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Description
Section 2.8 Quality of Service describes the different mechanisms, one of them being 802.1Q (VLAN tagging), just as if the user could just enable it. The feature is not supported. The PTP stack/TimeSync subsystem will be able to deal with VLAN tags on the application level, but VLAN tagging is not integrated in the V3/V5 firmwares on a general basis. Therefore, the attribute is disabled and the user cannot enable it.
This should be stated more clearly in the aforementioned section of the API manual.