this problem was not caused by the LwIP stack, but by the Ethernet driver. It
was solved with the help of the Ti support:
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.
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