AAA (Authentication, Authorization and Accounting) Server
In an Enterprise Network where there are hundreds of thousands of Switches so it is not possible to configure/change the passwords regularly & Secure the CLI on each and every device manually/locally.
AAA servers centralize the Authentication of the user of a Network (hold the usernames/passwords of all the devices of that Network).
It authenticates the password before a user can access the Network.
AAA Server like Cisco ACS (Access Control Server) Servers uses 802.1x Protocols like RADIUS (Remote Authentication Dial-In User Service) / TACACS+ (Terminal Access Controller Access-Control System Plus) for authentication [[202012050659 AAA 802.1x Protocols (Radius & TACACS+)]].
AAA Server may use Local Data Base or AD for storing those Usernames & passwords.
Flow:
PC (also running 802.1x compliant Software) will put in the Username & Password & try to login the Switch/Router (Authenticator) using Telnet/SSH.
Switch then forwards that frame (without reading) to the AAA Servers using 802.1x Protocols like RADIUS / TACACS+ for authentication/approval of those Username/Passwords.
If credentials matches then AAA Server will reply in affirmation to Switch (Authenticator) which then permits the client to the Network.
Reference:
CCNA 200-301 OCG, Volume 1, Pg. 136 - Wendell Odom.
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