Allied Telesis AT-S62 - Manual (Page 556)

Chapter 29: RADIUS and TACACS+ Authentication Protocols
Section VII: Management Security 556
When a switch receives a username and password combination from a
network manager, it sends the combination to the first authentication
server in its list. If the server fails to respond, the switch sends the
combination to the next server in the list, and so on.
If no authentication server responds or if no servers have been defined
and you are managing the switch locally, the management software
defaults to the standard manager and operator accounts.
Note
For more information on TACACS+, refer to the RFC 1492 standard.
For more information on RADIUS, refer to the RFC 2865 standard.