134
Figure 121 Traditional Ethernet frame format
IEEE 802.1Q inserts a four-byte VLAN tag after the DA&SA field, as shown in Figure 122.
Figure 122 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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