> Sounds to me like the card does not support VLAN tagging. What card is it ?
>
> I had same problem with a 3Com one which apparently did not have built-in
> vlan-support.
>

All the cards have realtek chipsets (rl0 and rl1), and they're
embedded within this hardware (a GTC Allwel Set Top Box).
I'm pretty sure they to not natively support VLAN tagging (btw, how
can i confirm this? there is a command to know exactly the hardware
model?) , so i know that MTU problems may occour if i still want to
use VLANs.
I'm just looking for a technical explaination of this, and asking if
it can be safe setting a lower MTU like mine (1496).

> I would try and change to a different card which clearly supports 802.1q.

Unfortunately this hardware has 2 embedded realtek cards, and no pci
slots available ;)

>
> Good luck!

Thank you for your attention,
Marco