Allied Telesis AT-S62 - Manual (Page 555)

AT-S62 Management Software Menus Interface User’s Guide
Section V: VLANs 555
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 548, 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:
ˆ VLAN Name
ˆ VLAN Identifier
ˆ Tagged and Untagged Ports
ˆ Port VLAN Identifier