Due to the problem in the 25G Ethernet Intel® FPGA IP User Guide, support for VLAN tag is not clear for Stacked VLAN frame.
The original description is:
Length/type = 0x8100—VLAN or stacked VLAN tagged frames. The MAC RX continues to check the frame and payload length.
The updated version should be:
Length/type = 0x8100—VLAN or stacked VLAN tagged frames (up to a total of two tags with value 0x8100). The MAC RX continues to check the frame and Payload length.