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Chapter 47: Port-based and Tagged VLANs
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Tagged VLAN Overview
The second type of 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 690, this number uniquely identifies each VLAN in a
network.
When the 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 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 describes 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 similar to those for a port-based VLAN.
They are:
VLAN Name
VLAN Identifier
Tagged and Untagged Ports
Port VLAN Identifier