Allied Telesis AT-S87 - Manual (Page 67)

AT-S87 Management Software User’s Guide
67
Tagged VLAN Overview
The second type of VLAN supported by the AT-S87 management software
is the tagged VLAN. VLAN membership in a tagged VLAN is determined
by information within the frames that are received on a port and the VLAN
configuration of each port.
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 Group ID of the VLAN to which the frame belongs
(IEEE 802.3ac standard). This number 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 whose Group ID
equals the VLAN tag.
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 parts of a tagged VLAN are:
ˆ VLAN Name
ˆ Group ID
ˆ Tagged and Untagged Ports
ˆ Port VLAN identifier (PVID)
Tagged and
Untagged Ports
When you specify that a port is a member of a tagged VLAN, you need to
specify that it is tagged or untagged. You can have a combination of
tagged and untagged ports in the same VLAN.
Packet transmission from a tagged port differs from packet transmission
from an untagged port. When a packet is transmitted from a tagged port,
the tagged information within the packet is maintained when it is
transmitted to the next network device. If the packet is transmitted from an
untagged port, the VLAN tag information is removed from the packet
before it is transmitted to the next network device.
The IEEE 802.1Q standard describes how the tagging information within a
packet is used to forward the traffic throughout the switch. The handling of
packets tagged with a VLAN ID coming into a port is straightforward, If the
incoming packet’s VLAN tag matches one of the Group IDs of which the
port is a member, the packet is accepted and forwarded to the appropriate
port(s) within that VLAN. If the incoming packet’s VLAN tag does not