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Command Descriptions
79
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
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