|  |  | Like TACACS+, RADIUS is a client-server model of a Triple A system that accepts a request from a RADIUS client and passes it to a RADIUS server for authentication. RADIUS supports: TACACS+ Kerberos local username lookup CiscoSecure ACS supports RADIUS on the following series routers: 1600, 2500,2600, 3600, 4000, 5000, and 7000. RADIUS supports PAP and CHAP. RADIUS standards are listed in Request for Comment (RFC) 2138 and 2139.