How 8021x Works

End User (Client)

Cisco Catalyst 2950 Series Switch (NAD)

Authentication Server (Cisco Secure ACS)

End User (Client)

Cisco Catalyst 2950 Series Switch (NAD)

EAPOL

Authentication Server (Cisco Secure ACS)

EAPOL

RADIUS

The actual authentication conversation occurs between the client and the authentication server using EAP. The authenticator is aware of this activity, but it is just an intermediary.

© 2007 Cisco Systems, Inc. All rights

The switch or the client can initiate authentication. If you enable authentication on a port by using the dotlx port-control auto interface configuration command, the switch must initiate authentication when it determines that the port link-state transitions from down to up. It then sends an EAP Request/Identity frame to the client to request its identity (typically, the switch sends an initial Request/Identity frame followed by one or more requests for authentication information). Upon receipt of the frame, the client responds with an EAP Response/Identity frame.

However, if during bootup the client does not receive an EAP Request/Identity frame from the switch, the client can initiate authentication by sending an EAPOL-start frame, which prompts the switch to request the identity of the client.

Note If 802.1x is not enabled or supported on the NAD, any EAPOL frames from the client are

When the client supplies its identity, the switch begins its role as the intermediary, passing EAP frames between the client and the authentication server until authentication succeeds or fails. If the authentication succeeds, the switch port becomes authorized.

The specific exchange of EAP frames depends on the authentication method being used. The diagram shows a message exchange initiated by the client using the one-time password (OTP) authentication method with a RADIUS server.

dropped. If the client does not receive an EAP Request/Identity frame after three attempts to start authentication, the client transmits frames as if the port is in the authorized state. A port in the authorized state effectively means that the client has been successfully authenticated.

© 2007 Cisco Systems, Inc.

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