Hello,
this problem was not caused by the LwIP stack, but by the Ethernet driver. It
was solved with the help of the Ti support:
https://e2e.ti.com/support/arm/sitara_arm/f/791/t/663155
The address lookup engine (ALE) processes all received packets to determine
which port(s) if any that the packet should the forwarded to. Configured as a
switch, the port state of all ports was set to forwarding, even if a port was
not connected. Setting the unconnected ports to blocking by default and setting
them to forward after Phy connect, solved the problem.