Administrator Guide

MAC Authentication Bypass
MAC authentication bypass (MAB) enables you to provide MAC-based security by allowing only known MAC addresses within the
network using a RADIUS server.
802.1X-enabled clients can authenticate themselves using the 802.1X protocol. Other devices that do not use 802.1X — like IP phones,
printers, and IP fax machines — still need connectivity to the network. The guest VLAN provides one way to access the network.
However, placing trusted devices on the quarantined VLAN is not the best practice. MAB allows devices that have known static MAC
addresses to be authenticated using their MAC address, and places them into a VLAN different from the VLAN in which unknown devices
are placed.
For an 802.1X-incapable device, 802.1X times out if the device does not respond to the Request Identity frame. If MAB is enabled, the port
is then put into learning state and waits indefinitely until the device sends a packet. Once its MAC is learned, it is sent for authentication to
the RADIUS server (as both the username and password, in hexadecimal format without any colons). If the server authenticates
successfully, the port is dynamically assigned to a MAB VLAN using a RADIUS attribute 81, or is assigned to the untagged VLAN of the
port. Afterward, packets from any other MAC address are dropped. If authentication fails, the authenticator waits the quiet-period and
then restarts the authentication process.
MAC authentication bypass works in conjunction and in competition with the guest VLAN and authentication-fail VLAN. When both
features are enabled:
1. If authentication fails, the port it is placed into the authentication-fail VLAN.
2. If the host does not respond to the Request Identity frame, the port transitions to MAB initiation state.
3. If MAB times out or MAC authentication fails, the port is placed into the guest VLAN.
If both MAB and re-authentication are enabled, when the re-auth period finishes and whether the previous authentication was through
MAB or 802.1X, 802.1X authentication is tried first. If 802.1X times out, MAB authentication is tried. The port remains authorized
throughout the reauthentication process. Once a port is enabled/disabled through 802.1X authentication, changes to MAB do not take
effect until the MAC is asked to re-authenticate or the port status is toggled.
MAB in Single-host and Multi-Host Mode
In single-host and multi-host mode, the switch attempts to authenticate a supplicant using 802.1X. If 802.1X times out because the
supplicant does not respond to the Request Identity frame and MAB is enabled, the switch attempts to authenticate the first MAC it
learns on the port. Afterwards, for single-host mode, traffic from all other MACs is dropped; for multi-host mode, all traffic from all other
MACs is accepted.
After a port is authenticated by MAB, if the switch detects an 802.1X EAPoL start message from the authenticated MAC, the switch re-
authenticates using 802.1X first, while keeping the port authorized.
NOTE:
If the switch is in multi-host mode, a MAC address that was MAB-authenticated but later was disabled from MAB
authentication, is not denied access but moved to the guest VLAN. If the switch is in single-host mode, the MAC
address is disallowed access.
MAB in Multi-Supplicant Authentication Mode
Multi-supplicant authentication (multi-auth) mode is similar to other 802.1X modes in that the switch first attempts to authenticate a
supplicant using 802.1X. 802.1X times out if the supplicant does not respond to the Request Identity frame. Then, if MAB authentication is
enabled, the switch tries to authenticate every MAC it learns on the port, up to 128 MACs, which is the maximum number of supplicants
that 802.1X can authenticate on a single port in multi-authentication mode.
If a supplicant that has been authenticated using MAB starts to speak EAPoL, the switch re-authenticates that supplicant using 802.1X
first, while keeping the MAC authorized through the re-authentication process.
Configuring MAC Authentication Bypass
To configure MAB in multi-supplicant authentication mode:
1. Configure the following attributes on a RADIUS Server:
• Attribute 1—User-name: Use the supplicant MAC address in hex format without any colons. For example, enter 10:34:AA:33:44:F8
as 1034AA3344F8.
• Attribute 2—Password: Use the supplicant MAC address, but encrypted in MD5.
• Attribute 4—NAS-IP-Address: IPv4 address of the switch that is used to communicate with the RADIUS server.
802.1X
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