User`s guide

AT-S45 User’s Guide

Untagged and

Tagged Ports

There are two kinds of ports that you can assign to an IEEE 802.1Q-

compliant 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

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