7.2.4. Using MAC Avalon ST i_tx_error Signal to Mark Packets Invalid
Because the core uses a cut-through interface, the core starts transmitting the packet data it is given as soon as possible. If the core discovers an error after the packet starts, e.g. in a bridging system where the receiver also uses a cut-through interface, you can use i_tx_error to invalidate the packet. You can also use i_tx_error for testing, to generate errored packets, and confirm that the other end of the link is able to reject the errored packets.
To invalidate an errored frame, end it with i_tx_endofpacket and assert i_tx_error. If the frame is good, deassert i_tx_error.