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PT-7728 User’s Manual Featured Functions
3-41
VLANs: Tagged and Untagged Membership
The PT-7728 supports 802.1Q VLAN tagging, a system that allows traffic for multiple VLANs to
be carried on a single physical (backbone, trunk) link. When setting up VLANs you need to
understand when to use untagged and tagged membership of VLANs. Simply put, if a port is on a
single VLAN it can be an untagged member, but if the port needs to be a member of multiple
VLANs, tagged membership must be defined.
A typical host (e.g., clients) will be untagged members of one VLAN, defined as Access Port in
PT-7728, while inter-switch connections will be tagged members of all VLANs, defined as Trunk
Port in PT-7728.
The IEEE Std 802.1Q-1998 defines how VLANs operate within an open packet-switched network.
An 802.1Q compliant packet carries additional information that allows a switch to determine
which VLAN the port belongs to. If a frame is carrying the additional information, it is known as a
tagged frame.
To carry multiple VLANs across a single physical (backbone, trunk) link, each packet must be
tagged with a VLAN identifier so that the switches can identify which packets belong in which
VLAN. To communicate between VLANs, a router must be used.
The PT-7728 supports two types of VLAN port settings:
y Access Port: The port connects to a single device that is not tagged. The user must define the
default port PVID that assigns which VLAN the device belongs to. Once the ingress packet of
this Access Port egresses to another Trunk Port (the port nePT all packets to carry tag
information), PT-7728 will insert this PVID into this packet to help the next 802.1Q VLAN
switch recognize it.
y Trunk Port: The port connects to a LAN that consists of untagged devices/tagged devices
and/or switches and hubs. In general, the traffic of the Trunk Port must have a Tag. Users can
also assign PVID to a Trunk Port. The untagged packet on the Trunk Port will be assigned the
port default PVID as its VID.
The following section illustrates how to use these ports to set up different applications.
Sample Applications of VLANs using PT-7728