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9-3
Catalyst 2950 and Catalyst 2955 Switch Software Configuration Guide
OL-10101-02
Chapter 9 Configuring IEEE 802.1x Port-Based Authentication
Understanding IEEE 802.1x Port-Based Authentication
information with the authentication server, and relaying a response to the client. The switch includes
the RADIUS client, which is responsible for encapsulating and decapsulating the EAP frames and
interacting with the authentication server.
When the switch receives EAPOL frames and relays them to the authentication server, the Ethernet
header is stripped, and the remaining EAP frame is re-encapsulated in the RADIUS format. The
EAP frames are not modified during encapsulation, and the authentication server must support EAP
within the native frame format. When the switch receives frames from the authentication server, the
server frame header is removed, leaving the EAP frame, which is then encapsulated for Ethernet and
sent to the client.
The devices that can act as intermediaries include the Catalyst 3750, 3560, 3550, 2970, 2955, 2950, 2940
switches, or a wireless access point. These devices must be running software that supports the RADIUS
client and IEEE 802.1x authentication.
Authentication Initiation and Message Exchange
During IEEE 8021.x authentication, the switch or the client can initiate authentication. If you enable
authentication on a port by using the dot1x port-control auto interface configuration command, the
switch initiates authentication when the link state changes from down to up or periodically as long as
the port remains up and unauthenticated. The switch sends an EAP-request/identity frame to the client
to request its identity. 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 client’s identity.
Note If IEEE 802.1x authentication is not enabled or supported on the network access device, any EAPOL
frames from the client are dropped. If the client does not receive an EAP-request/identity frame after
three attempts to start authentication, the client sends 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. For
more information, see the “Ports in Authorized and Unauthorized States” section on page 9-4.
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. If the authentication fails, authentication
can be retried, the port might be assigned to a VLAN that provides limited services, or network access
is not granted. For more information, see the
“Ports in Authorized and Unauthorized States” section on
page 9-4.
The specific exchange of EAP frames depends on the authentication method being used. Figure 9-2
shows a message exchange initiated by the client when the client uses the One-Time-Password (OTP)
authentication method with a RADIUS server.