Allied Telesis AT-S60 - Manual (Page 412)

Chapter 18: Virtual LANs
Section IV: VLANs 412
Tagged VLAN Overview
The second type of VLAN supported by the AT-8400 Series switch is the
tagged VLAN. Tagged VLANs use information inside tagged frames as
they are received on the ports to determine VLAN membership. This
contrasts with port-based VLANs, 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. As explained earlier in this chapter in VLAN Identifier on page
404, 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 share the
same VID.
A port to receive or transmit 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 within the VLAN
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 for connecting 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 that the
port is a tagged member of, 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.