Types of vlan – H3C Technologies H3C S12500 Series Switches User Manual
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The format of VLAN-tagged frames is defined in IEEE 802.1Q issued by the Institute of Electrical and
Electronics Engineers (IEEE) in 1999.
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
Figure 2 Traditional Ethernet frame format
IEEE 802.1Q inserts a four-byte VLAN tag after the DA&SA field, as shown in
Figure 3 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.
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The 3-bit priority field indicates the 802.1p priority of the frame. For more information about frame
priority, see ACL and QoS Configuration Guide.
•
The 1-bit CFI field specifies whether the MAC addresses are encapsulated in the standard format
when packets are transmitted across different media. Value 0 indicates that MAC addresses are
encapsulated in the standard format; value 1 indicates that 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 by the protocol, a VLAN ID actually is in the range of 1 to 4094.
A switch 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 VLANs
."
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.
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When a frame carrying multiple VLAN tags passes through, the switch processes the frame according
to its outer VLAN tag and transmits its inner tags as payload.
Types of VLAN
You can implement VLAN based on the following criteria:
•
Port
•
MAC address
•
Protocol
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IP subnet