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I have a small and inexpensive ethernet switch which is giving me some headache. For some reason some devices only work when I plug them into specific ports. The ports are not broken, as other devices work on the same ports. Also, if I just take the cable from one of the ports that doesn't work and plug it into another port, it works fine.

When I say the devices don't work, they actually show activity on the LEDs such as LINK and ACT, but I can't ping them, but I have no explanation. I also tried to restart the switch, computer, flush ARP cache etc.

I read that some switches have automatic MAC address aging and learning. Could that be related?

They reason why I am asking is because soon I will undergo a massive operation where we have to connect hundreds of these devices to the network, and we plan to buy 48 port switches for this purpose. If they start playing strange games like this, I am absolutely unsure what is going on.

Could this be a problem with the switch, is this expected, or do you believe it is rather a computer problem or router problem (the switch is connected to a router), but I am not using a gateway on these devices or the computer (and as said, as soon as I plugin the devices on a different port, they start working).

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  • What uptime does that switch has? It might be stuck and stop learning new MAC addresses. I would try to restart it and redo the tests. – laf Jul 25 '13 at 9:22
  • does the switch have any port settings like speed/duplex set , and does all the ports settings match ? – DrBru Jul 25 '13 at 11:30
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    "inexpensive" (aka consumer) switch... I'd start by fixing that problem first. – Craig Constantine Jul 25 '13 at 14:16
  • Is this a "dumb" switch - Does it have a management interface accessible via Telnet or HTTP for example? – jwbensley Jul 25 '13 at 14:44
  • What is the other end? Some possible causes - try same hard code port parameters like duplex speed and negotiations – engineerbaz Jul 25 '13 at 19:41
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Just because some devices will link doesn't mean the switch isn't still bad. Some NICs will be more tolerant of marginal signaling and/or timing.

Personal example... I have a Nortel ERS with a damaged clock circuit (power outage killed it.) I've verified with a scope that the clock for the second half of the switch is noisy and marginally out of spec. As a result, some network cards will link, and actually function. However, most, including the other half of the switch and every other switch I have, won't. 10M works fine, because the PHYs generate their own clock. 100M only works with "loose" NICs. 1000M won't even link with itself -- that signal is pure noise.

Cheap switches are cheap for a reason. I'm not going to say how many sub-50$ "hubs" I've thrown away.

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  • I have the same issues on identical new switches, so I don't think that it's a damaged clock circuit. Sometimes only power cycling ALL attached network equipment solves the problem, and it happens no matter what is connected (also computers etc). – Alex Jul 26 '13 at 0:20
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One problem that I have experienced with cheap switches is that many don't have good Auto MDI/MDIX support. This means that the switch cannot detect the difference between crossover or straight-trough wiring and cannot adjust to the current wiring situation. The most common setup that I've seen is switches with some of each. Usually, the port marked 'uplink' is wired for crossover connections to other switches/routers, and other ports are setup for straight-through connections to client devices. I've also seen some switches that have a hardware toggle to change the wiring scheme of a port.

What I would suggest is to experiment with crossover and straight-through cables and see if that fixes the issue for those ports. In the long term, inexpensive consumer hardware is just not equipped to deal with many use cases and you may want to look into upgrading as some commenters suggested.

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  • I am using identical brand new shielded straight through cables. They all work well. – Alex Jul 26 '13 at 0:20
  • try a crossover cable – JC Hulce Jul 26 '13 at 1:31
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As your switch doesn’t support MDX so use appropriate cable type, if other end is also switch then use cross cable

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  • Also don't post duplicate , kindly refer to the Q/A section as significant time has been spend making is user friendly. – DrBru Jul 25 '13 at 20:12
  • Acknowledged.It was not deliberately. Also There is some solutions as well in my previous reply ( What is the other end? Some possible causes - try same hard code port parameters like duplex speed and negotiations) – engineerbaz Jul 26 '13 at 7:52
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"Small and inexpensive" sounds to me that it's not a managed switch - so deactivated ports or ones configured to an incompatible mode can be ruled out.

The most likely issue is the cables you're using. Some cable connectors are rather short and some ports are rather deep, so when these two come together they don't link - or rather, they link only when the connector is pushed deep into the port and held that way.

Higher quality equipment has less tolerance and works more reliably.

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