R3303-HP HSR6800 Routers Layer 2 - LAN Switching Configuration Guide

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VLAN frame encapsulation
In order that a network device can identify frames of different VLANs, a VLAN tag field is inserted into
the data link layer encapsulation.
The format of VLAN-tagged frames is defined in IEEE 802.1Q issued in 1999.
As shown in Figure 32, in the header o
f a traditional Ethernet data frame, the field after the destination
MAC address and the source MAC address (DA&SA) field is the Type field, which indicates the upper
layer protocol type.
Figure 32 Traditional Ethernet frame format
IEEE 802.1Q inserts a 4-byte VLAN tag between the DA&SA field and the Type field to identify the VLAN
information, as shown in Figure 33.
Figure 33 Position and
format of VLAN tag
The fields of a VLAN tag are as follows:
• TPID—The 16-bit TPID field indicates whether a frame is VLAN-tagged. By default, the TPID value is
0x8100, which indicates that the frame is VLAN-tagged. Devices vendors can set the TPID to
different values. For compatibility with these devices, modify the TPID value so that frames carry a
TPID value identical to the value of a particular vendor, allowing interoperability with devices from
that vendor. The device determines whether a received frame carries a VLAN tag by checking the
TPID value. When the TPID value of a frame is the configured value or 0x8100, the frame is
considered as a VLAN-tagged frame. For information about commands used to modify TPID values,
see Layer 2—LAN Switching Command Reference.
• Priority—The 3-bit priority field indicates the 802.1p priority of the frame. For more information, see
ACL and QoS Configuration Guide.
• CFI—The 1-bit CFI field indicates whether the MAC addresses are encapsulated in standard format
when packets are transmitted across different media. A value of 0 indicates that MAC addresses
are encapsulated in standard format. A value of 1 indicates that MAC addresses are encapsulated
in a non-standard format. The value of this field is 0 by default.
• VLAN ID—The 12-bit VLAN ID field identifies the VLAN that the frame belongs to. The VLAN ID
range is 0 to 4095. Because 0 and 4095 are reserved, a VLAN ID is actually in the range of 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. For more information, see "Introduction to port-based VLAN."
Ether
net supports encapsulation formats Ethernet II, 802.3/802.2 LLC, 802.3/802.2 SNAP, and 802.3
raw. The Ethernet II encapsulation format is used here. For how the VLAN tag fields are added to frames
encapsulated in these formats for VLAN identification, see related protocols and standards.
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.
TPIDDA&SA
Type
Priority CFI
VLAN Tag
FCSData
VLAN ID