describes the fields contained in an 802.1Q tag.
Table 3-1
Fields contained in an 802.1Q tag
Field Leng
th
Name
Description
TPID
2
bytes
Tag Protocol Identifier (TPID),
indicating the frame type.
The value 0x8100 indicates an 802.1Q-
tagged frame. If an 802.1Q-incapable
device receives an 802.1Q frame, it will
discard the frame.
PRI
3 bits
Priority (PRI), indicating the
frame priority.
The value ranges from 0 to 7. The
greater the value, the higher the priority.
These values can be used to prioritize
different classes of traffic to ensure that
frames with high priorities are
transmitted first when traffic is heavy.
CFI
1 bit
Canonical Format Indicator
(CFI), indicating whether the
MAC address is in canonical
format.
If the value is 0, the MAC address is in
the canonical format. CFI is used to
ensure compatibility between Ethernet
networks and Token Ring networks. It
is always set to zero for Ethernet
switches.
VID
12
bits
VLAN ID (VID), indicating the
VLAN to which the frame
belongs.
VLAN IDs range from 0 to 4095. The
values 0 and 4095 are reserved, and
therefore VLAN IDs range from 1 to
4094.
Each frame sent by an 802.1Q-capable switch carries a VLAN ID. In a VLAN, Ethernet frames
are classified into the following types:
l
Tagged frames: frames with 4-byte 802.1Q tags.
l
Untagged frames: frames without 4-byte 802.1Q tags.
Link Types
, there are the following types of VLAN links:
l
Access link: connects a host to a switch. Generally, a host does not know which VLAN it
belongs to, and host hardware cannot distinguish frames with VLAN tags. Therefore, hosts
send and receive only untagged frames.
l
Trunk link: connects a switch to another switch or to a router. Data of different VLANs are
transmitted along a trunk link. The two ends of a trunk link must be able to distinguish
frames with VLAN tags. Therefore, only tagged frames are transmitted along trunk links.
Huawei AR530&AR550 Series Industrial Switch Routers
Configuration Guide - Ethernet Switching
3 VLAN Configuration
Issue 01 (2014-11-30)
Huawei Proprietary and Confidential
Copyright © Huawei Technologies Co., Ltd.
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