TX_ER in GMII interfaces
Typically, the MAC does not have enough FIFO to hold the entire packet. Therefore, it must start transmitting to the PHY before it has the entire packet. If an error condition, such as FIFO underflow, occurs, it may have already started transmitting. By asserting TX_ER, it causes the PHY to generate an unambiguously invalid frame, guaranteeing that no receiver will accept it.