A network engineer configures AAA on a router with the commands 'aaa authentication login default group tacacs+' and points to a single TACACS+ server. During a maintenance window the TACACS+ server becomes unreachable, and all administrators are now locked out of the device even though local usernames exist. What change will allow login using the local database when the TACACS+ server is down, without weakening security while the server is reachable?