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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 ?
 Date:  Thu, 10 Jul 2008 22:05:37 -0400
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.