3com 2928 Betriebsanweisung
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The format of VLAN-tagged frames is defined in IEEE 802.1Q-1999.
In the header of a traditional Ethernet data frame as shown in
In the header of a traditional Ethernet data frame as shown in
, the field after the destination
MAC address and the source MAC address fields (DA&SA in the figure) is the Type field indicating the
upper layer protocol type.
upper layer protocol type.
Figure 1-2
The format of a traditional Ethernet frame
IEEE 802.1Q inserts a four-byte VLAN tag before the Type field, as shown in
Figure 1-3
The position and format of VLAN tag
Type
TPID
DA&SA
Priority CFI
VLAN ID
VLAN Tag
A VLAN tag comprises four fields: tag protocol identifier (TPID), priority, canonical format indicator (CFI),
and VLAN ID.
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The 16-bit TPID field with a value of 0x8100 indicates that the frame is VLAN tagged.
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The 3-bit priority field indicates the 802.1p priority of the frame.
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The 1-bit CFI field specifies whether the MAC addresses are encapsulated in the canonical format
for the receiving device to correctly interpret the MAC addresses. Value 0 indicates that the MAC
addresses are encapsulated in canonical format; value 1 indicates that the MAC addresses are
encapsulated in non-canonical format. The field is set to 0 by default.
for the receiving device to correctly interpret the MAC addresses. Value 0 indicates that the MAC
addresses are encapsulated in canonical format; value 1 indicates that the MAC addresses are
encapsulated in non-canonical format. The field is set to 0 by default.
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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 by the protocol, the VLAN ID range available for assignment is 1 to
4094.
As 0 and 4095 are reserved by the protocol, the VLAN ID range available for assignment is 1 to
4094.
When receiving a frame, a network device looks at its VLAN tag to decide how to handle the frame.
The Ethernet II encapsulation format is used in this section. Besides this 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 used in these encapsulations for VLAN identification.
formats, including 802.2 LLC, 802.2 SNAP, and 802.3 raw, are also supported by Ethernet. The VLAN
tag fields are also used in these encapsulations for VLAN identification.
VLAN Types
You can create VLANs based on:
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Port
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MAC address
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Protocol
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IP subnet
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Policy
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Other criteria