Re: Ethernet Crossover Cables

"Wikipedia is the best thing ever! Anyone in the world can write
anything they want about any subject, so you know that you are getting
the best information..." - Michael Scott

Sorry, couldn't resist...

On Wed, Feb 3, 2010 at 7:45 PM, Scott Morris <smorris_at_ine.com> wrote:
> 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
>
> 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:
>
> 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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-- 
Ronald Angello
Network Architect
CCIE 17846
CCDP, CCIP, CCNP
Blogs and organic groups at http://www.ccie.net