GbE Switch Negotating Speed Down to 100Mb? What?

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GbE Switch Negotating Speed Down to 100Mb? What?

Postposted on Thu Nov 08, 2012 4:04 pm

I have a really odd thing happening on my home network. I live in a Condo and it's pre-wired with Cat5E to every room. When I plug all Cat5E cables into my Dlink DGS-1005D GbE switch, one of the links auto-negotiates down to 100Mb. I've confirmed that everything end - end supports GbE on this cable. When I plug this same cable directly into a laptop it correctly negotiates to 1000GbE but then when I plug the same line back into the GbE switch it negotiates down to 100Mb.

To me this looks like an issue with the switch. The thing is, other cables correctly negotiate to 1000GbE so I'm hesitant to point blame anywhere at this point. Has anybody ever seen something like this?
Would this be a case of the auto-negotiation mechanism in the switch being overly aggressive versus the laptop being a bit more forgiving with respect to line speeds.
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Re: GbE Switch Negotating Speed Down to 100Mb? What?

Postposted on Thu Nov 08, 2012 4:22 pm

Bum cable is my first guess.
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Re: GbE Switch Negotating Speed Down to 100Mb? What?

Postposted on Thu Nov 08, 2012 4:30 pm

bthylafh wrote:Bum cable is my first guess.

....yet the same bum cable works at GbE speeds when plugged directly into the laptop....so is it the cable? ;)
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Re: GbE Switch Negotating Speed Down to 100Mb? What?

Postposted on Thu Nov 08, 2012 5:35 pm

Jon wrote:
bthylafh wrote:Bum cable is my first guess.
....yet the same bum cable works at GbE speeds when plugged directly into the laptop....so is it the cable? ;)

Have you tried this with multiple ports on the switch?
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Re: GbE Switch Negotating Speed Down to 100Mb? What?

Postposted on Thu Nov 08, 2012 6:02 pm

Captain Ned wrote:
Jon wrote:
bthylafh wrote:Bum cable is my first guess.
....yet the same bum cable works at GbE speeds when plugged directly into the laptop....so is it the cable? ;)

Have you tried this with multiple ports on the switch?


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Re: GbE Switch Negotating Speed Down to 100Mb? What?

Postposted on Thu Nov 08, 2012 6:32 pm

Reckon a less-than-perfect cable too. Could be that the laptop NIC is just more tolerant of the connection than the switch.
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Re: GbE Switch Negotating Speed Down to 100Mb? What?

Postposted on Thu Nov 08, 2012 6:48 pm

If I understand correctly, when you go from laptop to wall jack to switch you get 100Mb and when you got laptop to switch using the same cable you plugged into the wall, you get 1Gb. Is that correct? If so, the prewired part of the connection is bad. Check the punch down if you have one and access. May just be a loose wire.
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Re: GbE Switch Negotating Speed Down to 100Mb? What?

Postposted on Thu Nov 08, 2012 7:25 pm

Unless you've used the prewired connection before or have the measurment protocol from installation, if there was one, it's not that uncommon that pre-wired cables can be bad at times. I've been to several places where wiring in homes was done by an electrician and not a network tech, and seen both cables that are bent to harshly as well as cables not properly connected or the pairs wasnt wired correctly. 100Mbit only requires 2 pairs, Gig requires all 4 pairs. Although negotiation only takes place over 2 pairs, which might lead to some odd things when it comes to autoneg and links, depending on the gear plugged in.
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Re: GbE Switch Negotating Speed Down to 100Mb? What?

Postposted on Fri Nov 09, 2012 8:38 am

^^ Exactly. My house had 4 out of 10 wall jacks with bad wiring, one of which had the 2 pairs working and the 4 pairs broken scenario. That socket would give different results (from 100Mb/s to no link) depending on the manufacturer of the NIC I plugged in.
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Re: GbE Switch Negotating Speed Down to 100Mb? What?

Postposted on Fri Nov 09, 2012 8:48 am

I'd suspect the wiring in the wall as well. I've found many a bad wire run in my time.

As far as electricians go, my own experience is that they do an excellent job. I've found they adhere to standards and testing a lot better than "network techs" do. I figure has something to do with the fact that they regularly run cable that can burn down buildings if installed incorrectly, whereas people who only install CAT5/6 runs, the worst thing that can generally happen is that it doesn't work. So I guess YMMV.
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Re: GbE Switch Negotating Speed Down to 100Mb? What?

Postposted on Fri Nov 09, 2012 11:12 am

If that switch port works properly when connected to a different Cat5e then yeah it sounds like a marginal cable (or jack) to me too. GbE is just this side of black magic; it's pretty miraculous that it works so well most of the time. If that cable run is right on the edge of being able to support GbE, minor differences in how the Ethernet ports on your switch and laptop are implemented could certainly make or break GbE.
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Re: GbE Switch Negotating Speed Down to 100Mb? What?

Postposted on Fri Nov 09, 2012 11:21 am

Yea I'm relegating to the assumption that the cable run in the wall is bad and that the GbE switch is a little less tolerant of less-than-perfect GbE cabling. Pity. It's certainly not a trainstopper as I'll simply move my network gear to another location in the Condo.

Thanks for all the input.
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