In the previous article, Cisco IOS Context-Sensitive Help, you learned how to use the built-in help system to discover new commands. We also talked briefly about error messages and how IOS abides by the rule of silence:
When a program has nothing surprising to say, it should say nothing.
Types of Error Messages
Cisco IOS is very unforgiving. When we issue a command to our routers or switches, the device will execute the command immediately. IOS won’t ask you for confirmation, even if it means that you’ll totally screw things up — and, just for reassurance, you will at some point; we all do.
When we issue commands that the router doesn’t understand or other cannot execute, it will return an error message. Usually the error messages are helpful and sometimes they are cryptic but, in general, they fall into one of three categories:
- IOS isn’t a mind reader
- You forgot something
- You really screwed up
IOS isn’t a mind reader, a.k.a. ambiguous command
This one will make a bit more sense once you learn some Cisco IOS Shortcuts but, in a nutshell, you didn’t type in enough characters for IOS to figure out which command you are referring to.
Router(config-if)# ip a % Ambiguous command: "ip a"
In other words, there are multiple commands you could issue here that begin with “t” and you didn’t provide enough information for IOS to figure out which one you meant. Be more specific.
You forgot something, a.k.a. incomplete command
You issued a command but IOS was expecting at least one (more) parameter that you didn’t supply. You were off to a good start, though. Try again.
Router(config-if)# ip address % Incomplete command.
You really screwed up, a.k.a. invalid input
In this case, IOS really has no idea what you’re talking about. Stop and think it over for a minute, then give it another shot.
Router(config-if)# ip address CHOCOLATE MILK ^ % Invalid input detected at '^' marker.
You’ll also run into this error when you attempt to execute a command in the wrong mode, such as if you attempt a show command in global config mode. I promise that you will do that at some point and you’ll be left wondering why a command that you know is 100% valid isn’t working.
IOS error messages range from the extremely detailed to the completely useless, but the ones you’ll run into the most often while studying for the CCNA test will likely fall into one of the above categories.
Now that we’ve got these out of the way, let’s take a look at some Cisco IOS Shortcuts to save you some typing and make you more efficient on the CLI.