nested vlans and ethernet frame size

[Ivan Alexandrovich]

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