Question Connection lost when I plug in a 2nd LAN cable in the router (but then, the 2nd connection works fine)

PlanK69

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Jan 12, 2014
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Hi. So I've been an IT tech for 3 years but I'm currently experiencing something that I've never seen before.

Here's a diagram of my setup at home: View: https://imgur.com/pddDYBg

Here's a 3min video that I recorded of the problem:
View: https://youtu.be/DKmVTxh2KJk


My ISP's 'dish' (NanoBeam 5AC Gen2) is on my roof and a LAN cable runs from there into the ISP's microfilter, and then a short LAN goes from there into my router. My router serves as the DHCP but it does so automatically (meaning I could factory reset my router, and I'd still have internet as the ISP's 'dish' (NanoBeam 5AC Gen2) actually does all the difficult stuff.. anyways, I then have a LAN cable running from my router into my PC, and another LAN cable running into another router in another part of my home, and THAT router has multiple mobile devices connected to it as well as a wired connection to my Xbox. All wifi connected devices on both routers have no problem at all.

So, here's the problem that literally started today (It had been running 100% fine this way for over 4months, with both routers having multiple devices connected to them and not having any problems of conflicts). My PC now gets disconnected as soon as I try to connect to my router with a LAN cable, yet the other router still works fine... BUT, if I plug the other router's LAN cable out then my PC's connection works 100% again. And then as soon as I plug the LAN cable to the other router in again, then my PC gets disconnected and the other router works again.

So it's like the other router's LAN cable is getting priority, and my PC's LAN cable immediately gets kicked when I plug the other one in. I have tried multiple LAN cables into all devices, with multiple different ports on the router, and even plugged my laptop in with my PC's LAN cable, yet the same thing happens to my laptop then... no connection on the laptop, until I unplug the LAN cable that goes to my other router. I have also changed the static IP of my PC to 192.168.0.200 and 192.168.0.210 and 192.168.0.150 just to make sure it's not conflicting with anything.

So... I've confirmed that it's NOT a faulty LAN cable, and it's NOT a fault port on my router. What could the cause of this be?
 

PlanK69

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Jan 12, 2014
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the link-light stays on at both the router and the PC's LAN port. Also, the 'connection status' on my PC shows 'connected but no internet access'. So my PC still knows that it's connected with a LAN cable, but it's not connected to the internet anymore once I plug the 2nd LAN cable in.
 
the link-light stays on at both the router and the PC's LAN port. Also, the 'connection status' on my PC shows 'connected but no internet access'. So my PC still knows that it's connected with a LAN cable, but it's not connected to the internet anymore once I plug the 2nd LAN cable in.
Thank you for the information. What about the the link light on the 2nd LAN cable that's being plugged in? Is it also on?
 

PlanK69

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Jan 12, 2014
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yeah that is also on (on my router, and on the other router to which the cable goes to). as soon as I plug the 2nd LAN cable in, then the link-light flashes like a mofo on that 2nd cable (obviously showing data being transmitted), and then the flashing stops on the 1st LAN cable that goes to my PC (showing that no data is being transmitted.. but the light stays on still, it just doesn't flash)
 
yeah that is also on (on my router, and on the other router to which the cable goes to). as soon as I plug the 2nd LAN cable in, then the link-light flashes like a mofo on that 2nd cable (obviously showing data being transmitted), and then the flashing stops on the 1st LAN cable that goes to my PC (showing that no data is being transmitted.. but the light stays on still, it just doesn't flash)
Thank you for the additional information--it definitely sheds some light on some things.

I suspect a duplicate IP or something of that nature (network loop?). Do you have another system that can ping -t your computer's IP?
 

PlanK69

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Jan 12, 2014
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I've thought about it being a duplicate IP maybe, but I changed my PC's IP from auto-obtain to static with various different IPs ending in .200 & .150 & .210 just to make sure that it's not a duplicate IP problem, yet the same thing still happens. And I know for certain that there's no devices with those IPs on the network since I did a local IP scan beforehand ( View: https://imgur.com/rGxpE46
) and those IPs are indeed available and I only have like 8 or so devices (phones, PCs, Xbox) connected on the network at any one time.

So as you can see by the screenshot, I'm 100% certain that my computer's IP is not taken or occupied.
 
Did you run this ping scan when it was "broken". Is it only the internet that does not work or will nothing respond to ping.

I assume you are running the second router as a AP ?. What IP address did you set the lan to on the second router.
 

PlanK69

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Jan 12, 2014
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The second router is auto-obtain, so I just factory-resetted it a couple of months ago, plugged the LAN cable into it from my 1st router, and it worked (I just put a password on the wifi for the 2nd router, that's all the settings I changed on it).

And I ran the IP scan from a laptop that's connected to the 1st router via wifi. And yeah, I ran the IP scan while it was 'broken' with no internet access on my PC still.
 
The second router is auto-obtain, so I just factory-resetted it a couple of months ago, plugged the LAN cable into it from my 1st router, and it worked (I just put a password on the wifi for the 2nd router, that's all the settings I changed on it).

And I ran the IP scan from a laptop that's connected to the 1st router via wifi. And yeah, I ran the IP scan while it was 'broken' with no internet access on my PC still.
If the second router is connected via the lan, then you've got 2 dhcp servers on the network which would cause some issues, but I don't think it would be like this. If you connected it to the wan and changed the subnet, then you should be okay even though the 2 networks would be separate.

I don't know the exact issue, but my hunch is that it is somewhere in the config of the 2x routers.
 

PlanK69

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Jan 12, 2014
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that's possible but it works absolutely fine for 4months+, and now all of a sudden it started doing it and it's only kicking my one machine off, and none of my other laptops/mobiles... it's super weird and I don't understand what it could be lol. But tonight when everyone is sleeping I'll factory reset my 1st router and change it from 'auto-obtain' to a static IP; I just hope it still works because my ISP's dish is actually the one that's assigning the connection rules, and my ISP can remotely log-in to their dish and by extension into my router to do diagnostics, but we'll see...
 

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