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Sam Wilson <> writes:
>> But, since this is a cisco group... there are cases when you do need to
>> make GigE copper cross-over cables when dealing with older Cisco
>> gear's GigE ports that didn't support auto MDI/MDIX.

>It's this I'm having trouble with - given the specification of
>1000base-T, why would there ever be a crossover cable?

Maybe the Cisco parts were developed before the GigE standard
stabalized before auto MDI/MDIX was implemented. Maybe Cisco didn't
feel like they needed to support it, or the 3rd part company they
bought the GBIC parts I've worked with wouldn't support it, but once
they started going down that path, they had to follow along.

I don't think its a huge deal overall. There's plenty of odd things in
the networking world .

In article <44f2f8f9$0$28263$ s.com>,
Doug McIntyre <> wrote:

> Sam Wilson <> writes:
> >> But, since this is a cisco group... there are cases when you do need to
> >> make GigE copper cross-over cables when dealing with older Cisco
> >> gear's GigE ports that didn't support auto MDI/MDIX.
>
> >It's this I'm having trouble with - given the specification of
> >1000base-T, why would there ever be a crossover cable?
>
> Maybe the Cisco parts were developed before the GigE standard
> stabalized before auto MDI/MDIX was implemented. ...

Look at the description of a GigE port and tell me why any kind of
MDI/MDIX is needed. AFAICS every port is wired the same and can be
connected to any other GigE port with a straight cable.

Sam

> > > It's this I'm having trouble with - given the specification of
> > > 1000base-T, why would there ever be a crossover cable?


Straight Through or "normal" cross over both work. 1000Base-T can do
MDI/MDIX by default.

Sam,
The reason why "normal" crossover cable will work is that EVERY cat5
(or better) cross-over cable always crossed all eight cables (in
pairs). So while only four cables are needed for 100Base-T, very very
few cables were made with just four cables.

If you pick up any cross over sold at any retail store (RadioShack,
CompUSA, BestBuys etc) you'll see that the pin out on Cisco's website
(http://www.cisco.com/en/US/products/...ducts_configur
ation_guide_chapter09186a00803959e5.html) is how they are built.

--

hsb


"Somehow I imagined this experience would be more rewarding" Calvin
**************************ROT13 MY ADDRESS*************************
Due to the volume of email that I receive, I may not be able to
reply to emails sent to my account. Please post a followup instead.
************************************************** ******************

Sam Wilson wrote:

>
> I've asked in another thread and received no answer: why is this? Given
> that 1000base-T transmits and receives on all four pairs simultaneously
> why is crossing pairs even considered? (And it can't be backwards
> compatibility with earlier crossover cables because a 1000base-T
> crossover is wired differently from any other cable!)
>
If you connect two like ports together the TX and RX lines must be
crossed or you end up connecting TX to TX and RX to RX. If one port is
MDI and the othe MDI-X the TX and RX pairs are on pins that match up
with a straight cable- an MDI port transmits on one set, an MDI-X on the
other.

A Gigabit X-Over is backwards compatible, but a old crossover wired for
10/100 usually only crosses 2 pairs of the wire (pins 1+2 and 3+6). In a
gigabit crossover all pairs are crossed, because as you say, all pairs
are used.

http://www.ertyu.org/steven_nikkel/ethernetcables.html

Hansang Bae wrote:
>>>> It's this I'm having trouble with - given the specification of
>>>> 1000base-T, why would there ever be a crossover cable?
>
>
> Straight Through or "normal" cross over both work. 1000Base-T can do
> MDI/MDIX by default.
>
> Sam,
> The reason why "normal" crossover cable will work is that EVERY cat5
> (or better) cross-over cable always crossed all eight cables (in
> pairs). So while only four cables are needed for 100Base-T, very very
> few cables were made with just four cables.
>
> If you pick up any cross over sold at any retail store (RadioShack,
> CompUSA, BestBuys etc) you'll see that the pin out on Cisco's website
> (http://www.cisco.com/en/US/products/...ducts_configur
> ation_guide_chapter09186a00803959e5.html) is how they are built.
>
I'm pretty sure I've come across some early Cisco GigE that wouldn't
auto MDI-MDIX and had to make a x-over.

"Chris Bartram" <> a écrit dans le message de
news:...
> Sam Wilson wrote:
>
> >
> > I've asked in another thread and received no answer: why is this? Given
> > that 1000base-T transmits and receives on all four pairs simultaneously
> > why is crossing pairs even considered? (And it can't be backwards
> > compatibility with earlier crossover cables because a 1000base-T
> > crossover is wired differently from any other cable!)
> >
> If you connect two like ports together the TX and RX lines must be
> crossed or you end up connecting TX to TX and RX to RX. If one port is
> MDI and the othe MDI-X the TX and RX pairs are on pins that match up
> with a straight cable- an MDI port transmits on one set, an MDI-X on the
> other.
>
> A Gigabit X-Over is backwards compatible, but a old crossover wired for
> 10/100 usually only crosses 2 pairs of the wire (pins 1+2 and 3+6). In a
> gigabit crossover all pairs are crossed, because as you say, all pairs
> are used.
>
> http://www.ertyu.org/steven_nikkel/ethernetcables.html


Thanks a lot Chris for your message and for the link.

Best Regards
Rahan

Chris Bartram wrote:
> I'm pretty sure I've come across some early Cisco GigE that wouldn't
> auto MDI-MDIX and had to make a x-over.

I have 11 2950T-48 swtiches that do not have auto MDIX and we bought
them brand new in April.

Chris Bartram wrote:
> I'm pretty sure I've come across some early Cisco GigE that wouldn't
> auto MDI-MDIX and had to make a x-over.

Yup. Very true. It gets worse. Ever lookup the command 'auto
negotiation' on CCO? It's enough to make your head spin.

--

hsb


"Somehow I imagined this experience would be more rewarding" Calvin
**************************ROT13 MY ADDRESS*************************
Due to the volume of email that I receive, I may not be able to
reply to emails sent to my account. Please post a followup instead.
************************************************** ******************

In article <>,
Chris Bartram <> wrote:

> Hansang Bae wrote:
> >>>> It's this I'm having trouble with - given the specification of
> >>>> 1000base-T, why would there ever be a crossover cable?
> >
> >
> > Straight Through or "normal" cross over both work. 1000Base-T can do
> > MDI/MDIX by default.

What's the difference between MDI and MDIX in 1000base-T? I can't find
a definition. I'll ask again, why would there ever need to be any kind
of crossover in 1000base-T?

> > Sam,
> > The reason why "normal" crossover cable will work is that EVERY cat5
> > (or better) cross-over cable always crossed all eight cables (in
> > pairs). So while only four cables are needed for 100Base-T, very very
> > few cables were made with just four cables.

The first part of this is simply not true. Until recently the vast
majority of crossover cables we've ever seen (made for 10/100base-T)
crossed over the 1-2 and 3-6 pairs but left 4-5 and 7-8 connected
straight through. There are many references to this kind of cable
online. [1]

> > If you pick up any cross over sold at any retail store (RadioShack,
> > CompUSA, BestBuys etc) you'll see that the pin out on Cisco's website
> > (http://www.cisco.com/en/US/products/...ducts_configur
> > ation_guide_chapter09186a00803959e5.html) is how they are built.

I can't find any published pinout for those other suppliers, but the
Cisco page is flawed. Table B-2 which claims to show a 10000base-T
crossover cable actually shows a straight through cable (with the signal
mislabelled on pin 2 side 2). Figure B-3 does show what might be a
crossover cable, but that takes me back to my question of why would you
bother?

> I'm pretty sure I've come across some early Cisco GigE that wouldn't
> auto MDI-MDIX and had to make a x-over.

What does MDIX mean on 1000base-T? That an MDIX interface sends and
receives databits 0 and 1 on pins pair 3-6 rather than 1-2, bits 2 and 3
on 1-2 rather than 3-6 and similarly for bits 4&5 and 6&7 on pairs 4-5
and 7-8? Why, when if you made all interfaces the same (MDI) they could
talk to any other with a straight through cable?

Sam, aka Baffled of Edinburgh


[1] for example <http://logout.sh/computers/net/gigabit/>,
<http://www.finnie.org/text/network-tips/> and
<http://www.derose.net/steve/guides/wiring/> (but why is the
brown/white-brown pair phase-crossed in the first two?)

In article <>,
Chris Bartram <> wrote:

> Sam Wilson wrote:
>
> >
> > I've asked in another thread and received no answer: why is this? Given
> > that 1000base-T transmits and receives on all four pairs simultaneously
> > why is crossing pairs even considered? (And it can't be backwards
> > compatibility with earlier crossover cables because a 1000base-T
> > crossover is wired differently from any other cable!)
> >
> If you connect two like ports together the TX and RX lines must be
> crossed or you end up connecting TX to TX and RX to RX. If one port is
> MDI and the othe MDI-X the TX and RX pairs are on pins that match up
> with a straight cable- an MDI port transmits on one set, an MDI-X on the
> other.

Not on 1000base-T - all pairs send are bidirectional - there's no TX and
RX to crossover. Look it up.

> A Gigabit X-Over is backwards compatible, but a old crossover wired for
> 10/100 usually only crosses 2 pairs of the wire (pins 1+2 and 3+6). In a
> gigabit crossover all pairs are crossed, because as you say, all pairs
> are used.
>
> http://www.ertyu.org/steven_nikkel/ethernetcables.html

I know what sites like that say, I just don't know why they say it -
allowing a crossover cable just makes life more complicated for everyone.

Sam