nested vlans and ethernet frame size

[Julian Elischer]

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 ? :)

basically you need go add smaller and smaller MTUs on your vlann interfaces
as they get more and more nested. You can't really go in the other direction of
increasing them in  a lot of scenarios.

> 
> 
> Thanks in advance,
> Ivan
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