TACACS+ User Authentication, Authorization, and Accounting

Terminal Access Controller Access-Control System Plus (TACACS+) is a centralized security protocol that provides authentication, authorization, and accounting (AAA) services for user access to network devices and management applications. Instead of maintaining user accounts and access policies on the device, you can use a TACACS+ server to centrally manage user authentication and access permissions.

You can use TACACS+ to manage user access to the device's Web interface and CLI. When TACACS+ is enabled, the device forwards Web interface and CLI user login requests to a TACACS+ server. The server validates the user's credentials and returns the authentication result to the device. If authentication succeeds, the TACACS+ server also provides authorization information that determines the user's management privileges (access levels) on the device.

TACACS+ provides the following services:

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Authentication: Verifies the identity of users attempting to log in to the device.

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Authorization: Determines the management privileges granted to authenticated users.

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Accounting: Records user login sessions and activities for auditing, security monitoring, and compliance purposes.

The device acts as a TACACS+ client, communicating with one or more external TACACS+ servers. After successful authentication, the TACACS+ server returns the user's authorization level. The device then uses its configured TACACS Authorization Mapping table to map the returned TACACS+ authorization level to a device user level (role). Based on the configured mapping, the user is assigned one of the user levels - Security Administrator, Administrator, or Monitor. The device subsequently enforces the permissions associated with the assigned user level.

To help ensure continuous administrative access, the device supports fallback authentication using locally configured user accounts. If the device cannot communicate with the TACACS+ server due to, for example, a timeout, connectivity problem, or other server availability issues, authentication can fall back to users configured in the device Local Users table. This enables authorized administrators to continue managing the device even when the TACACS+ service is temporarily unavailable.

Accounting tracks what the user does during their session. The device sends activity logs back to the TACACS+ server, recording information such as login times, logout times, and the specific CLI commands executed during the session. This information provides a reliable audit trail for security compliance and troubleshooting.

By centralizing authentication, authorization, and accounting functions, TACACS+ helps organizations enforce consistent access-control policies, simplify user management, improve security, and maintain detailed audit records of administrative access to the device.