Problems with 802.1x authentication and DisplayLink docks

This article covers typical issues that IT admins find with 802.1x authentcaition when deploying DisplayLink docks.

802.1x authentication is fully supported by DisplayLink docks, however it may require some further configuration of the Ethernet on the dock or the authentication to get it to work as expected.

If 802.1x is not working, please check the following common reasons for failure below

Check the version of DisplayLink software in use

The behaviour of the Ethernet driver with VLAN configuration was updated in v7.7 M4 to change the way VLANs were applied to the Ethernet connection from an 802.1x authentication. 

Previously VLANs were disabled and the VLAN ID was set to be 0. If VLANs were enabled the packets would have been priority tagged (VLAN set to zero). This was quite legal, but different behaviour to other Ethernet adapters.

The common behaviour is to have Priority and VLAN enabled, and the VLAN field set to zero, but still sent untagged traffic. When we changed our driver to have the same behaviour in 7.7 M4, it fixed these problems with 802.1x failures, as they could probably now enable VLANs and set the VLAN field in the same way is they did on other adapters.

MAC based authentication

If the 802.1x uses MAC address based authentication, this authentication will fail with a DisplayLink dock, as the MAC address is now the MAC address of the dock, not the MAC address of the laptop or tablet. To fix this issue, add the MAC addresses of the docking stations to the whitelist of allowed devices.

Check the MTU of the Ethernet interfaces

Some applications change the MTU of the laptop Ethernet interface, for example the Cisco VPN client. If the MTU is different on the DisplayLink adapter, the packets might be too large for the network infrastructure and trunkated when sent over some links. 

The Ethernet interface MTU settings can be checked with the windows command:

netsh interface ipv4 show subinterfaces

This shows an output similar to the one shown below

   MTU  MediaSenseState   Bytes In  Bytes Out  Interface
------  ---------------  ---------  ---------  -------------
  1500                2    2357246     829669  WiFi
  1500                5          0          0  Ethernet 4
  1500                1   29038224   12937531  Ethernet 12

If the MTU of the DisplayLink interface differs, the MTU can be changed to match the other interfaces. Eg:

netsh interface ipv4 set subinterface "Ethernet 4" mtu=1300 store=persistent

More information on setting the MTU can be found in this article.

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