You ever get a shiny new router out of the box, lovingly install a T3 card into the proper interface, fire up the router, and then realize that the T3 interface doesn’t show up in the configuration?
Must be the IOS version doesn’t recognize the card. Nope. A “show version” tells you that the serial interface is indeed recognized. So what then?
The answer, depending on the serial interface, is that you might have to tell IOS whether the serial interface will act as, in my case today, a T3 or an E3. You do that with the “card type” command. After that, off you go.
I was racking my brains on that one for a while before I spotted the command doing a “?” at the prompt and the bell went off in my head. Not everyday I build a new WAN router, and that detail is easy to forget.