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176 Software Developer’s Manual
802.1q VLAN Support
Table 9-2. 802.1q Tagged Frames
9.2 Transmitting and Receiving 802.1q Packets
Since the 802.1q tag is only four bytes, adding and stripping of tags can done completely in
software. (For transmits, software inserts the tag into packet data before it builds the transmit
descriptor list, and for receives, software strips the four byte tag from the packet data before
delivering the packet to upper layer software.)
However, because adding and stripping of tags in software results in more overhead for the host,
the Ethernet controller has additional capabilities to add and strip tags in hardware, as discussed in
the following two sections.
9.2.1 Adding 802.1q Tags on Transmits
Software can command the Ethernet controller to insert an 802.1q VLAN tag on a per packet basis.
If CTRL.VME is set to 1b, and the VLE bit in the transmit descriptor is set to 1b, then the Ethernet
controller inserts a VLAN tag into the packet that it transmits over the wire. The Tag Protocol
Identifier (TPID) field of the 802.1q tag comes from the VLAN Ether Type (VET) register, and the
Tag Control Information (TCI) of the 802.1q tag comes from the special field of the transmit
descriptor (TDESC.SPECIAL).
9.2.2 Stripping 802.1q Tags on Receives
Software can instruct the Ethernet controller to strip 802.1q VLAN tags from received packets. If
the CTRL.VME bit is set to 1b, and the incoming packet is an 802.1q VLAN packet (its Ethernet
Type field matched the VET register), then the Ethernet controller strips the 4-byte VLAN tag from
the packet, and stores the TCI in the Special field of the receive descriptor.
The Ethernet controller also sets the VP bit in the receive descriptor to indicate that the packet had
a VLAN tag that was stripped. If the CTRL.VME bit is not set, the 802.1q packets can still be
received if they pass the receive filter. In this case, the VLAN tag is not stripped and the VP bit is
not set. Refer to Table 9-3 for more information regarding receive packet filtering.
9.3 802.1q VLAN Packet Filtering
VLAN filtering is enabled by setting the RCTL.VFE bit to 1b. If enabled, hardware compares the
type field of the incoming packet to a 16-bit field in the VLAN EtherType (VET) register. If the
VLAN type field in the incoming packet matches the VET register, the 802.1q VLAN packet is
then compared against the VLAN Filter Table Array (VFTA) for acceptance.
The Virtual LAN ID field indexes a 4096 bit vector. If the indexed bit in the vector is 1b, there is a
Virtual LAN match. Software can set the entire bit vector to 1b’s if the node does not implement
802.1q filtering.
Octet 1 Octet 2
UP CFI VID