The three versions of Terminal Access Controller Access Control System (TACACS) are: TACACS, Extended TACACS (XTACACS), and TACACS+. Each version authenticates users and prohibits access to those without a verified username/password pairing.
- TACACS combines the authentication and authorization functions.
- XTACACS allows the separation of the authentication, authorization, and auditing functions, giving administrators more discerning control over its deployment.
- TACACS+ also allows the division of the authentication, authorization, and auditing but also provides two-factor authentication.
The authentication process with TACACS is similar to RADIUS and it parallels in functionality. However, RADIUS follows an Internet standard, and TACACS is a proprietary protocol. This difference has made TACACS less popular than RADIUS.