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I'm dealing with some weird networking issues at a workstation in a large warehouse. There is a preexisting drop that is being used for a machine.

I found the other end of the wire by using a tone generator. But when I connected the light tester, I received no lights. I returned to the workstation and found that when I plugged the network cable back into the workstation but did NOT plug it into the switch, I was still getting a link light.

This means there has to be another switch somewhere in the middle, right? Or am I missing something? Is there any way to do some type of trace to see how devices are connected?

Massimo
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Cade
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  • You need to get the cable ends properly labeled. The National Electric Code requires that low-voltage cables, including UTP network cables, be properly labeled and terminated on both ends and ready for use, otherwise it is considered abandoned cable and must be removed. Many jurisdiction adopt the NEC, and can red-tag the building, preventing occupation as well as levy fines until the problem is corrected. – Ron Maupin Dec 23 '15 at 01:17

2 Answers2

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Switches (and similar devices) are completely transparent to the network layer; there's just no way to discover the physical topology of your network from a single endpoint without actually following the cables; there could be any mix of switches, hubs, transceivers and so on between two devices, but as long as they are on the same IP subnet, the software running on them will be completely oblivious to how they are physically connected.

The only exception would be if the network devices are using some system (such as Cisco's CDP) to build and maintain a network map which you could then query using appropriate tools; but even this would not tell you if there is a cheap 5-ports switch between a wall socket and your core switch.

In your scenario, it's actually very likely there is something between your workstation and the core switch, because this would explain why your workstation appears to still be connected even when you disconnect the corresponding cable from the core switch; this could be either a small hub/switch, or even a couple of transceivers which move the like to fiber and back; there are actual use cases for this, such as connecting devices across bigger distances than what copper-based Ethernet could allow.

Massimo
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  • Also worth mentioning, the non-proprietary version of CDP (Cisco Discovery Protocol) is LLDP or (Link Layer Discovery Protocol) which most managed switches support. I only mention this because, although Massimo is correct to say that a cheap 5-port switch would probably not respond to either of those protocols, you would have a better chance that "no-name switch XXD" would respond to a LLDP query than CDP. – Get-HomeByFiveOClock Dec 22 '15 at 21:28
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How loud was the sound from the tone generator? I've seen them crosstalk when running next to other cabling for long enough. It's possible the two ends are different cables but close enough for tone interference.

You are correct - you shouldn't see lights unless the cable is plugged in on both sides.

Neil
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