802.1Q Vlan Tagging - HP PROCURVE 2610 Advanced Traffic Management Manual

Static Virtual LANs (VLANs)
Port-Based Virtual LANs (Static VLANs)

802.1Q VLAN Tagging

VLAN tagging enables traffic from more than one VLAN to use the same port.
(Even when two or more VLANs use the same port they remain as separate
domains and cannot receive traffic from each other without going through an
external router.) As mentioned earlier, a "tag" is simply a unique VLAN
identification number (VLAN ID, or VID) assigned to a VLAN at the time that
you configure the VLAN name in the switch. The tag can be any number from
1 to 4094 that is not already assigned to a VLAN. When you subsequently assign
a port to a given VLAN, you must implement the VLAN tag (VID) if the port
will carry traffic for more than one VLAN. Otherwise, the port VLAN assign­
ment can remain "untagged" because the tag is not needed. On a given switch,
this means you should use the "Untagged" designation for a port VLAN
assignment where the port is connected to non 802.1Q-compliant device or is
assigned to only one VLAN. Use the "Tagged" designation when the port is
assigned to more than one VLAN or the port is connected to a device that does
comply with the 802.1Q standard.
For example, if port A7 on an 802.1Q-compliant switch is assigned to only the
Red VLAN, the assignment can remain "untagged" because the port will
forward traffic only for the Red VLAN. However, if both the Red and Green
VLANs are assigned to port A7, then at least one of those VLAN assignments
must be "tagged" so that Red VLAN traffic can be distinguished from Green
VLAN traffic. The following illustration shows this concept:
Figure 2-24. Example of Tagged and Untagged VLAN Port Assignments
2-30
Blue
Blue
VLAN
Server
Red
Red VLAN: Untagged
6
5
VLAN
4
Green VLAN: Tagged
Switch
7
"X"
3
Red
2 1
Server
Green
Green
VLAN
Server
Ports 1-6: Untagged
Port 7: Red VLAN Untagged
Green VLAN Tagged
White
White
VLAN
Server
4 3
Switch
5
"Y"
1 2
Red Green
VLAN VLAN
Ports 1-4: Untagged
Port 5: Red VLAN Untagged
Green VLAN Tagged