In accordance with the IEEE 802.3-2008, clause 36 , state machines for the 1000BASE-X PCS,
gmii_rx_er can be driven High following
reception of the end frame in conjunction with gmii_rxd[7:0] containing the hexadecimal value of 0x0F to signal
carrier extension. This is shown in the following figure. See 1000BASE-X State Machines for more information.This is not an error condition and can occur even for full-duplex frames.
Figure 1. GMII Normal Frame Reception with Carrier Extension
