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I have a cheapo USB Ethernet adapter connected to Raspberry Pi Zero. When I log into my router's interface, I can see that the connection speed is 10 Mbps. However ethtool eth0 says that it's 100 Mbps full-duplex.

I've tried to disable auto-negotiation and forcefully downgrade to 10 Mbps half-duplex using this command:

sudo ethtool -s eth0 speed 10 duplex half autoneg off

but nothing changed. autoneg off alone didn't work too.

speedtest-cli measured download speed of 3.5 Mbps, but I guess it may be bottlenecked by RPi's low-end specs.

lsusb -t says:

/:  Bus 01.Port 1: Dev 1, Class=root_hub, Driver=dwc_otg/1p, 480M
    |__ Port 1: Dev 2, If 0, Class=, Driver=dm9601, 12M

If I read this correctly it means that the card is running at USB 1.1 speeds and it's just faking being 100 Mbps?

The device identifies itself as "ICS Advent DM9601 Fast Ethernet Adapter", I think I've also seen "10/100" mentioned somewhere. Is it possible that for some reason it chose USB 1.1, but could be switched to USB 2.0 to actually use 100 Mbps?

Additional information:

  • iperf over LAN measured 5 Mbps
  • The same adapter connected to a desktop PC has similar speedtest-cli results, reports as 100 Mbps but is using 12 Mbps USB link
  • uname -r returns 4.19.93+
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  • i also saw this sometimes, think this is a tiny bug in raspbian – ralf htp Jan 24 '20 at 22:18
  • What kernel and driver version are you using? – Jan Jan 25 '20 at 8:16
  • DM9601 in these cheap adapters are copies of the "real" DM9601 and thus kernel/drivers may act unpredictably. IMO it is luck it is working at all. – Jan Jan 25 '20 at 8:18
  • @Jan Kernel version is 4.19.93+. How do I check driver version? I've also added some more measurements. I didn't expect much from this adapter, but Fast Ethernet isn't exactly cutting edge nowadays... – gronostaj Jan 25 '20 at 8:27
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Most likely you got scammed, I got the same or similar cheap Chinese konck-off adapter as yours. It works on USB 1.1 on Linux and Windows 10. Windows couldn't find drivers for it, I manually downloaded and added "ICS Advent DM9601" driver to make it "work". I opened the casing of the adapter and there are no markings on the chip nor on the PCB.

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  • Yes, that's it indeed. I've ordered another similar one and it even has the same MAC address. – gronostaj Dec 6 '20 at 21:04

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