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Tag Header (NDIS 5.1)

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Note   NDIS 5. x has been deprecated and is superseded by NDIS 6. x. For new NDIS driver development, see Network Drivers Starting with Windows Vista. For information about porting NDIS 5. x drivers to NDIS 6. x, see Porting NDIS 5.x Drivers to NDIS 6.0.

The tag header is included in the 802.3 Ethernet header, which is between the MAC header and the data payload. Including the tag header in each packet serves the following purposes:

  • Each packet can carry user-priority information.

  • Each packet can carry a VLAN identifier (ID).

  • Each packet can indicate the format of MAC address information carried in MAC user data.

  • VLANs can be supported across different MAC types.

The following table shows the format of an 802.3 Ethernet header and how the tag header fits in the Ethernet header.

Octets (8-bit segment) Contents

1 - 2

Ethernet-encoded Tag Protocol Identifier (TPID). Set to 81-00 (hexadecimal).

3 - 4

The tag header. Also called Tag Control Information (TCI).

5 - 6

Length and type, which are values from the unmarked packet.
7 - N (maximum 36)

Embedded Source-Routing Information Field (E-RIF), present only if the Canonical Format Indicator (CFI) bit is set in TCI.

 

The TCI component is 2 octets long and contains the user-priority value, CFI, and VLAN ID information, as shown in the following figure:

The following is a description of the components shown in this figure:

  • Priority value
    Three-bit binary number that specifies a packet's priority. This number can represent eight priority levels, 0 through 7. The use and interpretation of this number is defined in ISO/IEC 15802-3.

  • CFI
    A single-bit flag value. If this bit is reset (that is, equal to 0), all MAC address information present in the packet is in canonical format (that is, the simplest form). If this bit is set (that is, equal to 1), E-RIF is present in the 802.3 Ethernet header. Currently, the only packets that are supported are those with CFI equal to 0; therefore, E-RIF is not present.

  • VLAN ID
    An unsigned twelve-bit binary number that identifies the VLAN to which a packet belongs. A computer can filter out VLAN ID-marked incoming packets whose VLAN IDs do not match the VLAN ID for which the computer is configured. If the VLAN ID is 0, the tag header contains only priority information. The following VLAN IDs are reserved:

    VLAN ID (hexadecimal) Meaning

    0

    The null VLAN ID. Indicates that the tag header contains only priority information; that is, no VLAN ID is present in the header.

    1

    		 The default VLAN ID. Used for classifying packets on entrance through a bridge port. Note that the specification of the entrance and exit rules for VLAN-aware bridges is such that a bridge does not propagate priority-marked packets. A priority-marked packet acquires a VLAN classification on entrance through a bridge; therefore the bridge either forwards the packet as unmarked or as marked with that VLAN classification, depending upon the exit configuration for that VLAN.

    FFF

    Reserved for implementation use.

     

    All other possible numbers are available for general use as VLAN IDs.

 

 

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