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828 Views 8 Replies Latest reply: Oct 23, 2013 10:59 AM by Joshua Johnson - CCNP R&S RSS

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Why do we need crossover cable for 1000Base-T

Oct 20, 2013 2:20 PM

Jamal 12 posts since
Apr 19, 2013

Hello,

 

I wonder if someone can help me with this issue. I understand for 100Base-T on a router or a PC we have pins 1and 2 for transmitting and 3 and 6 are for receiving. So when we need to connect similar devices we use a crossover cable.

 

With 1000Base-T it uses all four pairs to simultaneously transmit and receive data then why do we need acrossover cable?

 

Thanks

  • Jamie 13 posts since
    Feb 26, 2012
    Currently Being Moderated
    1. Oct 20, 2013 2:49 PM (in response to Jamal)
    Re: Why do we need crossover cable for 1000Base-T

    You need a cross over cable if you are connecting like devices, ie switch to switch, computer to computer. If you are connecting a switch to a router then you can use a straight cable. I hope this makes sense to you. . Here is an example of that cable;

    gigabit-crossover.jpg                                                                                                                                                                                                                                               

    Here is a link that will explain about each type of cable for your future reference.  http://www.networking-basics.com/twisted-pair-cable-types

     

    Message was edited by: Jamie

  • Hrishikesh 208 posts since
    Apr 25, 2013
    Currently Being Moderated
    2. Oct 20, 2013 6:17 PM (in response to Jamal)
    Re: Why do we need crossover cable for 1000Base-T

    Hi Jamal,

    I copied this from the book directly:,

    Crossover cable: If the endpoints transmit on the same pin pair

    Straight-through cable: If the endpoints transmit on different pin pairs.

     

    So, Now here You may get a confusion about the short definations..

    but what actually means is that,"Like devices transmit and recive on the same pair of pins"

    and "Unlike devices transmit and recive on the Different pair of pins", As we know switches transmitting at pins 3 and 6 and again if we connect another switch which transmits in the same 3 and 6 pins, there will be a problem, (And most probably, I think will be a collision), So To Avoid this, we need them to transmit and recieve in different pins.

    See this: X-over Cable-------->

    623px-Ethernet_MDI_crossover.svg.png(picture from wikipedia)

     

    Also see this from he book,

    123 (2).JPG

     

     

    HTH

    Regards,

    Hrishi

  • Wilson Crawford 33 posts since
    Jun 27, 2008
    Currently Being Moderated
    3. Oct 20, 2013 6:50 PM (in response to Jamal)
    Re: Why do we need crossover cable for 1000Base-T

    Jamie's reply explans it well.   (I liked the article as well)  When you are connecting like devices, like a switch to a switch you will need a crossover cable.

     

    Even for the 1000 BASE-T, because if you are connecting two devices and they are trying to tranmit on the same pins then you can imagine that it would cause issues.  And like the article explains, all the pins are crossed for 1000 BASE-T.

  • Darin 13 posts since
    Jul 25, 2013
    Currently Being Moderated
    4. Oct 20, 2013 11:07 PM (in response to Jamal)
    Re: Why do we need crossover cable for 1000Base-T

    Just because 1000Base-T has autoMDI-X written into the spec doesn't mean the thing it is connect to has it.  The same goes for auto-duplexing.  Sometimes auto-auto works and sometimes two manufacturers did something where it it's all borked and you have to "nail down" the duplexes manually.

     

    Maybe someday when we only have GigE or higher, we can stop thinking about wiring configurations, not yet though.

  • Currently Being Moderated
    6. Oct 21, 2013 10:57 AM (in response to Jamie)
    Re: Why do we need crossover cable for 1000Base-T

    Jamie wrote:

     

    You need a cross over cable if you are connecting like devices, ie switch to switch, computer to computer.


    While this is definitely true, I would make sure you are familiar with the pinouts for all of the "testable" devices.  For example:  What kind of cable would you use to connect a router directly to a computer that doesn't support auto MDIX?  These aren't like devices, but they have like pinouts.

  • Rob Riker CCNP R&S/CCNA Security 117 posts since
    Apr 20, 2012
    Currently Being Moderated
    7. Oct 21, 2013 12:14 PM (in response to Jamal)
    Re: Why do we need crossover cable for 1000Base-T

    You need a crossover cable anytime you are connecting like devices. Newer devices, mostly switches have the ability to auto detect and change, called Auto MDIX. Where it will detect which pins are sending and receiving and modify it's own port type to accommodate the issue.

  • Joshua Johnson - CCNP R&S 5,156 posts since
    Sep 22, 2009
    Currently Being Moderated
    8. Oct 23, 2013 10:59 AM (in response to Jamal)
    Re: Why do we need crossover cable for 1000Base-T

    Erick that's a good point, even though I'm sure Jamie knew that already... like devices from a pinout perspective, we need to be as specific as possible to ensure that those who are learning it get the right information...

     

    And don't just take the word of someone even if you trust them, you must always do your own research and validate the facts... if not just to get more study time in and better understanding.

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