R7800 poor WAN download/upload rate when a Fast Ethernet client is connected at 100Mbps to a Gigabit built-in switch (Workaround available)

[xyzzy]

Jumbo frames were a thing when gigabit ethernet first came out. CPUs weren't fast enough to handle the packets/sec at gigabit with 1500 byte packets in their TCP/IP stacks and ethernet cards didn't have features like interrupt coalescing to reduce IRQ load. But that was only for a bit. Nobody uses jumbo frames anymore.

 

[sppmaster]

I've opened a new thread here https://forum.openwrt.org/t/severe-...t-on-most-openwrt-versions/129135?u=sppmaster
because as it seems the issue is not specific only to the R7800 hardware. There are new interesting findings.

 

[sppmaster]

I have to announce really good news. After lots of tests and with the help of a lot of people I think the main reason for the issue has been identified.
It's the Flow Control settings of the switch. You can read more in the above thread. Fortunately there is a way (at least in my case but probably working for others too) to workaround the issue.
I've disabled the Flow Control of the PC server Realtek NIC and now all WAN/LAN throughput is at Full Duplex Gigabit speeds.
Cheers!