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 From:  Michel Servaes <michel at mcmc dot be>
 To:  monowall <m0n0wall at lists dot m0n0 dot ch>
 Subject:  Re: [m0n0wall] VLANs not really working ?
 Date:  Sat, 12 Jul 2008 21:47:11 +0200
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 ?
If this is a dumb remark, how should I change my MTU, lower than 1500 or 
higher than 1500...

I am connected to cable here, so the normal value would be 1500

kind regards,

Chris Buechler wrote:
> On Thu, Jul 10, 2008 at 2:39 PM, Michel Servaes <michel at mcmc dot be> wrote:
>> I just found a posting on making a backup of the config.xml, adding
>>  <mtu>1436</mtu> in between... but when going to the status window, my three
>> LAN cards are being given an MTU of 1500.
>> When loading this modified config.xml, the monowall reboots, loading it's
>> new settings... but somehow it doesn't accept them...
>> In status I still have 1500, and when downloading the config again, the
>> <mtu>1436</mtu> is still there... so it didn't clean out that parameter, and
>> I'm sure the config.xml is correctly uploaded to my system...
> I don't recall how MTU on interfaces other than WAN is handled, on WAN
> it's not really setting the MTU, it's configuring MSS clamping (which
> is what you really want). Might be the same case for other interfaces.
> You can confirm what you're seeing is indeed a MTU issue (more
> accurately described, likely a PMTUD black hole) by dropping the MTU
> on a client to 1400 and seeing what happens. If that resolves all the
> issues, that is indeed the cause. If not, need to look elsewhere.
> -Chris
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