AT-S41 User’s Guide
94
explained next, where VLAN membership is determined by information
within the frames themselves.) Frames received on untagged ports and
lacking any VLAN identifying information are referred to as untagged
frames.
When a switch receives a frame on an untagged port, it first examines
the PVID of the port on which the frame was received and then adds the
PVID to the frame itself. It then examines the destination MAC address of
the frame. If the destination address is in the MAC address table and if
the switch port where the destination node is located is part of the same
VLAN as the port that received the frame, the switch sends the frame out
the port to the destination node.
If the destination MAC address is not in the MAC address table, the
switch broadcasts the frame to all the ports that share the same PVID as
the port that received the frame.
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.
