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CHAPTER 10: SECURITY
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 the 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.
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 forwarded 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.
The advantage of MAC-based authentication over port-based 802.1X is that several clients can be connected to the same port
(e.g. through a 3rd party switch or a hub) and still require individual authentication, and the clients don’t need special supplicant
software to authenticate. The advantage of MAC-based authentication over 802.1X-based authentication is that the clients
don’t need special supplicant software to authenticate. The disadvantage is that MAC addresses can be spoofed by malicious
users— equipment whose MAC address is a valid RADIUS user can be used by anyone. Also, only the MD5-Challenge method is
supported. The maximum number of clients that can be attached to a port can be limited using the Port Security Limit Control
functionality.
RADIUS-Assigned QoS Enabled: When RADIUS-Assigned QoS is both globally enabled and enabled (checked) on a given port, the
switch reacts to QoS Class information carried in the RADIUS Access-Accept packet transmitted by the RADIUS server when a
supplicant is successfully authenticated. If present and valid, traffic received on the supplicant’s port will be classified to the given
QoS Class. If (re-)authentication fails or the RADIUS Access-Accept packet no longer carries a QoS Class or it’s invalid, or the
supplicant is otherwise no longer present on the port, the port’s QoS Class is immediately reverted to the original QoS Class (which
may be changed by the administrator in the meantime without affecting the RADIUS-assigned).
This option is only available for single-client modes, i.e.
- Port-based 802.1X
- Single 802.1X
RADIUS attributes used in identifying a QoS Class:
The User-Priority-Table attribute defined in RFC4675 forms the basis for identifying the QoS Class in an Access-Accept packet.
