Chapter 7: Virtual LANs
Tagged VLAN Overview
The second type of VLAN supported by the
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). This number, as explained earlier in “VLAN Identifier” on page 74, 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 that receives and transmits tagged frames is referred to as a tagged port. Any network device connected to a tagged port must be IEEE
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 much the same as those for a
VLAN. They are:
VLAN Name
VLAN Identifier
Tagged and Untagged Ports
Port VLAN Identifier
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