Allied Telesis AT-S41 - Manual (Page 93)

AT-S41 User’s Guide
93
Tagged Ports
The second type of port that can be a member of a VLAN is called a
tagged port. There are several principal differences between a tagged
port and an untagged port.
As explained earlier, a switch determines the VLAN membership of a
frame received on an untagged port by examining the PVID that you
assigned to the port.
But when a frame is received on a tagged port, the switch examines the
frame itself to 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).
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.
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 tagged ports is that they 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, an IEEE 802.1Q-compliant 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.
So how do you indicate which ports are to be tagged and which are to
be untagged when you create a VLAN? The rule is straightforward. If you
assign a port to only one VLAN, the switch assumes it is to be an
untagged port. If you assign a port to more than one VLAN, the switch
assumes that the port is to be both a tagged and untagged port.