OPTICAL SYSTEMS DESIGN
DOC ID: 10118101
OSD2512 OPERATOR MANUAL
PAGE 96
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 comes up will be the first one considered. If that supplicant doesn't
provide valid credentials within a certain amount of time, another supplicant will get a chance.
Once a supplicant is successfully authenticated, only that supplicant will be allowed access.
This is the most secure of all the supported modes. In this mode, the Port Security module is
used to secure a supplicant's MAC address once successfully authenticated.
MULTI 802.1X
Multi 802.1X is - like Single 802.1X - not an IEEE standard, but a variant that features many
of the same characteristics. In Multi 802.1X, one or more supplicants can get authenticated on
the same port at the same time. Each supplicant is authenticated individually and secured in
the MAC table using the Port Security module.
In Multi 802.1X it is not possible to use the multicast BPDU MAC address as destination
MAC address for EAPOL frames sent from the switch towards the supplicant, since that
would cause all supplicants attached to the port to reply to requests sent from the switch.
Instead, the switch uses the supplicant's MAC address, which is obtained from the first
EAPOL Start or EAPOL Response Identity frame sent by the supplicant. An exception to this
is when no supplicants are attached. In this case, the switch sends EAPOL Request Identity
frames using the BPDU multicast MAC address as destination - to wake up any supplicants
that might be on the port.
The maximum number of supplicants that can be attached to a port can be limited using the
Port Security Limit Control functionality.
MAC-BASED AUTH.
Unlike port-based 802.1X, MAC-based authentication is not a standard, but merely a best-
practices method adopted by the industry. In MAC-based authentication, users are called
clients, and the switch acts as the supplicant on behalf of clients. The initial frame (any kind of
frame) sent by a client is snooped by the switch, which in turn uses the client's MAC address
as both username and password in the subsequent EAP exchange with the RADIUS server.
The 6-byte MAC address is converted to a string on the following form "xx-xx-xx-xx-xx-xx",
that is, a dash (-) is used as separator between the lower-cased hexadecimal digits. The switch
only supports the MD5-Challenge authentication method, so the RADIUS server must be
configured accordingly.
