Altera TSE driver and example program for lwIP (1.3.2)

Ok - so that won't show if the inserted 'pad' bytes come from specific data bytes.

What I'd noticed is that the last frame ends with the correct byte (0x65) - so the problem isn't an obvious one where all 4 bytes are always written.

The IP header frame length (0x00 0x2c) is correct for the expected frame data - but not for that actually added.

An alternate hypothesis is that the MAC unit only looks at the byte enables in the fifo word at the end of frame. Whereas your data will have multiple unasserted byte enables embedded within the frame.

If so, the software would have to be willing to build an entire frame.