why a standard ethernet cable worked when a patch cable didn't

Caleb Call calebcall at me.com
Sun Dec 9 18:55:10 MST 2012


On Dec 9, 2012, at 6:27 PM, Michael Torrie <torriem at gmail.com> wrote:

> On 12/09/2012 05:25 PM, Wade Shearer wrote:
>> What's unique about that cable or about the Linsys switch that they work together?
> 
> I bet one of the pairs in the cable is bad, but not one of the the two
> pairs needed for 100 Mbit/s connection.  The Cisco switch and the
> airport extreme are almost certainly trying to negotiate for gigabit,
> and since gigabit requires all four pairs to be good.  I'm not sure why
> the switch won't drop down and negotiate for 100 Mbit when the gigabit
> fails.  I could be wrong though.
> 
> I recommend you invest in a good network tester.  They aren't cheap, but
> if you get one that can do wire maps and other forms of cable testing, I
> think you'll find it quite handy.  I bought myself a little lanscaper,
> and it has been extremely useful for my home networking projects.
> Besides being able to emit a powerful tone for tracing wires, it can do
> wire testing, and also test a port of a switch and determine what kind
> it is, and what protocols it supports (802.1q for example), vlans, etc.
> But mostly I use it for wire maps, and for telling me how long a cable
> run is.
> 
> My Lanscaper can only do 100 mbit when it comes to testing a switch or
> an active connection, but that seems to be enough for me. I think the
> pro version can do gigabit.  And if you're really rich there is the
> Fluke version!
> 
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The Airport Extreme will negotiate at any speed.  So if 100mb or 10mb is all that's available, it will still negotiate at those rates.  I'm willing to bet it's a limitation of the Cisco.  The issue is likely due to a bad pair, as has been said.  Something you may want to look at, but I believe the AE will only get a yellow/orange light if it negotiates at less than Gig and a green if it negotiates at Gig, that may be a quick indicator.  I have a Fluke LinkRunner that I used to use a ton back when I was dealing with cables.  Now it is limited to very little home use.  Even with the limited use I get out of it, I would still buy another.  It will tell me the length of each pair so I can track down the break (or bad punch usually) very quickly.  It will also tell me what it can negotiate at and if your cisco switch supports it, usually only managed cisco switches do, it can also tell me exactly what port it's plugged in to on the switch. 

If you want to borrow it, I'd be happy to let you use it.  


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