The vlan driver supports efficient operation over parent interfaces that can provide help in processing VLANs. Such interfaces are automatically recognized by their capabilities. Depending on the level of sophistication found in a physical interface, it may do full VLAN processing or just be able to receive and transmit frames exceeding the maximum Ethernet frame size by the length of a 802.1Q header. The capabilities may be user-controlled by the respective parameters to ifconfig(8), vlanhwtag and vlanmtu. However, a physical interface is not obliged to react to them: It may have either capability enabled permanently without a way to turn it off. The whole issue is very specific to a particular device and its driver.
By now, the list of physical interfaces able of full VLAN processing in the hardware is limited to the following devices: bge(4), em(4), ixgb(4), nge(4), re(4), ti(4), txp(4), and vge(4).
The rest of the Ethernet interfaces can run VLANs using software emulation in the vlan driver. However, most of them lack the capability of transmitting and receiving oversized frames. Assigning such an interface as the parent to vlan will result in a reduced MTU on the corresponding vlan interfaces. In the modern Internet, this is likely to cause tcp(4) connectivity problems due to massive, inadequate icmp(4) filtering that breaks the Path MTU Discovery mechanism.
The interfaces that support oversized frames are as follows: