802.1x works on a complex series of challenges and responses. This topic
covers the authentication initiation, message exchanges, and port states.
Authentication Initiation and Message Exchange
The switch or the
client can initiate authentication. If authentication on a port is enabled by
using the dot1x 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. Upon receipt
of the frame, the client responds with an EAP-response/identity frame.
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. This prompts the switch to request the identity of the client.
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. Figures
and
show a
message exchange initiated by the client using the One-Time-Password (OTP)
authentication method with a RADIUS server.
Ports in Authorized and
Unauthorized States
The switch port state determines whether or not the
client is granted access to the network. The port starts in the unauthorized
state. While in this state, the port disallows all ingress and egress traffic
except for 802.1x packets. When a client is successfully authenticated, the
port transitions to the authorized state, allowing all traffic for the client
to flow normally.
If a client that does not support 802.1x is connected
to an unauthorized 802.1x port, the switch requests the identity of the client.
In this situation, the client does not respond to the request, the port remains
in the unauthorized state, and the client is not granted access to the
network.
In contrast, when an 802.1x-enabled client connects to a port
that is not running 802.1x, the client initiates the authentication process by
sending the EAPOL-start frame. When no response is received, the client sends
the request for a fixed number of times. Because no response is received, the
client begins sending frames as if the port is in the authorized state.
The port authorization state can by manually configured using the
dot1x port-control interface configuration command. The
syntax for the dot1x port-control command is shown in
Figure
.
If the
client is successfully authenticated, the port state changes to authorized, and
all frames from the authenticated client are allowed through the port. If the
authentication fails, the port remains in the unauthorized state, but
authentication can be retried. If the authentication server cannot be reached,
the switch can retransmit the request. If no response is received from the
server after the specified number of attempts, authentication fails, and
network access is not granted.
When a client logs off, it sends an
EAPOL-logoff message, causing the switch port to transition to the unauthorized
state.
If the link state of a port transitions from up to down, or if an
EAPOL-logoff frame is received, the port returns to the unauthorized state.