Figure 131 Traditional Ethernet frame format

IEEE 802.1Q inserts a four-byte VLAN tag after the DA&SA field, as shown in Figure 132.

Figure 132 Position and format of VLAN tag

A VLAN tag comprises the following fields:

•Tag protocol identifier (TPID)—The 16-bit TPID field indicates whether the frame is VLAN-tagged and is 0x8100 by default.

•Priority—The 3-bit priority field indicates the 802.1p priority of the frame.

•Canonical format indicator (CFI)—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. The 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.

The Ethernet II encapsulation format is used in this section. In addition to the Ethernet II encapsulation format, Ethernet also supports other encapsulation formats, including 802.2 LLC, 802.2 SNAP, and

802.3raw. The VLAN tag fields are added to frames encapsulated in these formats for VLAN identification.

When a frame carrying multiple VLAN tags passes through, the device processes the frame according to its outer VLAN tag, and transmits the inner tags as payload.

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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