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74 AT-8800 Series Switch User Guide
Software Release 2.6.1
C613-02039-00 REV A
Figure 13: Format of user priority and VLAN data in an Ethernet frame.
Ethernet packets which contain a VLAN tag are referred to as tagged frames,
and switch ports that transmit tagged frames are referred to as tagged ports.
Ethernet packets which do not contain the VLAN tag are referred to as untagged
frames, and switch ports that transmit untagged frames are referred to as
untagged ports. VLANs can consist of simple logical groupings of untagged
ports, in which the ports receive and transmit untagged packets. Alternatively,
VLANs can contain only tagged ports, or a mixture of tagged and untagged
ports.
The switch is VLAN aware. It can accept VLAN tagged frames, and supports
the VLAN switching required by such tags. A network can contain a mixture of
VLAN aware devices, for example, other 802.1Q-compatible switches, and
VLAN unaware devices, for example, workstations and legacy switches that
do not support VLAN tagging. The switch can be configured to send VLAN
tagged or untagged frames on each port, depending on whether or not the
devices connected to the port are VLAN aware. By assigning a port to two
different VLANs, to one as an untagged port and to another as a tagged port, it
is possible for the port to transmit both VLAN-tagged and untagged frames. A
port must belong to a VLAN at all times unless the port has been set as the
mirror port for the switch.
Table 11: Reserved VID values .
VID value (hexadecimal) Meaning and use of reserved VID values
0 The null VLAN ID. Indicates that the tag header contains only
user priority information; no VLAN Identifier is present in the
frame. This VID value must not be configured in any Forwarding
Database entry, or used in any management operation. Frames
that contain the null VLAN ID are also known as priority-tagged
frames.
1 The default VID value used for classifying frames on ingress
through an untagged switch port.
FFF Reserved for implementation use. This VID value must not be
configured in any Forwarding Database entry, used in any
management operation, or transmitted in a tag header.
TPID
16 bits
1 bit
3 bits
12 bits
User
Priority
CFI VID
0x81-00
Preamble
Destination
Address
Source
Address
Type/
Length
Frame Data
CRC
64 bits
48 bits 48 bits
16
bits
368-12000
32 bits
SWITCH6