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Chapter 4: Security
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 ongoing 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 piggyback 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: 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 piggyback on the
successfully authenticated client and get network access even though they really aren't authenticated. To overcome this security
breach, use the Multi 802.1X variant.
Multi 802.1X is really not an IEEE standard, but features many of the same characteristics as does port-based 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 "xxxx-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.
When authentication is complete, the RADIUS server sends a success or failure indication, which in turn causes the switch to
open up or block traffic for that particular client, using the Port Security module. Only then will frames from the client be for-
warded on the switch. There are no EAPOL frames involved in this authentication, and therefore, MAC-based Authentication
has nothing to do with the 802.1X standard.
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