nested vlans and ethernet frame size

[Kevin Oberman]

> Date: Mon, 10 Sep 2007 20:01:40 +0100
> From: Tom Judge <tom at tomjudge.com>
> Sender: owner-freebsd-net at freebsd.org
> 
> Ivan Alexandrovich wrote:
> > Hi
> > 
> > I'd like to ask for advice - what is the right place for
> > setting maximum ethernet frame size in freebsd?
> > It needs 1526 bytes to allow two vlan tags per packet.
> > Those tags must be processed by ng_vlan code so
> > it seems that vlanmtu interface flag will be of no use here.
> > We're running freebsd6.2-stable and use some Intel pro 100
> > card for testing purposes along with QinQ-capable switch.
> > 
> > Currently frames of 1526 length are sent without problems but
> > incoming packets of such a size are lost. Since fxp driver
> > complains about 'discarding oversize frame' it can be supposed that
> > the problem in this case is not in nic hardware but in
> > driver.
> > 
> > As far as I can see in kernel sources (it's a pity I'm not a
> > programmer :)  many ethernet drivers use ETHERNET_MAX_FRAME macro
> > to define size checks.
> > Possibly I could replace "1518" by "1526" in sys/net/ethernet.h
> > but it must be considered brutal, isn't it ? :)
> > 
> > 
> > Thanks in advance,
> > Ivan
> 
> This fix does not support N depth nesting, also if you are exceeding 
> 1518 does that not imply at you require hardware that supports jumbo 
> frames to do VLAN tag nesting?

802.3 was updated to require support a minimum frame size of 1522
several years ago, so one VLAN tag should be OK on any but ancient
hardware. Of course, software is a different matter.

In either case, you really should decrease the IP MTU below 1500 if you
are going to stack tags. The recommended IP MTU for jumbo frames is 9000.
This is a US government recommendation and not a standard, but is used by
almost all major governmental and educational nets to allow lots of room
for VLAN and MPLS tags with a typical max frame size of at least 9100
octets.
-- 
R. Kevin Oberman, Network Engineer
Energy Sciences Network (ESnet)
Ernest O. Lawrence Berkeley National Laboratory (Berkeley Lab)
E-mail: oberman at es.net			Phone: +1 510 486-8634
Key fingerprint:059B 2DDF 031C 9BA3 14A4  EADA 927D EBB3 987B 3751
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