network segmentation without using vlans

James Knott james.knott-bJEeYj9oJeDQT0dZR+AlfA at public.gmane.org
Wed Feb 20 00:48:18 UTC 2008


Kristian Erik Hermansen wrote:
>   
>> As I mentioned, there is hand shaking that goes on, when a device is
>> plugged into a switch.  This is called the link integrity test.  Also,
>> most equipment now performs auto-negotiation, to determine speed &
>> duplex etc.  How will either of these work, if one device can't hear the
>> other?
>>     
>
> You don't need to enable auto-negotiation.  You can very easily set
> the link speed using a variety of tools used in your Linux distro :-)
> Additionally, there are many wires in a standard CAT6 cable.  Quiz:
> Which wires could you eliminate and still send data out, even linking
> properly to a switch using manual negotiation?  :-)  Are you sure it
> is not possible, or are you merely stating a claim which you have not
> verified...
>   

Please read again that part about the link integrity test.  It is 
performed periodically.  If there's no response the line is considered 
disconnected.  Will Linux or any other OS send or receive data on a 
disconnected ethernet port?

I based my post on "Ethernet - The Definitive Guide", by Charles E. 
Spurgeon, published by O'Reilly, on page 132 for the link integrity test 
"10BASE-T transceivers continually monitor the receive data path for 
activity as a means of checking whether the link is working correctly.  
The transceivers also send a link test signal to one another to verify 
the integrity of both twisted-pair links".  Information on 
auto-negotiation starts on page 85.  I have known about both for many 
years.  It's the link integrity test, that allows the NIC connect light 
to turn on.

I am quite familiar with ethernet wiring.  Pairs 1 & 3 (pins 4&5 and 
1&2) are the minimum required for ethernet to work.  As I mentioned 
above, without that link integrity test succeeding, data will not be sent.

Also, if you just want to monitor you can use a hub or a special device 
called an ethernet tap.  Also, some switches can configure a port as a 
monitor for other ports.  Linux, in the 2.4 kernel, provided a way to 
turn off the transmitter, while still receiving.  I don't know if that 
feature is present in 2.6.


Incidentally, I'm employed as a senior technician for a company that 
does specialized work for telecommunications companies, such as Bell, 
Telus and Allstream.  A part of my work involves networking, including 
routers, switches, VoIP and PPP links.  My career, in both 
telecommunications & computers, spans almost 36 years and I also studied 
electrical engineering at Ryerson.  I do have some idea of what I'm 
talking about, even without referring to texts.

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