...

See also: Cisco IOS

Log in or register to write something here or to contact authors.