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Software Developer’s Manual 175
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.
9.1 802.1q VLAN Packet Format
Table 9-1 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.
Table 9-1. VLAN Packet Format Comparison
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.
9.1.1 802.1q 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 9-2 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 Section 13.4.22.
• VLAN Identifier (VID)
802.3 Packet #Octets
802.1q VLAN
Packet
#Octets
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