Intel PCI-X 802.1q Vlan Packet Format, 1 802.1q Tagged Frames, Vlan Packet Format Comparison

	802.1q VLAN Support
802.1q VLAN Support	9

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

Note: The 82541ER Ethernet controller does not support VLAN tags.

9.1802.1q VLAN Packet Format

Table 9-1compares 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.

Table 9-1. VLAN Packet Format Comparison

802.3 Packet	#Octets
DA	6
SA	6
Type/Length	2
Data	46-1500
CRC	4

802.1q VLAN	#Octets
Packet
DA	6
SA	6
8021.q Tag	4
Type/Length	2
Data	46-1500
CRC*	4

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.

9.1.1802.1q Tagged Frames

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 Table 9-2for 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 Section 13.4.22.

•VLAN Identifier (VID)

Software Developer’s Manual 175