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We have two buildings connected by a 30m network cable with a switch on either end. Last week we upgraded some old equipment to support gigabit speeds. One of the two switches was an old 10/100 switch so we upgraded it. Right away the connection between the buildings stopped working.

What we've tried

  1. Plug the old 10/100 switch back in in-between the long run and new gigabit switch. This works, but obviously destroys the upgrade.

  2. The cable was a Cat5e that was installed 4 years ago. It's within the spec, but we thought it might be low quality / damaged, so we ran a brand new Cat7 cable. Same exact behavior as the old Cat5e (ie, it works on the 10/100 switch but not the gigabit switch).

  3. We tried using different gigabit switches on both sides of the cable. Same behavior.

  4. We plugged one end of the cable directly into a gigabit ethernet computer and the other end into the new switch. This worked fine, leading us to suspect there might be an Auto-MDI negotiation problem. We rewired the new Cat7 to be a crossover cable to avoid the need for Auto-MDI. Same behavior.

  5. We connected both of the new switches using a short (2m) patch and cross cables. The connection works perfectly with either one.

What other troubleshooting options do we have at this point?

Equipment details:

  • Old 10/100 switch - TP-Link TL-SF1005D
  • New gigabit switch - TP-Link TL-SG1008D (produced in 2018)
  • Old gigabit switch in building 2 - TP-Link TL-SG1008D (produced in 2016)
  • Replacement gigabit switches we used to test - DLink DGS-1008C and DLink DGS-1008A
just.another.programmer
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  • So you did everything to localize the problem, except the thing you should start with - testing the so-called "30 meters cable between the buildings" ? Is it still working - that is the question. – drookie Dec 17 '18 at 10:59
  • @drookie What kind of test? 1) We plugged the cable into the old switches - it works; 2) We plugged it into a cable test - it shows all pairs working; 3) We _replaced_ it just in case there was an issue w/ using Cat5e and did the same two tests again - both worked. What other test can we do? – just.another.programmer Dec 17 '18 at 11:04
  • Are there any other cables running between the two switches? – Davidw Dec 17 '18 at 11:04
  • @Davidw No. Just the original Cat5e and the new Cat7. (We unplugged the Cat5e, it's just sitting there until we resolve the problem). – just.another.programmer Dec 17 '18 at 11:05
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    Did you try any decent switches ? TP-Link produces garbage. By saying "decent switches" I also mean "managed" - because it's vital to see what's happening on interfaces. So far, by my understanding, you determine link state by only watching the port LED. – drookie Dec 17 '18 at 11:12
  • @drookie We're working on a tight budget - could you recommend a low cost managed / "decent" option? We're also located in Israel so we don't have access to all the same manufacturers available in the US. – just.another.programmer Dec 17 '18 at 12:02
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    Low-cost isn't really "decent". I'd say - get rid of all Compex/TP-Link/D-Link switches, but the problem described above can have nothing with the vendor (but the advice still stands). For the test - get some managed switch, plug the cable just to see what's happening on l2 interface. About your budget and low-cost - buy used cisco on e-bay. – drookie Dec 17 '18 at 12:54
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    There is no such thing as a gigabit crossover cable. Are you 100% sure the pins-to-pairs mapping on both cables was correct for gigabit ethernet? That's almost always the cause when you have this particular problem. Can you describe in detail how the cable was wired? What is the pin-to-pair mapping? – David Schwartz Dec 17 '18 at 18:31
  • I would suspect attenuation or interference from an electrical/RF source. Is the cable stranded or solid conductor? Is it shielded? – Greg Askew Dec 17 '18 at 18:33
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    We need to know the cable is pinned how too, T-568A/B Straight-Through Ethernet Cable ? Did you tried to do a crossover cable ? did you tested the cable after if the 4 pairs are ok ? – yagmoth555 Dec 17 '18 at 18:34
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    First, there is no such thing as Category-7 cable; I think you may mean Class-F cable, which is shielded, and the shield must be continuous and grounded, at least on each end, and by the equipment (consumer-grade equipment does not usually have such a ground), otherwise the cable will perform poorly. Also, in a cable channel, you are limited to 10 meters of stranded cable (5 meters on each end), and the horizontal cable needs to be solid-core. Modern network speeds require you to adhere to some strict specifications. – Ron Maupin Dec 17 '18 at 18:45
  • how long is the cable run? – SteamerJ Dec 17 '18 at 19:16
  • @DavidSchwartz It was originally wired as a standard cable. We switched it to cross (following a cat5e pinout) as a test. It's back to standard now. – just.another.programmer Dec 17 '18 at 19:47
  • @just.another.programmer Does "standard cable" mean T-568A or T-568B or just straight through? The mapping of pins-to-pairs matters. All pins connected straight through is *NOT* enough. – David Schwartz Dec 17 '18 at 19:51
  • @RonMaupin The store sells it as Cat 7. I assume your right it's Class F. Each pair is shielded and the entire cable is shielded. There's no ground on it - what do I need to do to add one? – just.another.programmer Dec 17 '18 at 19:56
  • @DavidSchwartz Straight through. T-568A on both ends. – just.another.programmer Dec 17 '18 at 19:58
  • There is some information about shielding [in this answer](https://networkengineering.stackexchange.com/a/51076/8499). Basically, not grounding the cable shield will cause interference and poor performance. ANSI/TIA/EIA defines the cable categories, and they have never certified Category-7. The currently certified categories are 5e, 6, 6a, and 8 (new, but many restrictions). Only Category-8 is shielded. ISO/IEC certified cable classes, many of which are shielded. – Ron Maupin Dec 17 '18 at 20:03
  • This is true, but also we're talking about a gigabit connection over only 30 meters. An unshielded cable should be able to at least connect and operate at gigabit speeds using cat5e stanards unless that cable is passing through a factory floor or something. – Spooler Dec 17 '18 at 20:06
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    @Spooler, not properly grounding the shield on shielded cable can degrade performance to below the Category-5e performance necessary to run 1000BASE-T. – Ron Maupin Dec 17 '18 at 20:10
  • Yeah, I guess not grounding it would make it into a big noise generator. I hereby concede the point. – Spooler Dec 17 '18 at 20:13
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    Such a lot of squitter over a squittery $20 switch. – user9517 Dec 17 '18 at 20:43
  • @drookie We bought a Netgear GS198Tv2 managed gigabit switch. I've never used a managed switch before. Can you point towards what information I need to troubleshoot this further? – just.another.programmer Dec 24 '18 at 14:11
  • With factory defaults every managed switch acts just like as unmanaged - all interfaces are in default 1st vlan. So you just plug it it, connect and access its web/cli interface. This is a good place to start: https://kb.netgear.com/31261/How-do-I-access-the-admin-page-of-my-ProSAFE-Web-Managed-Plus-or-Click-Switch . Also the default credentiuals are admin/1234. Plug the cables in and check the status of the inter-building link, that's your first quest. – drookie Dec 24 '18 at 14:40

4 Answers4

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Gigabit requires all 4 pairs, and 10/100 only requires 2 pairs. So "testing" it with the 10/100 switches doesn't really verify that all pairs of the cable are good.

For something like this, you will want to use a cable certifier, not a plain tester that just tests continuity.

30m really isn't that long of a run. If they are newer switches that support low powered Ethernet standards turn off Green Ethernet/EEE/whatever to see if this helps. If for whatever reason they are detecting the run as shorter than it is they might not be pushing enough power on the ports.

Is there any high voltage power running through the same conduit? Power and data should be at minimum 2" apart to avoid EMI (electromagnetic interference).

Jeter-work
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Brian D.
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  • on the EMI front, watch out of office fluorescent lighting tubes, has cause me problems with both long runs of GIG-E (baseband layer 1 ) and ADSL (broadband layer 1) – The Unix Janitor Dec 30 '18 at 12:58
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It seems likely that the (consumer grade) switch devices are not suitable for the job in hand. You should use better switches - it's the only way to be sure.

user9517
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I think the comment from @DavidSchwartz is the most likely culprit. That is, there really is no such thing as crossover when using gigabit. If you use a cable that has it's tx and rx crossed, it will simply not function when using gigabit. You need to use straight "normal" cables for everything gigabit and beyond (at least in the scope of Base-T).

UPDATE: I've never seen this in practice, but I don't usually go around trying to use crossover cables with gigabit. There is supposed to be a standard for handling this kind of scenario cleanly. From 1000BASE-T onwards the physical medium attachment sublayer (PMA) provides identification of each pair and usually continues to work even over cable where the pairs are unusually swapped or crossed. Beware, a "cheap" switch may not implement this correctly.

Also, watch out for auto-negotiation. Your question indicates that you've performed this upgrade "asymmetrically" -- that is, one of your sites was running a 10/100/1000 switch paired to a 10/100 switch. I would wager that auto-negotiation has been disabled on the "existing" 10/100/1000 switch to make it compatible with the 10/100. Gigabit must auto-negotiate, so be sure that there are no manual speed settings for that port on either 10/100/1000 switch.

Spooler
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  • If it was managed switches, you could force both ports in question to gigabit. – Jeter-work Dec 17 '18 at 20:55
  • Hm. They're unmanaged. I didn't actually think about that. You can't force a certain speed on an unmanaged switch, so my suspicion of having done so to force 10/100 is out the window.. However, you should never force a switch to use gigabit. Auto-negotiation is a required part of the gigabit spec. – Spooler Dec 17 '18 at 20:59
  • Well, I'm a lowly sysadmin, our network admins had to configure a connection (connects my lab network to the parent network) to static/forced 1GB connection in order to restore connectivity when the parent organization replaced their switch. – Jeter-work Dec 18 '18 at 15:56
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If it worked for a shorter cable, I'm thinking it might be a signal issue over distance with the new switches. You say you plugged it into a computer using the new switch and it worked, so i'm thinking that those new switches are getting a weaker signal. if you have a multimeter you can test this, or hook the other end back up to the computer and do a throughput test by copying a large file. If the bandwidth is poor it's likely due to the signal being weak and TCP is having to keep asking packets to be resent.

SteamerJ
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