9-3
Catalyst 2950 and Catalyst 2955 Switch Software Configuration Guide
78-11380-12
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 Initiation and Message Exchange
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 identity.
Note If IEEE 802.1x 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. 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 using the One-Time-Password (OTP) authentication
method with a RADIUS server.
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