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 From:  Manuel Kasper <mk at neon1 dot net>
 To:  Aaron Cherman <aaronc at morad dot ab dot ca>
 Cc:  m0n0wall at lists dot m0n0 dot ch
 Subject:  Re: [m0n0wall] MTU Trouble in FreeBSD 6
 Date:  Sat, 23 Dec 2006 00:28:17 +0100
On 22.12.06 09:17 -0700, Aaron Cherman wrote:

> kernel: em0: discard oversize frame (ether type 8100 flags 3 len
> 1518 > max 1418)

I'd try to do without setting any specific MTU (FreeBSD will take the
extra 4 bytes for the VLAN tag into account automatically) and
finding the real cause for your MTU problems with VLAN-tagged frames
- e.g. switches or NICs that don't handle them properly (usually not
an issue with Intel NICs) - your MTU setting most likely only makes
things worse. Explicitly setting an MTU below 1500 and/or using MSS
clamping should only ever be necessary for people who use PPPoE or
some sort of IP-based tunnel (e.g. GRE) to access the Internet.

> It does not appear that the MTU is actually getting forced.

Simply setting the MTU on an interface doesn't make the kernel
perform MSS clamping, if that's what you expected.

- Manuel