Operation Manual – VLAN
H3C S3100 Series Ethernet Switches Chapter 1 VLAN Overview
1-3
Figure 1-2 Encapsulation format of traditional Ethernet frames
IEEE 802.1Q inserts a four-byte VLAN tag after the DA&SA field, as shown in
Figure
1-3.
Figure 1-3 Format of VLAN tag
A VLAN tag comprises four fields: tag protocol identifier (TPID), priority, canonical
format indicator (CFI), and VLAN ID.
z The 16-bit TPID field with a value of 0x8100 indicates that the frame is VLAN
tagged. On the H3C series Ethernet switches, the default TPID is 0x8100.
z The 3-bit priority field indicates the 802.1p priority of the frame. Refer to the “QoS”
part of this manual for details.
z 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.
z 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
ranges from 1 to 4094.
Note:
The Ethernet II encapsulation format is used here. Besides the Ethernet II
encapsulation format, other encapsulation formats such as 802.2 LLC and 802.2 SNAP
are also supported by Ethernet. The VLAN tag fields are also added to frames
encapsulated in these formats for VLAN identification. Refer to section
Encapsulation
Format of Ethernet Data
for 802.2/802.3 encapsulation format.
VLAN ID identifies the VLAN to which a packet belongs. When a switch receives a
packet carrying no VLAN tag, the switch encapsulates a VLAN tag with the default
VLAN ID of the inbound port for the packet, and sends the packet to the default VLAN of
the inbound port for transmission. For the details about setting the default VLAN of a
port, refer to
Configuring the Default VLAN ID for a Port.
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