Could some networking guru please provide some hints to my question? I
am trying to test some tagged VLAN features. The machines that I am
using use Ethernet II. From what I know about tagged VLANs and
Ethernet II/802.1Q/1P, there ought to be a 4-byte tag field after the
source address in the Ethernet frame, containing the (TPID, PRIORITY,
CFI and VID). To test my set up, I am capturing and examining packets
with Wireshark, and 'pinging' each machine.

In all the packets I have examined (both going in and out of the
machines with VLAN supposedly activated), I do not see this 4-byte tag
field as mentioned above. I have ensured from the NIC card interface
that the 802.1Q/1P tagging is switched on the machines. I have re-
created the network 3 - 4 times, but I do not see the 4-bytes at all.

Is there something extra I need to do? I want to confirm there is
something wrong with the NIC cards, or there is some configuration
problem. Any would be greatly appreciated. Thanks in advance for
your .

On Sun, 14 Oct 2007 14:21:10 -0700, cpptutor2000@yahoo.com <cpptutor2000@yahoo.com> wrote:
> Could some networking guru please provide some hints to my question? I
> am trying to test some tagged VLAN features. The machines that I am
> using use Ethernet II. From what I know about tagged VLANs and
> Ethernet II/802.1Q/1P, there ought to be a 4-byte tag field after the
> source address in the Ethernet frame, containing the (TPID, PRIORITY,
> CFI and VID). To test my set up, I am capturing and examining packets
> with Wireshark, and 'pinging' each machine.
>
> In all the packets I have examined (both going in and out of the
> machines with VLAN supposedly activated), I do not see this 4-byte tag
> field as mentioned above. I have ensured from the NIC card interface
> that the 802.1Q/1P tagging is switched on the machines. I have re-
> created the network 3 - 4 times, but I do not see the 4-bytes at all.

Yes, but does the VLAN *work*? I.e. can you ping between IP addresses
on the VLAN? If so, why worry?

> Is there something extra I need to do? I want to confirm there is
> something wrong with the NIC cards, or there is some configuration
> problem. Any would be greatly appreciated. Thanks in advance for
> your .

I have noticed (on Linux) that tcpdump (and Wireshark, etc) tend not to
capture the VLAN header. It is stripped away on lower level than where
libpcap picks up the frames. A coworker of mine had to disable some
kind of optimization in the NIC or the driver to see the VLAN headers.

I can agree that this is a bit unfortunate from a debugging point of
view (and for curious people like you and me), but on the other hand
you have statistics on the interfaces and other tools like that. Plus
the fact that you can trust "tcpdump -iname_of_the_vlan_if" to only
pick up packets on that VLAN.

-Jorgen

--
// Jorgen Grahn <grahn@ Ph'nglui mglw'nafh Cthulhu
\X/ snipabacken.dyndns.org> R'lyeh wgah'nagl fhtagn!

>
> > Is there something extra I need to do? I want to confirm there is
> > something wrong with the NIC cards, or there is some configuration
> > problem. Any would be greatly appreciated. Thanks in advance for
> > your .
>
> I have noticed (on Linux) that tcpdump (and Wireshark, etc) tend not to
> capture the VLAN header. It is stripped away on lower level than where
> libpcap picks up the frames. A coworker of mine had to disable some
> kind of optimization in the NIC or the driver to see the VLAN headers.
>

RealTek cards seem to always preserve the tag for you to see, Intel
cards always seem to discard. If you want to test, set your source
and destination in separate VLANs and see if the Ping still works, if
it does, VLAN probably is not enabled, if it does not, then your VLAN
is there, you just can't see the tag.

Cheers,
Dan