Allied Telesis AT-S41 - Manual (Page 93)

AT-S41 User’s Guide
93
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 difference
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 to have 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 a switch will be untagged members of
the new VLAN. Consequently, you would assign Ports 1 to 4 PVIDs of 4,
the 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 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 a
connection 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
An AT-8326GB stack 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 ports are called untagged because the switch assumes that the
frames received on this type of port will not contain any information that
indicates VLAN membership and that VLAN membership will be
determined solely by a port’s PVID. (This contrasts with tagged ports,