Terminal Access Controller Access Control System (TACACS) is an authentication protocol that allows a remote access server to forward a user's login password to an authentication server to determine whether access can be allowed to a specific system. TACACS is an encryption protocol and therefore less secure than the later Terminal Access Controller Access Control System Plus (TACACS+) and RADIUS protocols.
TACACS+ and RADIUS have largely replaced earlier protocols in the newer or recently updated networks. TACACS+, which uses Transmission Control Protocol (TCP), is popular because TCP is thought to be a more reliable protocol. RADIUS combines authentication and authorization. TACACS+ separates these operations.