Tacacs+

Enable aaa (beware disables current auth methods) :

aaa new-model

Define tacacs+ server :

tacacs-server host 1.2.3.4
tacacs-server key 0null0

Define aaa authentication method for ‘VTY’ group :

aaa authentication login VTY group tacacs+

Apply to telnet lines :

line vty 0 5
login authentication VTY