Cisco - Configuration - Configure Banners

There are different banners:

MOTD banner: Will be displayed when attaching to the router, regardless of how you access the router.
Exec banner: Displayed when an EXEC process (line activation or incoming connection to a VTY line) is created.
- When starting a user exec session through a console port, exec banner will be activated.
Login banner: This banner is displayed after the MOTD banner but before the login prompts.
- Typically, this banner is used to display a permanent message to the users.

banner motd "Some message" #
banner exec "Some message" #
banner login "Some message" #

NOTE: The hash at the end is used to end the message.

motd example

conf t
Enter configuration commands, one per line.  End with CNTL/Z.
banner motd "Authorized access only" #

NOTE: After reloading the router, the MOTD is displayed.