lan9420 Standard Microsystems Corp., lan9420 Datasheet - Page 55

• Manufacturer Part Number: lan9420
• Description: Lan9420/lan9420i Single-chip Ethernet Controller With Hp Auto-mdix Support And Pci Interface
• Manufacturer: Standard Microsystems Corp.

Single-Chip Ethernet Controller with HP Auto-MDIX Support and PCI Interface
Datasheet
.
SMSC LAN9420/LAN9420i
3.5.2
3.5.3
PREAMBLE
PREAMBLE
(7 BYTES)
(7 BYTES)
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
VLAN Frame on page
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.
Address Filtering Functional Description
The Ethernet address fields of an Ethernet packet, consists of two 6-byte fields: one for the destination
address and one for the source address. The first bit of the destination address signifies whether it is
a physical address or a multicast address.
The address check logic filters the frame based on the Ethernet receive filter mode that has been
enabled. Filter modes are specified based on the state of the control bits in
Filtering
more detail in
If the frame fails the filter, the MAC does not receive the packet. The Host has the option of accepting
or ignoring the packet.
(1 BYTE)
(1 BYTE)
SOF
SOF
Modes", which shows the various filtering modes used by the MAC. These bits are defined in
DEST. ADDR.
(6 BYTES)
DEST. ADDR.
Section 4.4.1, "MAC Control Register (MAC_CR)," on page
(6 BYTES)
(2 BYTES)
55, the four bytes are inserted after the Source Address Field and before the
SOURCE ADDR.
TPID
(6 BYTES)
SOURCE ADDR.
(6 BYTES)
Figure 3.18 VLAN Frame
USER PRIORITY
DATASHEET
(3 BITS)
Tag Control Information
(2 BYTES)
Tag Protocol D: \x81-00
TPID
(2 BYTES)
55
TYPE
(TCI)
Ethernet frame with VLAN TAG
Ethernet frame
(1518 BYTES)
(2 BYTES)
(1 BIT)
TYPE
CFI
(1522 BYTES)
Indicates frame's priority
(2 BYTES)
TYPE
(46 - 1500 BYTES)
(12 BITS)
VLAN ID
DATA
Canonical Address Format Indicator
(46 - 1500 BYTES)
VID: 12 bits defining the VLAN
to which the frame belongs
DATA
120.
Table 3.13, "Address
Revision 1.1 (03-31-08)
(4 BYTES)
FCS
(4 BYTES)
Figure 3.18
FCS