Subject: Cable as Firewall?!

From: [email-masked]
Date: 12 Apr 2000 15:35:42 UT

I'm at the end of all my knowledge and I can't find any explaination
for the following issue. So I hope anyone of you Network Guru's can
give me an idea why this would happen.

I have this scenario in a Network:

A couple of PC's are connected to a 3300 Switch made by 3COM
which are connected using 3 metres long TP (normal CAT5) cables.
The stations have diffrent OS'es like Linux and Novell Netware.
However the problem seems to be the cable not the operating systems.

I use diffrent protocols over this network to establish a communication,
like IPX and of course TCP/IP. Now the weird thing is if I use one cable,
IPX data comes through, but no TCP/IP traffic. If I exchange the cables
again, both TCP/IP and IPX work. I tried with tcpdump (filtering UDP and
diffrent ethernet protocols), but I only get IPX through with this cable!
I used diffrent stuff to test this cable, like fluke 2000 dsp pro and
fluke 1000 dsp and a lot of diffrent volt/ohm testers. The cable seems
to function ok. I tried other switches such as 3COM 3300,
HP pro curve 4000, DLink 10/100, but I only get IPX through with that cable.
This cable doesn't have a chip and ethernet is ethernet, no? I would really
like to know what makes this cable diffrent. Its not the operating systems
since I tried some Windows and other Netware 4.x server too. Both only
get IPX through it. How can a cable filter specific ethernet protocols?

Best Regards,
Thomas Kuiper

Thomas Kuiper        |   [email-masked]   |  www.tobit.com     __
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