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With the Orange Pi One and the mainline kernel, I can't get it to work optimally when plugged into a gigabit switch.  The eth interface on the Pi One is 100Mb/s and it works perfectly when attached to a switch that is 100Mb/s.
 

When attached to a 1000Mb/s switch:

[112776.347520] dwmac-sun8i 1c30000.ethernet eth0: Link is Up - 100Mbps/Full - flow control off

When attached to a 100Mb/s switch:

[112882.431555] dwmac-sun8i 1c30000.ethernet eth0: Link is Up - 100Mbps/Full - flow control rx/tx

 

 

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2 hours ago, chrisf said:

Don't both sides of the link need to support it for it to work?

 

Sure. It's even part of Autonegotiation (part of GbE standards). So if the switch doesn't support to adjust settings and performance degradation happens the only way to solve the problem is to replace the switch with another one that allows to enable flow control there (managed switches should allow for this and the majority of 'smart managed' too).

 

I would first do a primitive test with iperf3 in the local net and speedtest if there's fast Internet connection to see whether it's an issue in reality (could become an issue with high latency connections like Internet access)

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Apparently flow control on full duplex gigabit links can cause the link to operate at 100M if it's talking with both gigabit and 100M nodes at the same time.

https://www.smallnetbuilder.com/lanwan/lanwan-features/30212-when-flow-control-is-not-a-good-thing

 

That could be a reason why some gigabit switches don't support it. when running in a mixed fast/gigabit network it can impacts transfers between two gigabit nodes if one of them is also talking to a fast node. as a manufacturer you don't want to field complaints about poor network performance that is "fixed" by swapping your switch with another brand.

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