Allied Telesis AT-S39 - Manual (Page 129)

AT-S39 User’s Guide
129
❑ Port VLAN Identifier
Note
For explanations of VLAN name and VLAN identifier, refer back to
VLAN Name and VLAN Identifier on page 121.
Tagged and Untagged Ports
You must specify which ports are members of the VLAN. In the case of a
tagged VLAN, VLAN members are usually a combination of both tagged
and untagged ports. When you create the VLAN, you specify which ports
are tagged and which ports are untagged.
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.
Port VLAN Identifier
As explained earlier in the discussion on port-based VLANs, the
management 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.
Because a tagged port determines VLAN membership by examining the
tagged header within the frames that it receives and not by the PVID,
you might conclude there is no need for a PVID. However, the PVID is
used if a tagged port receives an untagged frame—a frame without any
tagged information. The port forwards the frame based on the port’s
PVID. This is only in cases where an untagged frame arrives on a tagged
port. Otherwise, the PVID of a port is ignored on a tagged port.
General Rules to Creating a Tagged VLAN
Below is a summary of the rules to observe when creating a tagged
VLAN.
❑ Assign each tagged VLAN a unique VID. If a particular VLAN spans
multiple switches or stacks, each part of that VLAN on the
different switches or stacks must be assigned the same VID.
❑ A tagged port can be a member of multiple VLANs.
❑ An untagged port can be an untagged member of only one VLAN
at a time.