Ethereal-users: Re: [Ethereal-users] Regarding 802.1q

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From: Scott Lowrey <slowrey@xxxxxxxxxxx>
Date: Wed, 09 Mar 2005 12:53:18 -0500
Hub? First off, many modern hubs aren't really hubs in the traditional sense. They are switches. Beware. You'll see one packet go by and then that's it. The MAC address is stored and you'll see no more. :)

Some hubs/switches will drop VLAN-tagged frames because they don't understand the tag (which appears as a bogus type field to a non-VLAN-aware device). I have, however, seen small NetGear gigabit switches that will pass tagged packets even though they are not VLAN-aware.

If the packets are traversing the hub properly and you're not seeing them, then there are several possibilites:

- the hub is acting like a switch and therefore not sending the tagged frames to the port your PC is connected to

- Windows pcap isn't in promiscuous mode

- the Windows driver is dropping the frames because of the tag

Or it may be none of these....

I know the Intel PRO is VLAN capable but I'm not sure about Windows, and even if you do get the packets, you may not see the tags. See http://wiki.ethereal.com/CaptureSetup_2fVLAN for more info.
I'll update the Linux section...

somasundaram selvaraj wrote:

Hi
I have a windows 2000 pro wih intel pro 1000 MT adapter. I am trying to capture the vlan packets and i am not able to capture that. Basically I have two switches configured for vlan 100 and two trunk ports on both the switch. I ve connected a hub to one of the trunk port.one port of the hub is connected to a pc which can read 802.1q information.But etherpeek is not able to capture the vlan packets. I am able to ping between the switches. Can u assist me in fixing the issue.

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*Scott Lowrey*
Test Engineering Manager
NexTone Communications <http://nextone.com>
Gaithersburg, Maryland USA

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