Re: Ethernet Crossover Cables

 The 1000Base-T4 became 1000Base-T. 1000Base-TX exists as well.

I wouldn't call it a "commercial failure" at all, though I suppose it
depends on who wrote the wiki page and who they worked for!

There was a long pull at the beginning towards using all the pairs so
that people didn't have to purchase Level 6 cable (prior to Cat6
standard) or Level 7 cable... They could get by with Cat5 or Cat5e.

In short runs, it likely doesn't matter all that much what you use...
But at full length, it will make a difference! I haven't really paid
attention all that much, but Cisco switches are listed as TX.... My
macbook is listed as T. And yet it works fine at 1000M with a two-pair
cable. *shrug*

Not sure what to tell you as I really haven't read the specs to see who
is or isn't compatible with the other! Or if there's a downshift in
speed (still above 100?).

Remember that wikis are written by anyone. So there's room for social
commentary and not 100% accurate information 100% of the time!

Scott Morris, CCIEx4 (R&S/ISP-Dial/Security/Service Provider) #4713,

JNCIE-M #153, JNCIS-ER, CISSP, et al.

JNCI-M, JNCI-ER

evil_at_ine.com

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Ivan Walker wrote:

  Hi Scott,
  
  Thanks for the quick response. Again looking a wikipedia
  ( http://en.wikipedia.org/wiki/Gigabit_Ethernet ) I can find no reference to
  a 1000Base-T4 standard.
  
  I am specifically interested in why a *gigabit* link between two switches
  using a crossover with only 2 pairs crossed actually works.
  
  The wikipedia link above states "1000BASE-T requires all four pairs to be
  present." and also mentions a standard 1000BasseTX which only uses 2 pairs
  but "has been a commercial failure".
  
  Perhaps the Cisco switches are using 1000Base-TX???
  
  switch#show interfaces capabilities
  GigabitEthernet0/1
    Model: WS-C3560G-24PS
    Type: 10/100/1000BaseTX
    Speed: 10,100,1000,auto
    Duplex: half,full,auto
    Trunk encap. type: 802.1Q,ISL
    Trunk mode: on,off,desirable,nonegotiate
    Channel: yes
    Broadcast suppression: percentage(0-100)
    Flowcontrol: rx-(off,on,desired),tx-(none)
    Fast Start: yes
    QoS scheduling: rx-(not configurable on per port basis),
                           tx-(4q3t) (3t: Two configurable values and one
  fixed.)
    CoS rewrite: yes
    ToS rewrite: yes
    UDLD: yes
    Inline power: yes
    SPAN: source/destination
    PortSecure: yes
    Dot1x: yes
  
  Thanks
  
  Ivan

     Because there are two different standards. T4 uses all four pair, it's
    designed for use with lower-quality cables. TX only uses two pair like
    all other ethernet variants, but is supposed to be higher level cable.
    
    HTH,
    
    Scott Morris, CCIEx4 (R&S/ISP-Dial/Security/Service Provider) #4713,
    
    JNCIE-M #153, JNCIS-ER, CISSP, et al.
    
    JNCI-M, JNCI-ER
    evil_at_ine.com
    Internetwork Expert, Inc.
    http://www.InternetworkExpert.com
    Toll Free: 877-224-8987
    
    Outside US: 775-826-4344
    
    Knowledge is power.
    
    Power corrupts.
    
    Study hard and be Eeeeviiiil......
    
    Ivan Walker wrote:
    
      Wikipedia has a detailed explanation of crossover cables here http://en.wikipedia.org/wiki/Ethernet_crossover_cable#Crossover_cable_pinouts covering the pins outs for 1000Base-T and 100Base-TX. 1000Base-T
      crossover cables cross all four pairs where as 100Base-TX crossover
    cables
      cross only 2 pairs (can cross four but only 2 pairs are used anyway).
    
      Looking at some crossover cables I found I did indeed find some with all
      pairs crossed and some with only 2 pairs crossed. When testing these in
      some Cisco switches both worked fine at 1Gps. This was kind of
    unexpected
      as I anticipated that the crossover with only 2 pairs crossed would not
      work.
    
      I tried disabling mdix and speed/duplex negotiation etc but could not
    break
      it. Can anyone explain why a crossover cable with only 2 pairs crossed
      still works for 1000Base-T.
    
      Cheers
    
      Ivan

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Received on Wed Feb 03 2010 - 19:45:15 ART