This issue occurs when the PLB is running at a very high frequency with respect to the Ethernet line rate and large packets are sent (e.g., jumbo packets for MII 10 mbps links). This can cause the TEMAC Data FIFO to produce back pressure on the Local Link FIFO block which, in turn, might result in data corruption on TX data from the Local Link FIFO to the TEMAC.
The corrected ipic2llink.vhd file that fixes the issue is located at:
Unzip and place the ipic2llink.vhd file in the local "pcore" directory of your project.
This issue has been fixed in XPS_LL_FIFO v1_00_b scheduled to be released with EDK 10.1.