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Recently I got myself a TP-Link Archer C80 router because the default one from ISP didn't really work well for wireless. I am including my network topology diagram. Everything before the change was fine and I had internet speeds of 200mbit on all devices after which I changed the ISP router to act as a modem and plugged the new router in.

As soon as I plugged the router in, PC 1 and PC 2 dropped down to 100mbit while PC3 had full 200mbit. After a lot of troubleshooting, I replaced cable between router 1 and PC 1 from CAT 6 to CAT 7 which fixed the PC 1 issue (even though before it was all fine on CAT 6, PC1 had speeds of 200mbit). The problem now I am having is that PC2 is still stuck on 100mbit even after replacing cables and I have no idea how to fix it.

Network topology

I tried the following;

  • Changing all the cables around and even buying new CAT 7 and replacing all of them
  • Made sure switch, router, modem and all PC's had gigabit ports (again, it worked before)
  • Updated router and switch to the latest software versions
  • Tried going directly from router 1 to PC2 (which was still stuck on 100mbit) and then PC3 (worked perfectly on 200mbit, all tested on both CAT 6 and CAT 7 cables)
  • Went back and confirmed everything working perfectly fine when I switch the modem back to router
  • Forced both router and PC2 to negotiate speed of 1000mbit rather than auto negotiate
  • Disabled NAT Boost
  • Played around QoS, tried disabling it, enabling it, setting it to very low, very high and our actual internet speed (200mbit)
  • Restarting everything
  • Tried reinstalling network drives on all PC's
  • Updated Windows
  • Contacted TP-Link and they gave me a quite extensive list of troubleshooting steps but I have done pretty much everything on it and still nothing
  • Tried different ports on router and switch. Even moved cables from PC2 to PC3 and vice verse and still PC2 is the only one with the issue.

Thing I noticed is that the router tells me that the port connected to switch 1 has 1 gigbabit connection (which seems great) but as soon as I go to switch 1, connection to PC2 switches back to 100mbit while PC3 says has full gigabit connection and when plugging PC2 directly into the router, the router says connection is 100mbit. Windows 10 also says in PC2 that it has 100mbit connection rather than gigabit. Essentially now I have faster connection on my phone (200mbit) than PC2 (100mbit).

Any suggestions?

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    Did you buy pre-terminated cables, or did you do all of it yourself? Are wall jacks involved or is it always a direct connection? (Cat5e is usually enough for gigabit, so needing Cat7 seems slightly odd to me.) What kind of switch do you have? Does it have any configurable power-saving features? (Ethernet speed negotiation is only done between two ends of the cable -- if PC2's Ethernet port negotiates 100 Mbps, that's entirely between the PC and the switch, nothing to do with the router at all.)
    – grawity_u1686
    Commented Feb 21, 2021 at 13:51
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    First thing I would check: Install iperf3 network test on all PCs and test connections PC1<->PC2 and PC2 <->PC3 via iperf3 so you can see if the network connection between router and PC is working properly. Afterwards you can check the NAT performance of the router to the Internet.
    – Robert
    Commented Feb 21, 2021 at 13:55
  • 1)So the issue is only PC2 at this point right? Are you 100% sure it had 200mbit before? 2) You're sure switch has GB ports right? Maybe share the model. I verified new router does
    – gregg
    Commented Feb 21, 2021 at 14:10
  • @user1686 Yeah it was all bought brand new which is why it was so weird. Robert thanks for the program! Its really useful actually. and gregg yeah It definitely had 200mbit before. However turns out for some reason configuration was not properly configured so reinstalling the drivers second time helped! Thanks to all of you!
    – m1cker
    Commented Feb 21, 2021 at 15:45

1 Answer 1

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So if anyone stumbles on this question, I managed to find the answer (tried doing it for 3 days, posted the question here and nearly immediately got it).

So I used IPerf3 to confirm that connection between my two working PC's reached 1gigabit (or around 800mbit but still fantastic) while connection to PC2 were stuck in the 90's mbit so it must have been PC's fault. Before reinstalling Windows, I went ahead and removed the network adaptor drives in the device manager and restarted the PC (for the second time).

It seems like some configuration must have been kept since I have done it the first time but resetting it second time did the job and the PC booted up with gigabit connection.

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