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Date: Fri, 12 Nov 2004 11:36:13 +0000 (GMT)
From: Robert Watson <rwatson@FreeBSD.org>
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To: Attila Nagy <bra@fsn.hu>
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Subject: Re: cvs commit: src/sys/dev/em if_em.c if_em.h
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On Fri, 12 Nov 2004, Attila Nagy wrote:

> Robert Watson wrote:
> >   that the card isn't doing it on transmit.  Due to a feature of card
> >   operation, if the global VLAN prepend/strip register isn't set,
> >   setting the VLAN tag flag on individual packet descriptors will
> >   cause the packet to be transmitted using ISL encapsulation rather
> >   than 802.1Q VLAN encapsulation.
> 
> AFAIK, FreeBSD has no ISL support.  Would it be possible to set this
> flag somehow, so the user could switch between 802.1q and ISL? This way
> the NIC would do the tagging, so no kernel support would be needed I
> guess. 

Well, I think that would immediately run into the same problem the VLAN
support had: if the kernel doesn't know about the encapsulation then when
the card is put in promiscuous mode, you get back the un-encapsulated
packets rather than the encapsulated ones.  I don't know much/anything
about ISL encapsulation, but if it's similar to VLAN encapsulation it
probably wouldn't be bard to implement.  I identified the encapsulation
type being spewed out by the if_em cards using ethereal, which does a
pretty good job at identifying fairly arbitrary looking sets of bytes. 
:-) 

Robert N M Watson             FreeBSD Core Team, TrustedBSD Projects
robert@fledge.watson.org      Principal Research Scientist, McAfee Research