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 From:  =?iso-8859-1?Q?Fredrik_=D6stergren_-_IT-Kontakt_AB?= <fredrik dot ostergren at itkontakt dot se>
 To:  "'Marco Simioni'" <m dot simioni at gmail dot com>, <m0n0wall at lists dot m0n0 dot ch>
 Subject:  SV: [m0n0wall] VLAN and MTU, someone can explain this ?
 Date:  Tue, 13 Mar 2007 14:08:01 +0100
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-----

Skickat: den 13 mars 2007 11:34
Till: m0n0wall at lists dot m0n0 dot ch


> 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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