Vlan Tagging Information - HP 4108GL Management And Configuration Manual

Port-Based Virtual LANs (VLANs) and GVRP
Port-Based Virtual LANs (Static VLANs)
3. Click on [Add/Remove VLANs].
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VLAN Tagging Information

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. In the Switch 4108GL 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 assignment 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:
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