82583V Datasheet, PDF (166/374 Pages) Intel Corporation – Intel® 82583V GbE Controller

82583V GbE Controller—Inline Functions
The bit ordering is as follows:
Octet 1
UP
CFI
Octet 2
VID
7.5.2
Transmitting and Receiving 802.1q Packets
Since the 802.1q tag is only four bytes, adding and stripping of tags could be done
completely in software. (In other words, for transmits, software inserts the tag into
packet data before it builds the transmit descriptor list, and for receives, software strips
the 4-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 82583V has additional capabilities to add and strip tags in hardware. See
section 7.5.2.1 and section 7.5.2.2.
7.5.2.1
Adding 802.1q Tags on Transmits
Software might command the 82583V 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 82583V 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 VET register,
and the Tag Control Information (TCI) of the 802.1q tag comes from the special field of
the transmit descriptor.
7.5.2.2
Stripping 802.1q Tags on Receives
Software might instruct the 82583V 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 (for
example, it's Ethernet Type field matched the VET), then the 82583V strips the 4-byte
VLAN tag from the packet, and stores the TCI in the Special field of the receive
descriptor.
The 82583V 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, but the VLAN tag is not stripped and the
VP bit is not set.
7.5.3
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 Ether Type
(VET) register. If the VLAN type field in the incoming packet matches the VET register,
the packet is then compared against the VLAN filter table array for acceptance.
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