Intel PCI User Manual

175

The PCI/PCI-X Family of Gigabit Ethernet Controllers provide several specific mechanisms to 

support 802.1q VLANs:

•

Optional adding (for transmits) and stripping (for receives) of IEEE 802.1q VLAN tags

•

Optional ability to filter packets belonging to certain 802.1q VLANs

The 82541ER Ethernet controller does not support VLAN tags.

 compares the format of an untagged 802.3 Ethernet packet with an 802.1q VLAN tagged 

packet. The CRC for the 802.1q tagged frame is re-computed, so that it covers the entire tagged 

frame including the 802.1q tag header. 

Maximum frame size for a standard 803.3ac (802.1q VLAN and/or 802.1p priority) packet is 1522 
octets as opposed to 1518 octets for a normal 802.3 Ethernet packet. If jumbo frames are used, 

enabling 802.3ac adds 4 bytes to the packet to accommodate the q-tag. If multiple descriptors are 
required for a transmit, the q-tag information for the packet is extracted from only the last 

descriptor of the packet. VLAN tagging is supported independently of packet size.

For 802.1q, the Tag Header field consists of four octets containing the Tag Protocol Identifier 

(TPID) and Tag Control Information (TCI), each utilizing 2 octets. The first 16 bits of the tag 

header make up the TPID. It contains the “protocol type” which identifies the packet as a valid 
802.1q tagged packet.

The two octets making up the TCI contain three fields (see 

 for details):

•

User Priority (UP)

•

Canonical Form Indicator (CFI). The CFI should be 0b for transmits. For receives, the 
Ethernet controller has the capability to filter out packets that have this bit set. See the CFIEN 

and CFI bits in the RCTL as described in 

.

•

VLAN Identifier (VID)

DA

6

DA

6

SA

6

SA

6

Type/Length

2

8021.q Tag

4

Data

46-1500

Type/Length

2

CRC

4

Data

46-1500

CRC*

4