Due to a change in runt handling behavior in Quartus® Prime v16.1, the runt statistics register may read back as "X" in simulation in the Legacy 40/100G Ethernet Megacore®.
Normal core behavior is not affected, but when running the KR4 example testbench, the following messages may be seen:
# DUT 1 Sent 1027 packets
# DUT 2 Received 1027 packets
# Runts in RX data
# DUT 2 Sent 1027 packets
# DUT 1 Received 1027 packets
# Runts in RX data
These messages can be safely ignored.