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.



Michel Servaes wrote:
> I'm in over my head here... I don't get it.
> If I read this correctly, I need to change the MTU on both my LAN and 
> my client computer too... or what exactly?
>
> I am used to a normal LAN/DHCP setup... VLAN's are pretty new to me... 
> I have a VLAN setup at the office with pfSense, which just worked out 
> of the box... maybe my personal WLAN AP isn't just that good at VLANs ??
>
> Chris Buechler wrote:
>> On Sat, Jul 12, 2008 at 3:47 PM, Michel Servaes <michel at mcmc dot be> wrote:
>>  
>>> Sorry for (maybe a dumb) question, but I really can't see what 
>>> changing the
>>> MTU on the WAN side can have influence on an internal LAN / VLAN 
>>> issue here
>>> ?
>>>     
>>
>> I never said it would. I said change it on the client. If it's
>> strictly one VLAN to another, change it on both systems. That's just
>> to confirm this is really what you're seeing.
>>
>> -Chris
>>
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