LAN9730 Datasheet, PDF (63/222 Pages) SMSC Corporation – High-Speed Inter-Chip (HSIC) USB 2.0

LAN9730/LAN9730i
4.5.2 VIRTUAL LOCAL AREA NETWORK (VLAN) SUPPORT
Virtual Local Area Networks or VLANs, as defined within the IEEE 802.3 standard, provide network administrators one
means of grouping nodes within a larger network into broadcast domains. To implement a VLAN, four extra bytes are
added to the basic Ethernet packet. As shown in Figure 4-9, the four bytes are inserted after the Source Address Field
and before the Type/Length field. The first two bytes of the VLAN tag identify the tag, and by convention are set to the
value 0x8100. The last two bytes identify the specific VLAN associated with the packet; they also provide a priority field.
The device supports VLAN-tagged packets. It provides two registers which are used to identify VLAN-tagged packets.
One register should normally be set to the conventional VLAN ID of 0x8100. The other register provides a way of iden-
tifying VLAN frames tagged with a proprietary (not 0x8100) identifier. If a packet arrives bearing either of these tags in
the two bytes succeeding the Source Address field, the controller will recognize the packet as a VLAN-tagged packet.
In this case, the controller increases the maximum allowed packet size from 1518 to 1522 bytes (normally the controller
filters packets larger than 1518 bytes). This allows the packet to be received, and then processed by host software, or
to be transmitted on the network.
FIGURE 4-9:
VLAN FRAME
PREAMBLE
(7 BYTES)
SOF
(1 BYTE)
DEST. ADDR.
(6 BYTES)
SOURCE ADDR.
(6 BYTES)
Ethernet frame
(1518 BYTES)
TYPE
(2 BYTES)
DATA
(46 - 1500 BYTES)
FCS
(4 BYTES)
PREAMBLE
(7 BYTES)
Ethernet frame with VLAN TAG
(1522 BYTES)
SOF
DEST. ADDR. SOURCE ADDR.
TPID
TYPE
TYPE
(1 BYTE) (6 BYTES)
(6 BYTES)
(2 BYTES) (2 BYTES) (2 BYTES)
DATA
(46 - 1500 BYTES)
FCS
(4 BYTES)
Tag Control Information
(TCI)
TPID
(2 BYTES)
USER PRIORITY
(3 BITS)
CFI
(1 BIT)
VLAN ID
(12 BITS)
VID: 12 bits defining the VLAN
to which the frame belongs
Canonical Address Format Indicator
Indicates frame's priority
Tag Protocol D: \x81-00
 2012-2015 Microchip Technology Inc.
DS00001946A-page 63