We had this with old shields on a bottling line, two problems stacked on top of each other.
Forcing the switch port to 100/full is what killed traffic after you got a link. The W5100 still auto-negotiates. When the other side is forced, auto-negotiation can’t find out the duplex and falls back to half duplex. Switch on full, shield on half, and you get a duplex mismatch: link up, almost nothing passes. Put the ports back to auto, or force both ends (and you can’t force the W5100 from the Arduino library).
The original no-link problem is very often the reset on those clone shields. The W5100 comes out of reset before its supply is stable and its PHY sometimes never negotiates properly. The old switch was forgiving, the new one isn’t. Fix that worked for us:
// shield RESET pin wired to D8 with a jumper wire
pinMode(8, OUTPUT);
digitalWrite(8, LOW);
delay(50);
digitalWrite(8, HIGH);
delay(500); // let the PHY settle
Ethernet.begin(mac, ip);And if one still refuses, a cheap unmanaged 10/100 switch between the shield and the managed one ends the argument. Long term, W5500 shields don’t have this problem.