Allied Telesis AT-S25 - Manual (Page 148)

Introduction to Virtual LANs
148
Parts of a Tagged
VLAN
The parts of a tagged VLAN are much the same as those for a port-based
VLAN. They are:
❑ VLAN Name
❑ VLAN Identifier
❑ Port VLAN Identifier
❑ Tagged and Untagged Ports
Each item is described in the following sections.
Note
For explanations of VLAN name and VLAN identifier, refer back to
VLAN Name and VLAN Identifier on page 141.
Port VLAN Identifier
As explained earlier in this appendix in the discussion on port-based
VLANs, the AT-S25 software automatically assigns a PVID to each port
when a port is made a member of a VLAN. The PVID is always identical to
the VLAN’s VID, and that in a port-based VLAN packets are forwarded
based on the PVID.
But since a tagged port determines VLAN membership by examining the
tagged header within the frames that it receives, there would seem to be
no need for a PVID. But actually there is. If a tagged port receives an
untagged frame (that is, a frame without any tagged information), the
port will forward the frame based on the ports PVID. But this is only in
the case where untagged frames arrive on tagged ports. Otherwise, the
PVID of a port is ignored on a tagged port.
Tagged and Untagged Ports
You need to specify which ports will be members of the VLAN. In the
case of a tagged VLAN, it will usually be a combination of both untagged
ports and tagged ports. You will specify which ports will be tagged and
which untagged when you create the VLAN with the AT-S25
management software.
An untagged port, whether a member of a port-based VLAN or a tagged
VLAN, can be in only one VLAN at a time. However, a tagged port can be
a member of more than one VLAN. A port can also be an untagged
member of one VLAN and a tagged member of different VLANs,
simultaneously.