[VLAN] MTU problem between 10/100 & Gigabit cards

[Ard van Breemen]

Hi,
On Fri, Jun 29, 2007 at 05:19:06PM +0100, Gary Smith wrote:
> I have been running a VLAN configured system for some years using 10/100 
> Sundance driver cards without a problem.  I recently tried on a HP 
> proliant server using gigabit cards and found a problem related to the 
> MTU size for the interface I an using.
> 
> The 10/100 cards seem to need a MTU of 1504 bytes, but this size does 
> not work with the e1000 gig cards, or the US robotics, Dlink DGE, 
> broadcom etc.  I have to move the MTU down to 1500 to make fragmentation 
> work and allow it to pass the larger packets.

>From what I read, I guesss the vlan on sundance is software only,
and need a special MTU of 1504.
Most gigabit cards (not all!) already strip the vlan tag, so the
driver only sees a payload of 1500 and somewhere else in the
receiver buffer the vlan tag is found.

> Is this what I should expect from the e1000, broadcom, R8169 drivers for 
> gigabit cards?

I've been using vlans since 2.4.14 or so, and I have never had to
set the MTU size to 1504 for any of my cards. I do know however
that some of the drivers need the MTU size of 1504 to keep it
from discarding it as a long packet and to cope for the increased
buffer size needed.