If the MAC core is configured to pass the FCS field to you. It is handled as displayed in the following figure.
In this case, any padding inserted into the frame to meet Ethernet minimum frame length specifications is left intact and passed to you.
Even though the FCS is passed to you, it is also verified
by the MAC core and rx_axis_mac_tuser is asserted if the FCS check fails.
Figure 1. Frame Reception with In-Band FCS Field
