Allied Telesis AT-S25 - Manual (Page 147)

AT-S25 Version 1.4 User’s Guide
147
Tagged VLAN
The second type of VLAN is referred to as a tagged VLAN. With a tagged
VLAN, VLAN membership is determined by information within the
frames that are received on a port. This contrasts to a port-based VLAN,
where the PVIDs assigned to the ports determine VLAN membership.
The VLAN information within the frames 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 141, 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 main 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 and
interconnecting VLANs that span multiple switches. For example, a
server can be configured to accept and return packets from many
different VLANs simultaneously. Additionally, 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 will be 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 will be discarded.