Re: Ethernet Crossover Cables

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
>
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>
> Knowledge is power.
>
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>
> 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 Thu Feb 04 2010 - 13:09:34 ART