Allied Telesis AT-S62 - Manual (Page 395)

AT-S62 User’s Guide
Section V: Virtual LANs 395
Tagged VLAN Overview
The second type of user-configured VLAN is the tagged VLAN. VLAN
membership in a tagged VLAN is determined by information within the
frames that are received on a port. This differs from a port-based VLAN,
where the PVIDs assigned to the ports determine VLAN membership.
The VLAN information within an Ethernet frame is referred to as a tag or
tagged header. A tag, which follows the source and destination
addresses in a frame, contains the VID of the VLAN to which the frame
belongs (IEEE 802.3ac standard). As explained earlier in this chapter in
VLAN Identifier on page 388, this number uniquely identifies each VLAN
in a network.
When a switch receives a frame with a VLAN tag, referred to as a tagged
frame, the switch forwards the frame only to those ports that are
members of the VLAN whose VID matches the tag in the frame.
A port receiving or transmitting tagged frames is referred to as a tagged
port. Any network device connected to a tagged port must be IEEE
802.1Q-compliant. This is the standard that outlines the requirements
and standards for tagging. The device must be able to process the
tagged information on received frames and add tagged information to
transmitted frames.
The benefit of a tagged VLAN is that the tagged ports can belong to
more than one VLAN at one time. This can greatly simplify the task of
adding shared devices to the network. For example, a server can be
configured to accept and return packets from many different VLANs
simultaneously.
Tagged VLANs are also useful where multiple VLANs span across
switches. You can use one port per switch to connect all VLANs on the
switch to another switch.
The IEEE 802.1Q standard deals with how this tagging information is
used to forward the traffic throughout the switch. The handling of
frames tagged with VIDs coming into a port is straightforward. If the
incoming frame’s VID tag matches one of the VIDs of a VLAN of which
the port is a tagged member, the frame is accepted and forwarded to
the appropriate ports. If the frame’s VID does not match any of the
VLANs that the port is a member of, the frame is discarded.
The parts of a tagged VLAN are much the same as those for a port-based
VLAN. They are:
❑ VLAN Name
❑ VLAN Identifier