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Date:      Mon, 10 Sep 2007 20:01:40 +0100
From:      Tom Judge <tom@tomjudge.com>
To:        Ivan Alexandrovich <ivsan@ngs.ru>
Cc:        freebsd-net@freebsd.org
Subject:   Re: nested vlans and ethernet frame size
Message-ID:  <46E59494.7040507@tomjudge.com>
In-Reply-To: <web-95343829@mx1.intranet.ru>
References:  <web-95343829@mx1.intranet.ru>

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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?

Tom



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