The Ethernet encoded IEEE Std 802.1Q VLAN tag includes:
- 16-bit tag protocol identifier (TPID):
8100h. - 16-bit tag control information (TCI): first three priority bits, then CFI bit, then 12 VID bits.
The VLAN tag is inserted at the 13th byte of the frame adding an extra four bytes to the frame length. To support these extra four bytes, the GEM can accept frame lengths up to 1,536 bytes by setting the network_config [receive_1536_byte_frames, 8] bit = 1.
If the VLAN identifier (VID) is null (0000h), a
priority-tagged frame is indicated.
The following bits in the RX buffer descriptor status Word [1] provide information about VLAN tagged frames:
- Set bit [21] if the receive frame is VLAN tagged (specific type ID of
0x8100). - Set bit [20] if receive frame is priority tagged (specific type ID of
0x8100and null VID). If bits [20] is set, bit [21] is also set. - Set bits [19], [18], and [17] to priority if the bit [21] is set.
- Set bit [16] to CFI if bit [21] is set.
The controller can be configured to reject all frames except VLAN tagged frames by setting the discard non-VLAN frames bit in the network configuration register.