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102

The format of VLAN-tagged frames is defined in IEEE 802.1Q issued by the Institute of Electrical and

Electronics Engineers (IEEE) in 1999.

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, which indicates the upper layer protocol type, as shown in Figure

32.

Figure 32 Format of a traditional Ethernet frame

Type DataDA&SA

IEEE 802.1Q inserts a four-byte VLAN tag after the DA&SA field.

Figure 33 Position and format of VLAN tag

TPIDDA&SA TypePriority CFI VLAN ID

VLAN Tag

The fields of a VLAN tag are tag protocol identifier (TPID), priority, canonical format indicator (CFI), and

VLAN ID.

 The 16-bit TPID field with a value of 0x8100 indicates that the frame is VLAN-tagged.

 The 3-bit priority field indicates the 802.1p priority of the frame. For more information about frame

priorities, see the ACL and QoS Configuration Guide.

 The 1-bit 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.

 The 12-bit VLAN ID field identifies the VLAN that the frame belongs to. The VLAN ID range is 0 to

4095. Because 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, other

encapsulation formats, including 802.2 LLC, 802.2 SNAP, and 802.3 raw, are also supported by Ethernet. The

VLAN tag fields are also added to frames encapsulated in these formats for VLAN identification.

 When a frame carrying multiple VLAN tags passes through, the switch processes the frame according to its outer

VLAN tag, and transmits the inner tags as payload.

Types of VLANs

You can implement VLAN based on the following criteria:

 Port

 MAC address

 Protocol

 IP subnet

 Policy

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