Question Internet cuts out on one PC when another boots, wakes from sleep or shuts down

Mar 23, 2019
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Hello!

I'm having a very strange issue and haven't found a solution to my problem yet so I would like to ask for some help.

First of all my networking setup is the following:
  • I have a Sagemcom F@ST 3686 cable modem from my ISP with default settings except I've changed the login details.
  • I have two PCs, an IPTV Box and two mobile phones connected to the cable modem. The two PCs and the IPTV box are connected to the modem via separate ethernet cables directly and the two phones are connected via WiFi.
  • I have no extra modem or router installed in my network.
The two PCs are the following:

PC 1.:
Lenovo ThinkCentre M58e running Windows 7 with the latest updates and drivers, including the latest Marvell Yukon 88E8057 PCI-E Gigabit Ethernet Controller.
PC 2.: A Desktop build with a Gigabyte GA-B85M-D3H mobo, and Realtek GbE LAN chip. This PC is running Windows 10 with the latest updates and all the drivers are up to date.

Now my problem is the following:

The whole thing started sometime in February, I can't pinpoint the date. Previously I had another desktop instead of the Lenovo PC and haven't experienced this problem but it got outdated so it was replaced with the Lenovo in December. Every time PC 1. (the Lenovo) boots up, wakes up from sleep or shuts down I lose network connection on PC 2. (the custom desktop) for 20-60 seconds and get the red X on the network icon in the taskbar. I've tried using the Windows troubleshooter to see if it could fix the problem but it said the generic message that "one of the ethernet cables aren't plugged in". When I lose the connection on PC 2. I can't even reach the admin page of the cable modem. Seemingly this doesn't affect the IPTV box as it merrily continues to play the TV broadcast but it might not be noticeable due to the IPTV buffering in advance. I haven't noticed the internet dropping on either the phones but I will test this later. Both PCs are virus free, I've checked them with Malwarebytes and Eset Antivirus. So far I've checked the ethernet cables, all are properly plugged in both in the PCs and in the cable modem. I've tried turning off the Power Saving for LAN on PC 1. (the Lenovo) but I couldn't fix the problem. I've tried resetting the cable modem to factory settings but that didn't help either. Both PCs are using the DHCP server of the cable modem so it shouldn't be an IP addres conflict.

Can somebody help me diagnose and solve this issue please as it's driving me mad.

Edit: added that connection is lost on PC 2. when PC 1. is turned off too.
 
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Ralston18

Titan
Moderator
You mentioned that you checked that the Ethernet cables were properly plugged in. Well and good. However, try swapping the Ethernet cables between PC 1 and PC 2. Determine if the problem follows the cable or stays with the PC.

What about the IP addresses on the other devices: i.e., the IPTV box and the two mobile phones?

How are they configured? What IP addresses are they using?

On both PC's, run "ipconfig/all" (without quotes) via the Command prompt.

Compare the results and likewise look at the network configuration settings in the Sagecom cable modem. Check the mobile phones as well.
 
Mar 23, 2019
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You mentioned that you checked that the Ethernet cables were properly plugged in. Well and good. However, try swapping the Ethernet cables between PC 1 and PC 2. Determine if the problem follows the cable or stays with the PC.
I've swapped the two PCs and started monitoring the connection on PC 2. The issue remained the same. The moment PC 1. booted or woke up from sleep PC 2. lost network connection for about 20 seconds.

What about the IP addresses on the other devices: i.e., the IPTV box and the two mobile phones?

How are they configured? What IP addresses are they using?
Both the mobiles and the IPTV box are using the same DHCP server of the Sagemcom cable modem and all of them have a different one. All IP adresses expire in one hour as default except the IPTV one. That one shows the begining of unix time as the expiration date so I think it's permanent as the DHCP server never offers a new IP address to it nor is the one it currently uses is renewed. I don't have an admin access to the IPTV box as it's locked down by the ISP.

On both PC's, run "ipconfig/all" (without quotes) via the Command prompt.

Compare the results and likewise look at the network configuration settings in the Sagecom cable modem. Check the mobile phones as well.
The "ipconfig/all" command results and the Sagemcom settings are matching on both PCs. What I've seen on the phones their settings are matching the settings on the cable modem as well.
 
Mar 23, 2019
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ipconfig /all shouldn't match exactly. The IP address for each host should be unique.
I meant that the the settings on Sagemcom for PC 1. is the same as on PC 1. when I use the ipconfig /all command, and the settings on Sagemcom for PC 2. is the same as on PC 2. when I use the ipconfig /all command.

Ipconfig /all is different between PC 1. and PC 2. They have different MAC addresses, IPv4 and IPv6 addresses.

Edit: So after messing a bit more with the two PCs I've noticed that I also lose connection on PC 2. when PC 1. gets turned off.
 
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The red x generally means a hardware type of issue. If you were to just lose internet you get the yellow exclamation point. Maybe the pc event viewer has a code that will give you more idea into what the failure it but many times it just tells you what you already know.

This is very strange because networks are actually designed to prevent devices from having impact on each other.

My first concern would be that the router is failing since your pc do not directly connect. Not sure how you are going to test this. If the ipconfig has a disconnected status when you get this failure then the hardware thinks the physical port is down.

If the port is not actually disconnected you could try static ip addresses but the failure you describe does not make me think you have a duplicate ip issue but there really is little harm in using static ip on desktop machines that are not being moved to different networks all the time.

You might be able to get into the router via a wifi connection and maybe see if it thinks the lan ports are down. Not sure on your router how much status information it gives on lan ports.
 
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Mar 23, 2019
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The red x generally means a hardware type of issue. If you were to just lose internet you get the yellow exclamation point. Maybe the pc event viewer has a code that will give you more idea into what the failure it but many times it just tells you what you already know.
I've checked the event viewer but it has no warning, error or information log at the time PC 2. gets the red X on the network icon.

This is very strange because networks are actually designed to prevent devices from having impact on each other.

My first concern would be that the router is failing since your pc do not directly connect. Not sure how you are going to test this. If the ipconfig has a disconnected status when you get this failure then the hardware thinks the physical port is down.
The cable modem is about 1,5 years old. Of course there is a chance that it has some issues, but I can't prove it to my ISP since I don't have an access to another cable modem to see if I could reproduce the issue on that one.

If the port is not actually disconnected you could try static ip addresses but the failure you describe does not make me think you have a duplicate ip issue but there really is little harm in using static ip on desktop machines that are not being moved to different networks all the time.

You might be able to get into the router via a wifi connection and maybe see if it thinks the lan ports are down. Not sure on your router how much status information it gives on lan ports.
I've set the DHCP server up so that PC 1. gets 192.168.0.37 and PC 2. gets 192.168.0.48 every time so they should never have duplicate IPs.

I've tried setting up static IPs on the PCs and I could do it on the phones too, but the DHCP server of the modem overrides them. If I turn the DHCP off then the IPTV box isn't going to get a new IP whenever it needs one and I can't set it up to use a fix IP as the admin access is locked down by the ISP.

I've also tried freeing up the IP of PC 1. and PC 2. in the admin menu of the cable modem, that way the modem says that they have static IP addresses but it lasts only until the PC is restarted, after that the DHCP server offers an IP for the one which was restarted.

Snooping around in the admin menu of the Sagemcom I've noticed in the modem log that PC 2. gets offered the same IP address it's already using the moment PC 1. turns on, turns off or wakes up from sleep. I don't know if this has any importance because other times it's offered the same IP and those times I don't lose connection on PC 2.
 
I would hard code the ip on the pc that is having issues and make sure it outside the dhcp range of the router. Normally a router allocates from the bottom so something link 192.168.0.235 will be safe even with dhcp on since it should never get that high. Best if it is in the excluded range but I have seen some routers that do not have that option.

The pc should completely ignore any dhcp with a static ip.
 
Mar 23, 2019
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I would hard code the ip on the pc that is having issues and make sure it outside the dhcp range of the router. Normally a router allocates from the bottom so something link 192.168.0.235 will be safe even with dhcp on since it should never get that high. Best if it is in the excluded range but I have seen some routers that do not have that option.

The pc should completely ignore any dhcp with a static ip.
I've set up a static IP on PC 1. and PC 2. I will test the results soon.

I've also encountered another lost connection before setting up the static IP on PC 2. and I was fast enough to do an ipconfig /all whine I had no connection. I've got the following results, not sure if it could help:
 
Mar 23, 2019
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Disconnect is very strange it means it is down at a hardware level
Which hardware could it be though, the ethernet port on the mobo or the cable modem? What I find strange is that when PC 1. is turned off I have no issues with the connection on PC 2.

I have set up fix IPv4 addresses on both PCs but I still get the problem.

Could I get disconnections like these if the IPv6 addresses are matching?
 
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That is what I expected to a point. I am betting the port is electrically going down as if you unplugged it from the router. I can't think of a software signal that would tell the pc to turn off its port. Some pc have power save modes etc but I can''t see that being triggered externally.

See if the event viewer give you any clues. You should see something even if it is not real useful.

If you have a switch or even another old router see if place that in between. A switch will be simpler but a lan ports on a router are a switch.

If it is some issue with your PC whatever signal is causing the port to go down should still be cause it to do down. A switch in general is completely transparent. If it is a issue with the router the port to the pc will stay active and the port to the switch should drop. If it happens very quickly you may not see the light on the switch go out but the pc should not see the outage. No traffic will pass but the pc port should not go down. You should not see any indication in the event viewer.

It is almost like the switch chip in the router is resetting for some reason.
 
Mar 23, 2019
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That is what I expected to a point. I am betting the port is electrically going down as if you unplugged it from the router. I can't think of a software signal that would tell the pc to turn off its port. Some pc have power save modes etc but I can''t see that being triggered externally.

See if the event viewer give you any clues. You should see something even if it is not real useful.
I was digging through the event viewer on PC 2. and I've found some things. Looks like the issue started on the 20th of February as there are no logs like these before it. I'm not sure this helps or not, if you need translation of the event details too I can provide it tomorrow, just let me know.

LevelDateSourceEvent IDTask category
Information2019.03.25. 12:47:07NetworkProfile10001None
Information2019.03.25. 12:47:09NetworkProfile4004None
Information2019.03.25. 12:47:23NetworkProfile20002None
Information2019.03.25. 12:47:23NetworkProfile4001Waiting for identification
Information2019.03.25. 12:47:24NetworkProfile10000None
Information2019.03.25. 12:47:24NetworkProfile4002Waiting for identification
Information2019.03.25. 12:47:24NetworkProfile20002None
Information2019.03.25. 12:47:24NetworkProfile10000None
Information2019.03.25. 12:47:25NetworkProfile4004None


As an extra I'm providing a screenshot of the Pingplotter tracing from the above time period, maybe it will say something that I can't see (first hop is the cable modem, the second one is the CMTS of the ISP):


If you have a switch or even another old router see if place that in between. A switch will be simpler but a lan ports on a router are a switch.

If it is some issue with your PC whatever signal is causing the port to go down should still be cause it to do down. A switch in general is completely transparent. If it is a issue with the router the port to the pc will stay active and the port to the switch should drop. If it happens very quickly you may not see the light on the switch go out but the pc should not see the outage. No traffic will pass but the pc port should not go down. You should not see any indication in the event viewer.

It is almost like the switch chip in the router is resetting for some reason.
Sadly I don't have an acces to a switch or a router currently so I can't test this for now.
 
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I am no expert at the event codes. I know the 10000 means connect and 10001 means disconnect. It does not say why it disconnected though which does little good since you already know that fact.

You have me stumped on this one. Maybe we get lucky and someone else see this thread that knows more.
 

kanewolf

Titan
Moderator
I would hard code the ip on the pc that is having issues and make sure it outside the dhcp range of the router. Normally a router allocates from the bottom so something link 192.168.0.235 will be safe even with dhcp on since it should never get that high. Best if it is in the excluded range but I have seen some routers that do not have that option.

The pc should completely ignore any dhcp with a static ip.
I know it is a long shot, but if the PC has the MAC address overridden, that would cause BIG problems.

I think you will have to get wireshark running on one or both PCs and see the the traffic that does happen.
 
Mar 23, 2019
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I know it is a long shot, but if the PC has the MAC address overridden, that would cause BIG problems.

I think you will have to get wireshark running on one or both PCs and see the the traffic that does happen.
I haven't modified the MAC addresses of my devices so that should be out of question.

Never used Wireshark before, I will check out some guides for it. Do you have any tips what to look out for which could help identifying the issue?
 
You need to get lucky and see something different. I would unplug the cable and plug it back in and try to compare that with one of these failures.

Be very sure to examine all the mac addresses as well as the ip addresses in the packets to be sure they match.

Your problem is very strange because the port is actually going all the way down. It is very different than some dhcp or ip conflict.

Wireshark is the sledge hammer to solve problems like this. It will take some practice since it captures so much data and in this case you can't really use filters since you don't know what you are looking for.
 

kanewolf

Titan
Moderator
You need to get lucky and see something different. I would unplug the cable and plug it back in and try to compare that with one of these failures.

Be very sure to examine all the mac addresses as well as the ip addresses in the packets to be sure they match.

Your problem is very strange because the port is actually going all the way down. It is very different than some dhcp or ip conflict.

Wireshark is the sledge hammer to solve problems like this. It will take some practice since it captures so much data and in this case you can't really use filters since you don't know what you are looking for.
Yeah, I agree, wireshark is a 12GA shotgun, but you and I are out of 22 cal ideas :)
 
Mar 23, 2019
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So I've started running Wireshark alongside Pingplotter on PC 2.
Went down to the cable modem, set up my phone recording the ethernet ports on it to see if the leds turn off on it, then turned PC 1. on, waited about 5 mintes then turned it off. The only leds turned off were for the port of PC 1. for about 10-15 seconds.
Came back to PC 2. and neither Pingplotter or the event viewer log showed disconnections. The only thing changed on PC 2. was that I've started Wireshark.
 
Mar 23, 2019
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I've successfully reproduced the issue once again while running Wireshark on PC 2.
I couldn't see anything suspicious or things similar to the results with unplugged ethernet cable. From the Wireshark logs it looks like PC 2. loses connection for a moment and starts reconnetcing to the internet again.
 
That likely means that the router is dropping the connection but I have no idea how or why it would do that. The only other option is the hardware port in the pc has some strange issue that is not software controlled so the OS does not see it.

I doubt the router has any indication maybe you get lucky and it has a log. Since this device has a cable modem you can not generally upgrade firmware which is the fairly standard recommendation when routers are being bad. The ISP pushes any firmware updates to devices like this on whatever their schedule is.
 
Mar 23, 2019
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That likely means that the router is dropping the connection but I have no idea how or why it would do that. The only other option is the hardware port in the pc has some strange issue that is not software controlled so the OS does not see it.

I doubt the router has any indication maybe you get lucky and it has a log. Since this device has a cable modem you can not generally upgrade firmware which is the fairly standard recommendation when routers are being bad. The ISP pushes any firmware updates to devices like this on whatever their schedule is.
Sadly the hardware log link is removed in the modem's UI probably by the ISP and I get a time out when I try to use an URL to see it. MS released an update for Windows 7 but I highly doubt that will fix my issue as I couldn't find anybody else with this problem before and not even sure if Windows is the one to blame.

Looks like we've hit a brick wall here. I will try to grab a PCI-Express Network Adapter and plug it in PC 2. for testing purposes to see if the issue still persist with that. In the meantime while it arrives if anybody has some ideas that I could try, please let me know.
 

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