Information Technology Reference
In-Depth Information
FIguRE 2-1
802.1Q Tag and
802.1p Field
Type/
Length
(2)
802.1Q
Destination
Address (6)
Source
Address (6)
TPID
(2)
TCI
(2)
Preamble (8)
Data
FCS (2)
Frame:
User
Priority (3)
CFI
(1)
VLAN ID
(12)
As shown in
Figure 2-1,
the IEEE 802.1Q tag sits between the Source Address and Type/Length fields of the original Ethernet frame.
The tag itself consists of Tag Protocol ID (TPID) and Tag Control Information (TCI) fields.
The TPID field is a 2-byte field and contains a fixed value of 0x8100 that indicates that this is a tagged (802.1Q) frame.
The TCI field is a 2-byte field that contains three subfields:
■
User Priority:
Three bits in length and indicates the QoS priority of the frame. IEEE 802.1p defines the operation of this
field.
■
Canonical Format Indicator (CFI):
This 1-bit field indicates whether the information in this frame is carried in a canonical
(Ethernet) or noncanonical (Token Ring) format.
■
VLAN ID:
This 12-bit subfield indicates the VLAN.
Figure 2-2
shows the ToS byte and DS field, which are contained in the IP packet header.
0
1
2
3
4
5
6
7
FIguRE 2-2
ToS Byte and DS Field
Precedence
ToS
MBZ
0
1
2
3
4
5
6
7
DS Field, DSCP
ECN Field
