A question about placing 2 ethernet adapters on the same network

I had always been taught that you do not want to place 2 ethernet adapters in one machine on the same subnet unless you have some sort of NIC teaming software because this would cause conflicts.

However, I was testing something completely different (I'll spare you the long story), and on a Mini PC with 2 Realtek gigabit Ethernet adapters, I plugged them both into the same network. I should note that most machines on my network are 2.5 Gb, so I am able to see speed increases over normal gigabit Ethernet by transferring files to / from a machine with a 2.5 Gb adapter.

Completely unexpectedly, not only did networking work just fine with the adapters connected, but my speed actually doubled. When I copy files from a machine with a 2.5 Gb Ethernet connection, I jump from transferring about 113 MB/s to about 225 MB/s. When I look at the individual adapters, I see that each has its own IP address. So, this clearly indicates that there is some sort of load balancing operation going on here.

Documentation I am finding still suggests NOT connecting 2 adapters to the same network but I don't know if this is simply old info or something my Realtek drivers work around.

With this in mind, here are my questions:

1) Is this capability something new in Windows or is this capability provided by the Realtek drivers?

2) If this is a Windows capability without the need for special drivers, does it depend upon having 2 identical adapters or could you use a mix of adapters, for example, 1 Intel and 1 Realtek?

3) I know that newer versions of Windows 10 (2004+) and Windows 11 introduced a new driver model for Network adapters, known as "CX" drivers. Is this at all responsible for this?

Side note: This is an ultra-cheap Intel N95 based Mini PC. I was a bit surprised to be able to get this kind of performance out of it.

I'm just guessing, but it sounds like your cards/drivers are setup and supporting smb multichannel

Many moons ago, I played with a general purpose desktop PC with 2 PCIe NICs (I still have the 2 expansion cards!) and Sophos UTM software. I forgot all the details, but you may want to read the following. There may be other info on the website.

Ive had Gigabyte boards with duel Ethernet inputs. Possibly as far back as Win 8.
Never had to do anything special, always seemed faster.

I've performed a good deal of testing since yesterday, so I thought that I would just share my results.

The headline is that it seems that Windows can simply handle multiple NICs on the same LAN segment automatically. I have tried multiple systems that had 2 realtek gigabit Ethernet interfaces and connected both to the same network. In order for this test to be valid, it's necessary that the network be faster than gigabit to see if performance is actually improved by doing this. My network is a 2.5GB network. Sure enough, performance doubled. Since these were two realtek adapters I thought it possible that the realtek driver was responsible.

Next test: Still 2 NICs in this test, but I connected a USB 2.5Gb adapter to a system with a built-in GB Ethernet interface. To see if any performance increase would be observed I forced the speed of the 2.5GB adapter down to 1GB. Sure, this also worked.

Finally, for the most difficult test, I tested a system that has both an Intel and a Realtek NIC and even this worked.

In each case, I also fired up Performance Monitor and could clearly see that traffic was being sent / received on both interfaces.

Bottom line is that somewhere along the line, Microsoft snuck in this capability without telling me . This may also explain why the Interface metric seems to make no difference - it may simply be that since both interfaces will be active there is no need for one to take priority over the other.

hsehestedt said:

I've performed a good deal of testing since yesterday, so I thought that I would just share my results.

The headline is that it seems that Windows can simply handle multiple NICs on the same LAN segment automatically. I have tried multiple systems that had 2 realtek gigabit Ethernet interfaces and connected both to the same network. In order for this test to be valid, it's necessary that the network be faster than gigabit to see if performance is actually improved by doing this. My network is a 2.5GB network. Sure enough, performance doubled. Since these were two realtek adapters I thought it possible that the realtek driver was responsible.

Next test: Still 2 NICs in this test, but I connected a USB 2.5Gb adapter to a system with a built-in GB Ethernet interface. To see if any performance increase would be observed I forced the speed of the 2.5GB adapter down to 1GB. Sure, this also worked.

Finally, for the most difficult test, I tested a system that has both an Intel and a Realtek NIC and even this worked.

In each case, I also fired up Performance Monitor and could clearly see that traffic was being sent / received on both interfaces.

Bottom line is that somewhere along the line, Microsoft snuck in this capability without telling me . This may also explain why the Interface metric seems to make no difference - it may simply be that since both interfaces will be active there is no need for one to take priority over the other.

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I don't doubt the validity of your experiments, but I don't think that the answer is as simple as 'Windows does it'. In the 7-layer OSI model, Ethernet is in the second layer from the bottom, the Data Link Layer, so Windows as Operating System only controls it via network drivers > the answer thus resides in the pertinent IEEE standards on the Data Link Layer, but I'm too lazy to dig further, the days I was involved in such matters was something like 2 reincarnations ago LOL LOL

hsehestedt said:

So, this clearly indicates that there is some sort of load balancing operation going on here

Click to expand...

That's what I also concluded today!!! With or without a network bridge, Windows 11 seems to natively balance the traffic among all the adapters!!!

Haydon said:

In the 7-layer OSI model, Ethernet is in the second layer from the bottom, the Data Link Layer, so Windows as Operating System only controls it via network drivers > the answer thus resides in the pertinent IEEE standards on the Data Link Layer

Click to expand...

After reading your post, I guess my conclusion may not be correct!!!

Multiple NICs is common when it comes to server platforms, assign each NIC an IP address and as an example you would have incoming (download) traffic through one NIC and sending traffic (upload) through the other, also not uncommon to have backup traffic through a third NIC.

NOTE: The NIC Teaming feature is available only on Windows Server 2012 and later server operating systems. This feature is not available on Windows 10 or any other client operating systems. The NIC Teaming Windows PowerShell cmdlets are available on Windows 10 and other client operating systems to enable the management of NIC teams on remote computers that are running Windows Server 2016, Windows Server 2012, R2 and Windows Server 2012.

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Note

For any Windows Client editions, these NetSwitchTeam cmdlets can be used only for remote management of the Hyper-V Extensible Switch that is running on Windows Server editions. Windows Client editions do not support running the Hyper-V Extensible Switch on the client itself.

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In short, having two NICs installed in a Windows 11 PC causes all existing network connections to be closed by Windows 11 immediately the moment when Windows 11 decides to make the switch, from the NIC that it currently uses, to the other NIC. So, when two NICs are able to be used by it, it automatically chooses the NIC with the highest Connection Priority Order.
It also is why a Windows 11 PC equipped with both a wireless (WLAN) adapter and a LAN adapter cannot be connected via both at once. Windows 11 simply lacks the capacity to do so.

hdmi said:

In short, having two NICs installed in a Windows 11 PC causes all existing network connections to be closed by Windows 11 immediately the moment when Windows 11 decides to make the switch, from the NIC that it currently uses, to the other NIC. So, when two NICs are able to be used by it, it automatically chooses the NIC with the highest Connection Priority Order.

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Clearly this is false. I now have 5 systems all using multiple Ethernet adapters simultaneously. Yes, I can 100% guarantee that both adapters are in use simultaneously.

I can understand WiFi and Ethernet not coexisting because they are fundamentally different, but clearly multiple Ethernet adapters on the same subnet now work great.

hdmi said:

In short, having two NICs installed in a Windows 11 PC causes all existing network connections to be closed by Windows 11 immediately the moment when Windows 11 decides to make the switch, from the NIC that it currently uses, to the other NIC. So, when two NICs are able to be used by it, it automatically chooses the NIC with the highest Connection Priority Order.

Click to expand...

I don't think it works exactly this way because both adapters exhibit similar loads when there's intense network activity!!!

Edit: By the way, my problem went away after I disabled the "Power Saving" and "Wake-Up LAN" features for both adapters!!!

I've run 2 NICs from the same machine into a Network Switch -- with multiplexing and full duplex you should be able to increase traffic throughput to the target destination machine, You need decent Internet / LAN connection at home though -- any decent modern fibre system should be more than sufficient for typical domestic network traffic.A network switch is actually a good idea in any case - even a cheap unmamaged one -- costs around € 8 - 10 -- worth it -- that way you only need 1 wire to the router point from the switch !!

Cheers
jimbo

hsehestedt said:

I had always been taught that you do not want to place 2 ethernet adapters in one machine

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Hannes,

I had not thought it was possible.

Inspired by your post, I have just connected two USB-Ethernet adapters to my main computer and transferred several large files to it from another computer.
- I could see straight away that both were in use. They both continued to be used throughout the transfer.
- The fact that both report different speeds demonstrates that this is not some Windows confusion misreporting the same adapter twice.
- I did not do anything special, I just connected both Ethernet adapters and started the file transfer.
-

I routinely transfer large files [10GB is commonplace & was used in this test] from that other computer to my main computer every morning.
I routinely have this second Ethernet adapter hanging around doing nothing so I'll start plugging it for the big transfer from now on.
Now I'll barely have time to make a pot of Earl Grey during the transfer.

Thank you so much for posting about your experiences.


All the best,
Denis

Try3 said:

Thank you so much for posting about your experiences.

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And thank you for the comment. Remember though that this is only going to help where everything along the path supports the greater throughput. As an example...

The way I even discovered this was that I recently upgraded everything in my network from gigabit to 2.5 gigabit. But I have several mini PCs that have 2 separate gigabit Ethernet built-in. I connected both to same switch (a 2.5 gigabit switch) and was then able to transfer files between other machines much faster because I had 2 gigabit bandwidth to that mini PC. Note that if anything along the path between that mini PC was not only gigabit, then this would have limited me to only gigabit speed. But in my case, my switch is 2.5 gigabit and the machine I was sharing files with has a 2.5 gigabit adapter, so it dramatically helped me.

Understood.

Actually, I'm just making the best of some adapters & cables that I was given last year. I had previously been using WiFi [ax across the board] but even the single USB-Ethernet connection was almost ten times as fast***.
I did not measure the average transfer speed for this test. If I find the speed test records I made when I got the bits & pieces then I'll be able to do some sensible comparisons.

*** If I remember correctly, you found that your ax WiFi was as fast as your [single] Ethernet connection but my ax WiFi was nowhere near as fast as even a single Ethernet connection.


All the best,
Denis

Try3 said:

*** If I remember correctly, you found that your ax WiFi was as fast as your [single] Ethernet connection but my ax WiFi was nowhere near as fast as even a single Ethernet connection.

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Yes, but there is a difference. Bear in mind that I am on WiFi 6e. So, I have not only the 2.4 and 5Ghz bands, but also the 6Ghz band.

Of course, I'm now starting to get the itch for WiFi 7

hsehestedt said:

Clearly this is false. I now have 5 systems all using multiple Ethernet adapters simultaneously. Yes, I can 100% guarantee that both adapters are in use simultaneously.

I can understand WiFi and Ethernet not coexisting because they are fundamentally different, but clearly multiple Ethernet adapters on the same subnet now work great.

Click to expand...

Have you actually measured (e.g. by using ROBOCOPY with the -MT option) the time it takes for a large file to be copied over the network with just a single NIC, and compared that to the time it takes for this same large file to be copied over this same network with NIC Teaming "enabled" on Windows 11 (all else having been kept equal during the comparison test)? Nothing is impossible of course, but AFAIK it usually doesn't take very long to figure out that combining two Gigabit Ethernet NICs in a single Windows 11 PC in this manner does not really give you more than the usual 1000 Mbps transfer speed after all. (I can't test this myself right now, as I only have one Realtek USB GbE Family Controller to work with.)

Try3 said:

but my ax WiFi was nowhere near as fast as even a single Ethernet connection

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With my Asus RT-AX92U 2-Pack using WiFi 6 (AX6100) with the 160MHz option set to enabled, I am getting pretty close to Gigabit Ethernet speeds as long as the WiFi signal is kept close enough to its maximum strength. In practice, it easily averages around the 960 Mbps mark. This is with an Intel AX200 card in a desktop PC and two different laptops each one of which has an Intel AX201 card inside.