Gigabit Ethernet Speed/Duplex

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Nov 12th, 2013
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Hi All,


I read this in most of the documents that I read about Gig ethernet that it uses all four pairs of a UTP cable. That would mean that if I connect a cross cable with only the Orange and Green pairs crossed and have Auto MDIX disabled on two 3750 switches the ports that are set to auto negotiate these ports must negotiate to 100 Mbps instead of 1000. I tried it and the switcports negotiated to 1000 Mbps. Anyone knows what I am missing here ?


Thanks in Advance

Umesh          

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John Blakley Tue, 11/12/2013 - 08:25
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Why don't you hardcode the ports to 100/full? Technically, you could create a cable with only the 2 pairs of wire in a cat 5e (1,2,3, and 6) and it shouldn't negotiate at 1000/full.


Try wiring on both sides - ow/o/gw/g (1/2/4/6)


I've never tested this, but I suppose it could work...It would be better if you could hard code both devices, but if you simply can't, you could try this..


HTH,
John

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Umesh Shetty Tue, 11/12/2013 - 09:44
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Thnx for your response John,


I specifically tried this to test of the Gigabit ethernet negotiates to 100 Mbps with only the Green and the Orange pairs crossed. I was quite surprised to know this fact when I read this recently so decided to test it, but the Gig ports negotiated to 1000/Full with only two pairs crossed ? If the Gigabit ethernet uses all 4 pairs this setting should have caused the Gig ports at both ends to negotiate to 100 which did not happen surprisingly . Any idea why ?



Thanks

Umesh

Peter Paluch Tue, 11/12/2013 - 08:54
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John, Umesh,


I owe Umesh an apology for not responding sooner. Umesh is asking about a particular specific behavior in Gigabit Ethernet. It has been my best knowledge that in order for the Gigabit Ethernet autonegotiation to complete successfully, the cable between two devices either has to be a straight-through cable, or a full cross-over cable, crossing both orange, green, blue and brown pairs, and in addition, reversing the polarity on the 4-5/7-8 pair:


http://en.wikipedia.org/wiki/Ethernet_crossover_cable#Crossover_cable_pinouts


Umesh has reported that despite using a non-fully-crossed cable, switches were nevertheless capable of negotiating full Gigabit connectivity, something I have not expected to happen. My only answer here is only my personal opinion. I guess that because this kind of miscabling can be quite common (as fully crossed crossover cables are relatively rare), networking hardware manufacturers may have implemented a proprietary capability to cope even with non-fully-crossed cables and provide full Gigabit connectivity. Considering the fact that Gigabit Ethernet uses all 8 wires for simultaneous transmission, it is kind of expected, anyway. I would not expect any Gigabit Ethernet interface to be capable of this, however.


Best regards,

Peter

Umesh Shetty Mon, 11/18/2013 - 09:09
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Hey Peter,


Thnx for taking your time and adding so much value to this discussion, I must agree your inputs have been quite insightful.

I've gone through the wikepedia page and read soemting there " Although the Gigabit crossover is defined in the Gigabit Ethernet standard, in practice all Gigabit PHYs feature an auto-MDIX capability and are designed for compatibility with the existing 100BASE-TX crossovers. The IEEE-specified Gigabit crossover is generally seen as unnecessary." Does this mean that current Gigabit Phy's do not follow the IEEE standards and use only 2 pairs instead of all 4. Atleast thats what I was able to conlcude from what I did.


Thnx

Umesh

Leo Laohoo Mon, 11/18/2013 - 13:24
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Does this mean that current Gigabit Phy's do not follow the IEEE standards and use only 2 pairs instead of all 4.

GigabitEthernet requires three pairs minimum.  Two pairs only gives you FastEthernet.


So the "answer" to your question depends on who you are talking to.  If you ask someone from Marketing, they might respond that the manufacturer follows the IEEE standard.  If you ask someone technical, the respond will be standards are NOT being followed. 

Umesh Shetty Mon, 11/18/2013 - 19:26
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Thnx Leo,


So from what I tried few days back, I used a cross over cable with only 2 pairs crossed ie pairs 1-2 & 3-6. I was still able to get 1000 Mb with Auto Negotiation. Is that not a bit strange .


Thnx Umesh

Peter Paluch Tue, 11/19/2013 - 02:55
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Leo,


I am losing you in a couple of statements you have posted. Can I ask you for more explanation on these?


GigabitEthernet requires three pairs minimum.


What do you mean exactly? IEEE 802.3 is very clear about 1000BaseT that is must be implemented using four pairs. Three pairs will never give you GigabitEthernet.


However, when it's being implemented in switches, like Cisco, Pair D  determines whether or not the interface is running GigabitEthernet or  FastEthernet. 


Is this documented anywhere? In addition, how exactly does the D pair determine that the link is going to be GigabitEthernet?


Thanks!


Best regards,

Peter

Leo Laohoo Tue, 11/12/2013 - 12:47
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I read this in most of the documents that I read about Gig ethernet that it uses all four pairs of a UTP cable.

Yes and no.  IEEE standards states all four pair but switches only "sees" Pair A, B and D.  Pair C is used for PoE.

Peter Paluch Tue, 11/12/2013 - 13:27
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Leo,


Yes and no.  IEEE standards states all four pair but switches only "sees" Pair A, B and D.  Pair C is used for PoE. 


Ummm... with or without PoE, Gigabit Ethernet always uses all four pairs. Ethernet on TP uses differential signalling; a voltage bias on a particular pair has no influence on the data signal intelligibility.


Best regards,

Peter

Leo Laohoo Tue, 11/12/2013 - 13:31
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Ummm... with or without PoE, Gigabit Ethernet always uses all four pairs.

No arguments there.  This is the IEEE standards.


However, when it's being implemented in switches, like Cisco, Pair D determines whether or not the interface is running GigabitEthernet or FastEthernet.


Another thing, I believe there's a stupid "bug" somewhere between Cisco and/or IEEE.  Let me explain:


Get a GigabitEthernet-capable client, a GigabitEthernet switch and a FastEthernet cable (Pair A and B terminated, Pair D shorted).  Plug them in.  Logic dictates that the link should be FastEthernet, right?  Not in the eyes of Cisco.


Do this setup and you'll see the link go down and stay down (like a boxer knocked out cold).  The link will not go up until you issue the interface command "speed auto 10 100".


PS:  Hey Peter!  Haven't seen you for some time.

Leo Laohoo Mon, 11/18/2013 - 20:07
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I used a cross over cable with only 2 pairs crossed ie pairs 1-2 & 3-6.

Can you do the following commands: 


1.  Command:  test cable tdr interface ;

2.  Wait for about 5 to 7 seconds;

3.  Command:  sh cable tdr interface ;

4.  Post the output to #3. 

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