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CHAPTER 10: SECURITY
Allow Guest VLAN if EAPOL Seen: The switch remembers if an EAPOL frame has been received on the port for the lifetime of the
port. Once the switch considers whether to enter the Guest VLAN, it will first check if this option is enabled or disabled. If disabled
(unchecked; default), the switch will only enter the Guest VLAN if an EAPOL frame has not been received on the port for the lifetime
of the port. If enabled (checked), the switch will consider entering the Guest VLAN even if an EAPOL frame has been received on the
port for the life-time of the port. The value can only be changed if the Guest VLAN option is globally enabled.
Port Configuration
Port: The port number for which the configuration below applies.
Admin State: If 802.1X is globally enabled, this selection controls the port’s authenticationmode. The following modes are available:
- Force Authorized: In this mode, the switch will send one EAPOL Success frame when the port link comes up, and any client on
the port will be allowed network access without authentication.
- Force Unauthorized: In this mode, the switch will send one EAPOL Failure frame when the port link comes up, and any client on
the port will be disallowed network access.
- Port-based 802.1X: In the 802.1X-world, the user is called the supplicant, the switch is the authenticator, and the RADIUS server
is the authentication server. The authenticator acts as the man-in-the-middle, forwarding requests and responses 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).
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.
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. 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.
