LAN9420 Datasheet, PDF (55/171 Pages) SMSC Corporation – Single-Chip Ethernet Controller with HP Auto-MDIX Support and PCI Interface

Single-Chip Ethernet Controller with HP Auto-MDIX Support and PCI Interface
Datasheet
3.5.2
.
appropriate values set in all the different fields (as defined in the 802.3x specification) and transmits
the frame (via the PHY). The transmission of the control frame is not affected by the current state of
the Pause timer value that is set because of a recently received control frame. Refer to Section 4.4.8,
"Flow Control Register (FLOW)," on page 130 for more information on enabling flow control in the
MAC.
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 3.18
VLAN Frame on page 55, 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 8100h. The last two bytes identify the specific VLAN associated with the packet; they also
provide a priority field.
The MAC supports VLAN-tagged packets. The MAC provides two registers which are used to identify
VLAN-tagged packets. One register should normally be set to the conventional VLAN ID of 8100h. The
other register provides a way of identifying VLAN frames tagged with a proprietary (not 8100h)
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
the application, or to be transmitted on the network.
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
USER PRIORITY
(2 BYTES)
(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
Figure 3.18 VLAN Frame
SMSC LAN9420/LAN9420i
55
DATASHEET
Revision 1.6 (07-18-11)