Planet Networking & Communication WGSW-24040 User Manual page 232

User's Manual of WGSW-24040 / WGSW-24040R
between the supplicant and the authentication server. Frames sent between the
supplicant and the switch are 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 are 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 AAA 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.
Single 802.1X
In port-based 802.1X authentication, once a supplicant is successfully
authenticated on a port, the whole port is opened for network traffic. This allows
other clients connected to the port (for instance through a hub) to piggy-back on
the successfully authenticated client and get network access even though they
really aren't authenticated. To overcome this security breach, use the Single
802.1X variant.
Single 802.1X is really not an IEEE standard, but features many of the same
characteristics as does port-based 802.1X. In Single 802.1X, at most one
supplicant can get authenticated on the port at a time. Normal EAPOL frames are
used in the communication between the supplicant and the switch. If more than
one supplicant is connected to a port, the one that comes first when the port's link
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