Setting a Telnet password
By default, the device does not require a user name or password when you log in to the CLI using Telnet. You can assign a password for Telnet access using one of the following methods.
Set the password "letmein" for Telnet access to the CLI using the following command at the global CONFIG level.
device(config)#enable telnet password letmein
Syntax: [no] enable telnet password string