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Existence of switch disturbs network: cable is fine straight to laptop but not through a switch

za flag
Gulyás Mihály Iskola Csorvási

This will be a bit complicated to read, but I hope it will be clear. English is not my first language, sorry.

In our school we have an HP 2530-24G switch connected to an unmanaged tp-link tl-sf1005d switch. We'll call the latter Switch 6. From this switch, the network is expanded through similar unmanaged switches to two directions: each branch goes through classrooms. A PC in a room is connected so that we have a cable coming into the room, going into a tp-link switch which connects the PC and we have another cable which goes out of the switch to connect to the next classroom. So the switches are linked into each other; there's a switch in each classroom.

Now, when I got here I had it working well. Suddenly, there were no network in branch 1. As it turned out, the cable from Switch 6 to the first switch of branch 1 was faulty but I replaced the ethernet connectors and it's now good. When I plug it in to a laptop, it works fine. But, if tehere's a switch between the cable and the laptop, there's no network beyond the switch (because the laptop shows a connection but I cannot ping other devices on the network). I tried 3 different switches, many patch cables, with no luck. I connected two PCs to the switch and they could ping each other.

The interesting thing is, when I connect the PC through a switch, there is a connection for a few seconds. Just enough to load a webpage or ping something, then it goes away. Another mystery is that if a switch is connected, it disturbs the network in Switch 6 and the other branch. Straight or crosslink shouldn't be an issue because the tp-link tl-sf1005d is auto-mdix as far as I know.

If anyone has a clue on what causes this, I'd appreciate if they could share it... Thanks for your time.

Endre

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in flag
NiKiZe
Your setup means that if one switch dies then that whole segment is gone. Best is to use a star, that way you have one main switch with as many other switches as possible connected to that one switch. Have you checked link speed? there is a difference between in which pairs is used in 10/100Mbps and 1000Mbps links. It might also be that you have created a network loop somewhere, and one of the switches detects this and disconnects the link.
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za flag
Gulyás Mihály Iskola Csorvási
Tanks for your reply. Well I know it's far from ideal... it's what I have to work with as the cables go through walls and it would be enormous work with the maintenance involved. I haven't checked speed. Do you mean the cable? The tester shows each wire as good. I also thought of a loop but I havent't discovered any yet.
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Larryc avatar
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Larryc
Yes, it sounds like a loop. Check your wiring. It could be a case where you have a device wired to the switch that also has a wireless connection established.
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za flag
Gulyás Mihály Iskola Csorvási
@Larryc AFAIK devices can't connect wirelessly to the network. When I tried the switch, there were no connections except the cable coming in, and the PC. connecting with a patch.
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Zac67 avatar Server
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Zac67

As you've already noticed, chaining Ethernet switches is not a good idea. There may be limitations in the cabling but even that should be corrected some day.

Generally, Ethernet switches should be arranged in a tree (or multi-star if you like): a central switch and all other switches connected to that central switch. This topology has two main advantages: if any switch but the central one dies, just a small part of your network is down (same goes for the cabling itself), and each switch gets the full bandwidth of its uplink port (rather than sharing that across a chain).

Connecting two switches to each other only works if at least one of them supports Auto MDI-X, otherwise a crossover cable is required. Auto MDI-X is pretty much standard today but not ubiquitous. It seems to be supported by the TL-SF1005D, however[*1]. On the HP 2530-24G it's active by default but can be deactivated. Check whether the port LEDs light up when plugging the cable. If they don't light up a crossover is likely required.

Another possibility for connectivity loss is a network loop. I'm not sure that can be ruled out from your description, but if more than the connection at hand is impacted it's almost certainly a loop - do you see the traffic LED go solid?

The 2530 supports both spanning tree protocol RSTP/MSTP and loop detection - both are off by default. RSTP/MSTP logically blocks a detected loop, you won't see the port lights go off! Loop detection deactivates the port altogether for a configured time period and the port lights will go dark. Check the 2530's config to make sure (show running-config). Also check the 2530's log for any abnormalities (log -r).

[*1] You can quickly check whether a switch supports Auto MDI-X: disconnect everything(!) and then plug a standard patch cable between any two ports. If the LEDs light up, Auto MDI-X is used.

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