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I know that IEEE 802.1Q compliant switches can have their ports configured as access (just accept traffic from one specific VLAN) or trunk (carry traffic from more than one VLAN, it must be specified which ones). But how about a host/station? Is it allowed by the standard to be configured as trunk?

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Depends on your network card. Intel cards are legendary for this feat, VIA usually works well. Others, you'll have to research. The card has to be able to hand over raw data so the OS can see more than one vlan.

Under Windows, use Intel's Pro drivers, has a config util.

Under Linux, vconfig

FreeBSD has it's own ipconfig sub-command

After that, it looks like more than one network card is connected.

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My Networks professor told me the same, and that in the general case, I should assume that the host is multihomed, or can emulate that behavior somehow –  dario_ramos Jul 18 '11 at 14:21

IEEE 802.1Q-2005 primarily concerns itself with defining VLAN-capable bridges (switches), but in a few places it does acknowledge the possibility of VLAN-aware end stations. In fact, clause 11.2.1.1 is titled "Behavior of end stations". It describes how end stations can use a protocol called GVRP, defined earlier in the document, which allows VLAN-aware switches and end stations to communicate which VLANs they'd like to have trunked over to them, so that static configuration doesn't have to be used.

I'm not sure if anyone actually uses GVRP to dynamically manage the topology of VLANs, but plenty of vendors allow static VLAN trunk configuration on Ethernet cards.

One thing to watch out for on really old or cheap NICs is whether they support the extra 4 bytes per frame necessary for supporting the VLAN header while keeping a 1500 byte MTU. Before VLANs, a full-size frame was 14 bytes of header, 1500 bytes of payload, and 4 bytes of Frame Check Sequence as a trailer, for a total of 1518 bytes. Add in the 4 byte VLAN header, and your card needs 1522-byte frame buffers in order to support standard 1500 byte MTUs. The increase of the frame size from 1518 to 1522 was ratified by the IEEE almost a year before 1000BASE-T was ratified, so all 1000BASE-T capable cards should support it, but 10/100 cards from the 1990's, or really cheap crap NICs, might not.

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