Throughput is reduced when In-Band FCS passing is enabled for frames that are multiples of 8 bytes in length. This is frames of 64, 72, 80, 88 and so forth bytes in length from the start of the Destination Address to end of the FCS. Instead of transmitting the minimum 12 bytes of FCS (including the terminate), 16 bytes are transmitted.
This problem is not seen for frames sizes that are not multiples of 8 bytes. This problem is also not seen if In-Band FCS is not enabled.
This issue will be fixed in the LogiCORE 10-Gigabit Ethernet MAC v8.5 which is currently scheduled to be released in 10.1 IP Update 2 due out in the summer of 2008.