I have OMRON PLC act as TCP client, and a custom w5500 based board as TCP server. Basically, the w5500 ignores all arp request from OMRON, although it accepts arp request from PC.
while trying to solve the issue, I opened a raw mac socket on w5500, listening and responding to ARP requests. BUT THE RESULT IS THE SAME!
the w5500 chip filtered out all the ARP reply I sent to OMRON, but everything I sent to PC is ok.
I tried the chip on another Semiens PLC, the same result.
Now I really have no idea how to solve this issue then replacing w5500 with a difference phy chip.