How does command abbreviation work in Cisco IOS?

You can shorten a command to the fewest characters that still uniquely identify it (e.g. conf for configure); too few characters gives an "Ambiguous command" error.

Commands can be shortened to the minimum number of characters that identify a unique selection.

Examples:

• configure can be shortened to conf (unique identifier)
• con alone is ambiguous (configure vs connect)

Router#con
% Ambiguous command: "con"
Router#con?
configure  connect
Router#conf t
Router(config)#