Which industrial protocol does not support VLANs?

The correct answer is C. EtherCAT .

The study guide states that for industrial Ethernet protocols, “such as Ethernet/IP and Modbus/TCP, you can use VLANs to segment your physical LAN into multiple logical LANs.” This directly confirms that Ethernet/IP and Modbus/TCP support VLAN-based segmentation in the OT context.

By contrast, the guide explains that “EtherCAT skips layers 3 to 6 to deliver real-time communication” and describes it as an “Open-Software Modified-Ethernet” approach. Because it does not operate like the standard Ethernet/IP model used for normal VLAN-based segmentation, EtherCAT is the protocol identified here as not supporting VLANs in the way Ethernet/IP and Modbus/TCP do.

So, based on the study guide comparison, the verified answer is EtherCAT .