HP-UX VLAN Administrator's Guide, February 2007

Figure 1-3 IEEE 802.1Q VLAN Tag in Ethernet Frame
Destination
Address
Source
Address
802.1Q
VLAN Tag
Type/Len
Data
4 Bytes
2 Bytes 2 Bytes (Tag Control Information)
Frame
Check
Tag
Protocol
ID
User
Priority
(3 Bits)
VLAN ID
(12 Bits)
Canonical
Format
Indicator
(1 Bit)
Figure 1-4 VLANs Overlapping or Sharing the Same LAN Card Port
PowerRun Attn. Fault Remote
Server
VLAN0 VLAN5
VLAN-Aware Ethernet
LAN Card Port
When a VLAN-aware switch receives data from an end station, the switch determines where the
data is to go and whether the VLAN ID is retained. If the data is to go to a device that can
recognize the VLAN tag, the VLAN tag is retained. If the data is to go to a device that has no
knowledge of VLANs (a VLAN-unaware device), the switch sends the data without the VLAN
tag.
You must configure VLAN tagging on switch ports that interface to end stations that have tagged
VLANs. If a switch or end-station port is member of only a single, port-based VLAN, tagging is
not required.
To transmit tagged frames, you must configure a VLAN on the end station with a VLAN ID that
matches the following:
• The VLAN ID of a tagged VLAN on the switch port
• The VLAN ID of a VLAN at the remote end station
Tagging has several advantages – a major one is that VLAN association need be applied only
once at an end station or at an edge switch, so that downstream switches all the way to the
destination are relieved of the burden of classifying frames. Tagging at end stations is particularly
attractive because the overhead of frame classification is distributed.
Overview of VLANs 17