Allied Telesis AT-S39 - Manual (Page 128)

Section II: Local and Telnet Management
128
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 122, this number uniquely identifies each
VLAN in a network.
When a tagged port 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. This is true only when a tagged port is tagged to the
destination VLAN.
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 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.
The parts of a tagged VLAN are much the same as those for a port-based
VLAN. They are:
❑ VLAN Name
❑ VLAN Identifier
❑ Tagged and Untagged Ports
❑ Port VLAN Identifier
Note
For explanations of VLAN name and VLAN identifier, refer back to
VLAN Name and VLAN Identifier on page 122.