Question PC loses connection when mac connected / put to sleep

jalundan

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Nov 6, 2015
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I have a problem that is basically in the topic.

My setup is:
Network comes to my house, and I have VDSL modem as a first point. It is located in the electrical center of the house and fromt here there are RJ45 connections built in to the walls to every room. Those are connected to the modem. Modem is in NAT mode.

I have a PC and Mac in my work room. Mac is for work and PC is my personal. Both are connected via RJ45 cable to a switch and that switch is connected via RJ45 to the wall.
The problem is that after I stop working and put the mac to sleep the PC loses its cable connection. Then if I disconnect mac the PC immediately gets its cable connection back.
If I put mac on and take out the cable in mac and put the cable back on while both machines are on, then both connections work. With Wifi either work fine in every situation.

I have bought a new switch and that did not help. I also have reinstalled windows so it should not be driver issue. I have also resetted the modem in electrical center.
I am running out of options here. Could it be defective modem? I doubt that since the network is working simultaneously at some cases, so the port and NAT should work ok.
And this all started happening few days ago. And I have no clue what caused this. This setup has been working for over a year.


Mac is 2018 macbook pro with external dongle. PC has Gigabyte Aorus Pro Wifi Z390 (intel version) and it is connected to the intel LAN port on the motherboard. I have tried 3 different drivers for the motherboard LAN, updated also bios, etc.

EDIT: I have also tried the cables in the othe machine (using macs on PC and vice versa)
 
Last edited:

Lutfij

Titan
Moderator
By reinstalling the OS, did you recreate you bootable USB installer? Make sure that your drivers are up to date. You can try installing the drivers for your NIC in an elevated command, i.e, Right click installer>Run as Administrator.

Make and model of your modem? Did it come from the ISP?
 

jalundan

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Nov 6, 2015
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Make is Inteno and model is DG200-AC.

I don't understand the bootable USB meaning. I used my authentic windows 10 DVD and authentic cd-key. Tried installing latest drivers straight from Intel and also 2 different older versions. All have the same effect to functionality.
I also don't see the point on installing the drivers in the administrator mode, since the connection works always until there is lan cable connected to a different machine. Is there something I am missing in here?
 
Very few people install via optical media anymore I think is why the USB question.

What happens if you just unplug the mac from the switch rather than putting it to sleep.

This has to be some kind of IP address conflict.

Use the IPCONFIG /all command and see if anything changes when it works and when the mac breaks it. You also want to use the ARP -a command and see if the mac address that is mapped to IP is different. What I suspect is the mac address is somehow hijacking the router IP.
 

jalundan

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Nov 6, 2015
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Very few people install via optical media anymore I think is why the USB question.

What happens if you just unplug the mac from the switch rather than putting it to sleep.

This has to be some kind of IP address conflict.

Use the IPCONFIG /all command and see if anything changes when it works and when the mac breaks it. You also want to use the ARP -a command and see if the mac address that is mapped to IP is different. What I suspect is the mac address is somehow hijacking the router IP.
Everything works on PC if I unplug the mac. Either from the switch or from the mac.
I also tried setting static IP on the windows but same thing happens.
I also tried with external network card but same thing happens (this gets different IP than the one on mobo ofc).
I also ran the ARP -a command and compared to the modem configuration page and those are the same in there.

I noticed that there is listed all LAN connections on the modem configuration page, but when the issue happens the page shows "No current LAN connections". So it actually vanishes everything, not just my PC.

Could this be really a defective modem? All other hardware is changed already (except on the mac).
 
It is not a hardware thing. The ports on the switch in a way create a separate connection between he router and the end devices. It should not matter if the port is up or not or what speed it runs. The ports on the switch should have no impact on each other.

The network is actually designed to prevent this

So a basic overview of how this is support to work.

Each device including the router has a IP address but the IP address is only partially used for communication on a local network. What happens is the PC will look up the hardware address (mac address) of the IP. After that point it will send the traffic to the mac address. The router also does this so it will have a entry for each of the 2 devices on your network.

The switch is really stupid it has no concept of IP addresses. What it does it keep track of what mac address is on what port.

So it will for example say port 1 has max address xxxxx port 2 has mac address yyyy and port 3 has mac address zzz. It is not any configuration it just sees what is the sending mac address.

So say the router is on port 1 and your pc is on port 2. Your communication should flow between those ports and not care about any other port traffic.

So it is actually impossible for your problem to occur.

Because it does occur something, like the mac, is violating some rule either intentionally or because of malware or a bug. Although you find some kids run a attack called ARP poison in their house to try to get more bandwidth for themselves there really is no valid reason to change anything.

What you have to determine is what is contaminating the flow.

My best guess would be that the mac is stealing the router IP address and giving you its mac address rather than the routers mac address (this is what arp poison does). If you have vpn software on the mac and it does not cleanly disable itself you can see this problem.

In any case you need to carefully watch the ARP tables and see if any of these values change. Now it could be the mac address of your pc in the router that is being compromised but that is almost impossible to see on consumer grade routers. Some tiny handful let you display the ARP table.
 

jalundan

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Nov 6, 2015
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It seems that this is now solved. What I did was that I shut down all, removed all cables and all machines from the bottom to top. Then started first modem and let it to fully boot, then adding switch, pc and then mac and then rest of the network. Now it seems to work which.
 

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