Allied Telesis AT-S25 - Manual (Page 126)

Section II: Local and Telnet Mangement
126
VLAN Name For an explanation of VLAN Name, refer to VLAN Name on page 120.
VLAN Identifier For an explanation of VLAN Identifier, refer to VLAN Identifier on page
120.
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 specify which ports will be tagged and
which untagged when you create the VLAN.
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, and that in a port-based VLAN packets are forwarded based
on the PVID.
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. The PVID is used 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 port’s PVID.
But this is only in cases where untagged frames arrive on tagged ports.
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.
❑ The AT-8300 Series switch can support up to 255 VLANs.
❑ Each tagged VLAN must be assigned a unique VID. If a particular
VLAN spans multiple switches, each part of the VLAN on the
different switches 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.
❑ Each tagged port is required to have a PVID. If the port’s untagged
membership is to be removed from a port, the switch will
automatically set the PVID to ‘one’. This may not be obvious
unless you view the port status or reboot the switch.