3Com Switch 4800G 24-Port Configuration Manual page 85

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VLAN Classification
IEEE802.1Q defines a four-byte VLAN Tag between the DA&SA field and the Type
field to carry VLAN-related information, as shown in Figure 27.
Figure 27 The position and the format of the VLAN Tag
DA&SA TPID
The VLAN Tag comprises four fields: the tag protocol identifier (TPID) field, the
Priority field, the canonical format indicator (CFI) field, and the VLAN ID field.
The TPID field, 16 bits in length and with a value of 0x8100, indicates that a
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packet carries a VLAN tag with it.
The Priority field, three bits in length, indicates the 802.1p priority of a packet.
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For information about packet priority, refer to "Introduction to QoS Policies"
on page 870.
The CFI field, one bit in length, specifies whether or not the MAC addresses are
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encapsulated in standard format when packets are transmitted across different
medium. With the field set to 0, MAC addresses are encapsulated in standard
format; with the field set to 1, MAC addresses are encapsulated in
non-standard format. The filed is 0 by default.
The VLAN ID field, 12 bits in length and with its value ranging from 0 to 4095,
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identifies the ID of the VLAN a packet belongs to. As VLAN IDs of 0 and 4095
are reserved by the protocol, the value of this field actually ranges from 1 to
4094.
A network device determines the VLAN to which a packet belongs to by the VLAN
ID field the packet carries. The VLAN Tag determines the way a packet is
processed. For more information, refer to section "Introduction to Port-Based
VLAN" on page 87.
The frame format mentioned here is that of Ethernet II. Besides Ethernet II
encapsulation, other types of encapsulation, including 802.2 LLC, 802.2 SNAP,
and 802.3 raw are also supported by Ethernet. The VLAN tag fields are also added
to packets adopting these encapsulation formats for VLAN identification.
Based on how VLANs are established, VLANs fall into different categories. The
following types are the most commonly used:
Port-based
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MAC address-based
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Protocol-based
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IP-subnet-based
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Policy-based
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Other types
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The the Switch 4800G support port-based VLAN, MAC address-based VLAN,
protocol-based VLAN, and IP-subnet-based VLAN.
VLAN Tag
Priority CFI VLAN ID
Introduction to VLAN 85
Type