100BaseT, is there a minimum cable length ?

Unanswered Question
Mar 19th, 2008


I am connecting two ethernet switches together via CAT5e crossover, used for a 100BaseT network. The switches will physically sit ontop of each other.

Obviously I want to keep the cable as short as possible, (20cm-30cm)

I have spent hours & hours trawling the IEEE specificiation and can find nothing on "minimum" cable length.

Also, not sure if it's wise to have the Ethernet Transiever ports too close together..... due to ...

"NEXT [Near End Cross Talk]; A measurement of interference between the conductors of a cable"

any ideas ?

thank you.


I have this problem too.
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scottmac Wed, 03/19/2008 - 14:18

One meter is the suggested minimum.

In many cases, you can go as short as 18" / half meter.

Between active devices, 18" / 50cm is the absolute shortest cable you should run, but FWIW, I'd stay with one meter.

The other issues, to a lesser degree, are FEXT and the power sum numbers. The minimum length is more for the feedback loop than attenuation.

Good Luck


mcroft Wed, 03/19/2008 - 14:58

Thanks for your response Scott.

Can I ask where you find these facts ?

'Cause I am unable to find any "official" documents ..


Rick Morris Thu, 03/20/2008 - 10:45

We have made cables as short as 6" and not had any trouble. Also, the cable we are using is Cat6 too. 5e should due the same. Here is something directly from BICSI:

Cross-connect Jumpers and

Patch Cords

Telecommunications Distribution Methods Manual

on CD-ROM, 8th edition, © 1998 BICSI®

Chapter 4 - Horizontal Cabling Systems










Length Requirements

Horizontal cross-connect jumpers and patch cords should not exceed a length

of 6 m (20 ft.) per link. System designers should plan for a combined cable

length of 10 m (33 ft.) for patch cords and for equipment connections in the

work area and telecommunications closet. This length is in addition to the

90 m (295 ft.) of cable allowed between the horizontal cross-connect and the

telecommunications outlet/connector.


page 50

Now there is nothing specific to shortness, rather it is the twist to be concerned with.

scottmac Fri, 03/21/2008 - 07:00

It was a "best practice" rule we used while I worked in the Lab at Anixter. It was just a generally accepted convention, but one that was pretty much accepted as "The Law."

When we did proof-of-concept testing for customers, or if we were doing anything related to performance of some networking component, staying within best practice convention (especially when there was no specific written rule) was very important.

I made a quick pass through the EIA/TIA 568 documentation and didn't see anything specific to minimum cable length. My other cable-related books & docs are buried and not easily accessible right now.

If this is a critical point for you, then I would take it within the context of the following:

The EIA/TIA provides specific thresholds related to transmission characteristics of Category-rated cabling. As long your configuration conforms to those specific characteristics, then the length of the cable (longer than 100M, shorter than conventionally accepted best practice)is moot.

The "100 meter" rule is merely a guideline, such that if you use Category-compliant components, and you stay within the recommended 100M overall length (5m stranded/jumper + 90m solid / "in-wall" or permanent + 5m stranded jumper), and all other rules (MBR, stretch, termination ...) are followed, your cabling system should perform at maximum performance levels.

There are always "I did it this way and it worked" (when "this way" is not necessarily by the spec)people: Just getting bits down the wire is not the goal.

For the time and money expended to create the network (at least in a commercial environment), you want to get the maximum possible performance. Operating outside the spec generally means sub-optimal performance (i.e., money wasted from being lazy, ignorant, cheap, or stupid).

You might be getting some bits down the wire, but not as many as you probably could get.

If I get time, I'll try to dig up the docs.

Good Luck


Danilo Dy Fri, 03/21/2008 - 08:08


This is the problem. Different Networking Technology specifies that there should be minimum distance between systems/hosts. However, it does not specify what is the distance.

There was an old "standard" (not endorsed or come from by IEEE) that the minimum Ethernet cable length is 2.5meters.

It's case to case basis, sometimes I experienced problem with 6" cable...

- The connection doesn't even goes up between two Catalyst 3750 Switches (GLC-T)

- The connection did goes up between two Catalyst XL Switches but too many errors.

To avoid problem (and overhead), I use longer cables and just coil them neatly. I use minimum 1.5 meters, so far this does not give me any problems - I don't want to test all length to find out the absolute minimum that will satisfy all technologies. i.e. 10BaseT, 100BaseT, 1000BaseT, half-duplex, full-duplex, Cat5, Cat5e, and Cat6. I leave this to the guys that supposed to set the standards.

Most of network professional problem is how to make the cabling neat, that's why they want to cut them in their precise length between systems/hosts. Well, it takes years of perfection, descipline, and good mentoring.



Paolo Bevilacqua Fri, 03/21/2008 - 15:13

Perhaps that's the reason cisco makes that short cable with sfp's on both ends, it's short enough to be tidy but won't cause errors in any case.


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