H3C Technologies H3C SecPath F1000-E - Manual (Page 54)

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In the header of a traditional Ethernet data frame, the field after the destination MAC address and the
source MAC address is the Type field indicating the upper layer protocol type, as shown in Figure 14 .
Figure 14 Traditional Ethernet frame format
IEEE 802.1Q inserts a four-byte VLAN tag after the DA & SA field, as shown in Figure 15 .
Figure 15 Position and format of VLAN tag
A VLAN tag comprises the following fields:
• TPID—The 16-bit Tag protocol identifier (TPID) field with a value of 0x8100 indicates that the frame
is VLAN-tagged.
• Priority—The 3-bit priority field indicates the 802.1p priority of the frame.
• CFI—The 1-bit canonical format indicator (CFI) field specifies whether the MAC addresses are
encapsulated in the standard format when packets are transmitted across different media. A value
of 0 indicates that MAC addresses are encapsulated in the standard format; a value of 1 indicates
that MAC addresses are encapsulated in a non-standard format. The value of the field is 0 by
default.
• VLAN ID—The 12-bit VLAN ID field identifies the VLAN the frame belongs to. The VLAN ID range
is 0 to 4095. As 0 and 4095 are reserved, a VLAN ID actually ranges from 1 to 4094.
A network device handles an incoming frame depending on whether the frame is VLAN tagged and the
value of the VLAN tag, if any. For more information, see "
Introduction to port-based VLAN ."
NOTE:
•
The Ethernet II encapsulation format is used here. Besides the Ethernet II encapsulation format, Ethernet
also supports other encapsulation formats, including 802.2 LLC, 802.2 SNAP, and 802.3 raw. The
VLAN tag fields are added to frames encapsulated in these formats for VLAN identification.
• For a frame with multiple VLAN tags, the device handles it according to its outer-most VLAN tag and
transmits its inner VLAN tags as payload.
VLAN types
You can implement VLANs based on the following criteria:
• Port
• MAC address
• Protocol
• IP subnet
• Policy
• Other types, such as voice VLAN