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 a.

a.Traditional Ethernet frame format

DA&SA

Type

Data

 

 

 

IEEE 802.1Q inserts a four-byte VLAN tag after the DA&SA field, as shown in b.

b.Position and format of VLAN tag

A VLAN tag comprises the following fields: 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.

∙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 the MAC addresses are encapsulated in canonical format. A value of 1 indicates that the 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 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.

VLAN types

You can implement VLANs based on the following criteria:

∙Port

∙MAC address

∙Protocol

∙IP subnet

∙Policy

∙Other criteria

The web interface is available only for port-based VLANs, and this chapter introduces only port-based VLANs.

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