I did some quick search on ‘EtherCard jumbo frames’ as well as ‘ENC28J60 jumbo frames’ and didn’t find much useful information. I will need to get a router or switch with jumbo frame support to do further testing. I suspect the problem may be that since a jumbo frame can be as large as 9K (and ENC28J60 has only 8K TX/RX buffer), it may have caused a buffer overflow problem. This is just my wild guess and I have to perform some experiments to investigate further.