Download User's Guide - Allied Telesis

Section II: Local and Telnet Management
Untagged and
Tagged Ports
There are two kinds of ports that you can assign to an IEEE 802.1Qcompliant VLAN: tagged ports and untagged ports. The basic different
between the two is that an untagged port can be a member of only one
VLAN at a time while a tagged port can be a member of multiple VLANs.
Untagged Ports
When the ports on an Ethernet switch are divided into independent
VLANs, the switch needs a mechanism for determining which ports
belong to which VLANs. For instance, if a switch needs to broadcast a
frame to the ports of a particular VLAN, it needs to know which ports
comprise the VLAN.
In a VLAN that consists of untagged ports, port membership is
determined by what is referred to as the port VLAN identifier (PVID). This
is a number that you must assign to a port when you assign it as an
untagged member of a VLAN. The PVID of a port will be the same as the
VID of the VLAN in which the port is to be an untagged member.
Here is an example. Let’s assume that you are creating a new VLAN
called Sales and that you assigned the VLAN a VID of 4. You have
decided that Ports 1 through 4 on the switch will be untagged members
of the new VLAN. Consequently, you would assign Ports 1 to 4 PVIDs of 4,
same as the VID. Now, when the switch receives a frame on one of the
ports on the Sales VLAN and it needs to broadcast the frame to the other
ports of the VLAN, it will know that the Sales VLAN consists of Ports 1 to
4.
A VLAN that consists of only untagged ports is referred to as an untagged
VLAN. In order for frames from untagged VLANs to cross a VLAN
boundary, there must be a Layer 3 switch or router providing an
interconnection between the VLANs.
You can assign each port only one PVID. Consequently, a port can be an
untagged member of only one VLAN at a time.
Note
The AT-9410GB Switch is pre-configured with one untagged VLAN,
called the Default VLAN. All ports on the switch are members of this
VLAN. The Default VLAN has a VID of 1. Consequently, all the ports in
the VLAN have a PVID value of 1.
The reason the ports are called untagged is because it is assumed that
the frames received on this type of port will not to contain any
information that indicates VLAN membership, and that VLAN
membership will be determined solely by a port’s PVID. (This contrasts
90