Antaira Technologies - Industrial Ethernet Switches
LNX-2012G-SFP Series User Manual V1.0
61
Overview of 802.1X (Port-Based) Authentication
In the 802.1X-world, the user is called the supplicant, the switch is the authenticator, and the
RADIUS server is the authentication server. The switch acts as the man-in-the-middle,
forwarding requests and responses between the supplicant and the authentication server.
Frames sent between the supplicant and the switch is special 802.1X frames, known as
EAPOL (EAP Over LANs) frames. EAPOL frames encapsulate EAP PDUs (RFC3748). Frames
sent between the switch and the RADIUS server is RADIUS packets. RADIUS packets also
encapsulate EAP PDUs together with other attributes like the switch's IP address, name, and
the supplicant's port number on the switch. EAP is very flexible, in that it allows for different
authentication methods, like MD5-Challenge, PEAP, and TLS. The important thing is that the
authenticator (the switch) doesn't need to know which authentication method the supplicant
and the authentication server are using, or how many information exchange frames are needed
for a particular method. The switch simply encapsulates the EAP part of the frame into the
relevant type (EAPOL or RADIUS) and forwards it.
When authentication is complete, the RADIUS server sends a special packet containing a
success or failure indication. Besides forwarding this decision to the supplicant, the switch uses
it to open up or block traffic on the switch port connected to the supplicant.
*Note:
Suppose two backend servers are enabled and that the server timeout is configured to
X seconds (using the Authentication configuration page), and suppose that the first server in
the list is currently down (but not considered dead). Now, if the supplicant retransmits EAPOL
Start frames at a rate faster than X seconds, then it will never get authenticated, because the
switch will cancel on-going backend authentication server requests whenever it receives a new
EAPOL Start frame from the supplicant. And since the server hasn't yet failed (because the X
seconds haven't expired), the same server will be contacted upon the next backend
authentication server request from the switch. This scenario will loop forever. Therefore, the
server timeout should be smaller than the supplicant's EAPOL Start frame retransmission rate.
