The IEEE 802.1x standard defines three participants in an authentication conversation (see Figure 1 which shows an example with the 7450 ESS).
The supplicant — This is the end-user device that requests access to the network.
The authenticator — Controls access to the network. Both the supplicant and the authenticator are referred to as Port Authentication Entities (PAEs).
The authentication server — Performs the actual processing of the user information.
The authentication exchange is carried out between the supplicant and the authentication server, the authenticator acts only as a bridge. The communication between the supplicant and the authenticator is done through the Extended Authentication Protocol (EAP) over LANs (EAPOL). On the back end, the communication between the authenticator and the authentication server is done with the RADIUS protocol. The authenticator is therefore a RADIUS client, and the authentication server a RADIUS server.
The messages involved in the authentication procedure are shown in Figure 2. The router initiates the procedure when the Ethernet port becomes operationally up, by sending a special PDU called EAP-Request/ID to the client. The client can also initiate the exchange by sending an EAPOL-start PDU, if it does not receive the EAP-Request/ID frame during bootup. The client responds on the EAP-Request/ID with a EAP-Response/ID frame, containing its identity (typically username + password).
After receiving the EAP-Response/ID frame, the router encapsulates the identity information into a RADIUS AccessRequest packet, and sends it off to the configured RADIUS server.
The RADIUS server checks the supplied credentials, and if approved returns an Access Accept message to the router. The router notifies the client with an EAP-Success PDU and puts the port in authorized state.