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802.1X adopts a client/server architecture with three entities: a supplicant system, an

authenticator system, and an authentication server system, as shown in the following figure.

Figure 11-17 Architecture of 802.1X authentication

（1） Supplicant System: The supplicant system is an entity in LAN and is authenticated by the

authenticator system. The supplicant system is usually a common user terminal computer.

An 802.1X authentication is initiated when a user launches client program on the

supplicant system. Note that the client program must support the 802.1X authentication

protocol.

（2） Authenticator System: The authenticator system is usually an 802.1X-supported network

device, such as this TP-LINK switch. It provides the physical or logical port for the

supplicant system to access the LAN and authenticates the supplicant system.

（3） Authentication Server System: The authentication server system is an entity that

provides authentication service to the authenticator system. Normally in the form of a

RADIUS server. Authentication Server can store user information and serve to perform

authentication and authorization. To ensure a stable authentication system, an alternate

authentication server can be specified. If the main authentication server is in trouble, the

alternate authentication server can substitute it to provide normal authentication service.

¾ The Mechanism of an 802.1X Authentication System

IEEE 802.1X authentication system uses EAP (Extensible Authentication Protocol) to exchange

information between the supplicant system and the authentication server.

（1） EAP protocol packets transmitted between the supplicant system and the authenticator

system are encapsulated as EAPOL packets.

（2） EAP protocol packets transmitted between the authenticator system and the RADIUS

server can either be encapsulated as EAPOR (EAP over RADIUS) packets or be

terminated at authenticator system and the authenticator system then communicate with

RADIUS servers through PAP (Password Authentication Protocol) or CHAP (Challenge

Handshake Authentication Protocol) protocol packets.

（3） When a supplicant system passes the authentication, the authentication server passes the

information about the supplicant system to the authenticator system. The authenticator

system in turn determines the state (authorized or unauthorized) of the controlled port

according to the instructions (accept or reject) received from the RADIUS server.