When you parameterize a 40GBASE-KR4 variation of the LL 40GbE IP core, you must turn on at least one of auto-negotiation (AN) and Link Training (LT). Variations with neither of these features turned on might be non-functional. They might fail to lock to incoming 40GbE data, or might show a high packet error rate. This issue is seen in configurations with and without forward error correction (FEC) enabled.
This issue has no workaround.
This issue will be fixed in a future version of the Low Latency 40- and 100-Gbps Ethernet MAC and PHY IP core.