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You are correct.  If the OS is doing the VLAN tagging, rather than  
the hardware (as what happens when you turn  on vlans on a card that  
doesn't support it in hardware - the rl cards fit this), then you  
need to make room for the vlan tag.  Going to 1496 should solve your  
problem.

	Chris

On 13 Mar 2007, at 21.08 , Fredrik Östergren - IT-Kontakt AB wrote:

> When i tested around with changing MTU I (at the 3com-card) did not  
> think
> that it improved the quality of the service. I was able to switch  
> card as I
> was using a PCI-slot.
>
> The reason for it, correct me if I'm wrong, is that standard MTU  
> being used
> is 1500 and all cards does not handle the extra 4 bytes that vlan  
> tagging
> demands and this causes problems with functionality. When changing  
> the MTU
> to 1496 you can fit the extra 4 bytes for tagging into the 1500- 
> limit and
> therefore receive the function you need.
>
> As far as safe to set 1496, I don't know actually, I wouldn't want  
> it in a
> production-environment thanks to the experience I had with the  
> 3comcard, but
> I guess it's up to you and your "demands" ;-)
>
> Best Regards
>
> / Fredrik
>
> -----Ursprungligt meddelande-----
> Från: Marco Simioni [mailto:m dot simioni at gmail dot com]
> Skickat: den 13 mars 2007 11:34
> Till: m0n0wall at lists dot m0n0 dot ch
> Ämne: Re: [m0n0wall] VLAN and MTU, someone can explain this ?
>
>> 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
>
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