Data Sheet

Ethernet Controller I210 —Inline Functions
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7.4.1 802.1Q VLAN Packet Format
The following diagram compares an untagged 802.3 Ethernet packet with an 802.1Q VLAN tagged
packet:
Note: 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. Also, max frame size for an 802.1Q VLAN packet is
1522 octets as opposed to 1518 octets for a normal 802.3z Ethernet packet.
7.4.2 802.1Q Tagged Frames
For 802.1Q, the Tag Header field consists of four octets comprised of the Tag Protocol Identifier (TPID)
and Tag Control Information (TCI); each taking 2 octets. The first 16 bits of the tag header makes 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:
• User Priority (UP)
• Canonical Form Indicator (CFI). Should be 0b for transmits. For receives, the device has the
capability to filter out packets that have this bit set. See the CFIEN and CFI bits in the RCTL
described in Section 8.10.1.
• VLAN Identifier (VID)
The bit ordering is as follows:
7.4.3 Transmitting and Receiving 802.1Q Packets
7.4.3.1 Adding 802.1Q Tags on Transmits
Software might command the I210 to insert an 802.1Q VLAN tag on a per packet or per flow basis. If
the VLE bit in the transmit descriptor is set to 1b, then the I210 inserts a VLAN tag into the packet that
it transmits over the wire. 802.1Q tag insertion is done in different ways for legacy and advanced Tx
descriptors:
Table 7-54. Comparing Packets
802.3 Packet #Octets 802.1Q VLAN Packet #Octets
DA 6 DA 6
SA 6 SA 6
Type/Length 2 802.1Q Tag 4
Data 46-1500 Type/Length 2
CRC 4 Data 46-1500
CRC* 4
Table 7-55. TCI Bit Ordering
Octet 1 Octet 2
UP CFI VID