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 From:  "Chris Buechler" <cbuechler at gmail dot com>
 Cc:  monowall <m0n0wall at lists dot m0n0 dot ch>
 Subject:  Re: [m0n0wall] VLANs not really working ? -solved!
 Date:  Sun, 13 Jul 2008 17:41:41 -0400
On Sun, Jul 13, 2008 at 1:48 PM, Michel Servaes <michel at mcmc dot be> wrote:
> I think I solved my issue here... since VLAN tagging is 4 bytes more than
> normal Ethernet (I've been digging), I gathered that having a larger MTU at
> VLAN level could have some troubles at the LAN being 1500.
> So I lowered the MTU value of all my VLAN's to 1496, and kept LAN/WAN at
> 1500 - and voila, my troubles went away !!  (didn't even have to reboot
> anything - nor the monowall, nor my laptop, it just started working after :
> ifconfig vlan0 mtu 1496
> ifconfig vlan1 mtu 1496
> ...
> I can now access the internet without any troubles on each VLAN.

That's what the new NIC should have resolved, it should allow the
larger frames with the VLAN tagging.  Apparently it was an issue with
your switch rather than your NIC. This is a suitable work around since
it fixes PMTUD, so you should be fine running this way.