I can reproduce a throughput issue on the AS21–MxL path with both MxL firmware 1.0.70 and 1.0.85. The vendor image shows the same behavior. With the iperf3 server connected to AS21 PHY24 and the client to an MxL port, iperf3 -c <server> -P4 -t4 -M1450 -R loses about 70% throughput and produces heavy retransmissions, while MSS 1449 works normally. The same behavior occurs with USXGMII+RA and 10GBASE-R with RA disabled. When both sides are fixed to 2500BASE-X/2.5G, the problem almost disappears. Could anyone reproduce this, or might it be a hardware issue specific to my board?