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Management Switch Card User’s Guide
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VLAN Screens
15.1 VLAN Introduction
A VLAN (Virtual Local Area Network) allows a physical network to be partitioned into multiple logical
networks. Devices on a logical network belong to one group. A device can belong to more than one
group. With VLAN, a device cannot directly talk to or hear from devices that are not in the same
group(s); the traffic must first go through a router.
In MTU (Multi-Tenant Unit) applications, VLAN is vital in providing isolation and security among the
subscribers. When properly configured, VLAN prevents one subscriber from accessing the network
resources of another on the same LAN, thus a user will not see the printers and hard disks of
another user in the same building. If you have enabled port isolation in the
Switch Setup screen,
you do not need to configure the VLAN to isolate subscribers.
VLAN also increases network performance by limiting broadcasts to a smaller and more
manageable logical broadcast domain. In traditional switched environments, all broadcast packets
go to each and every individual port. With VLAN, all broadcasts are confined to a specific broadcast
domain.
15.2 IEEE 802.1Q Tagged VLAN
The IEEE 802.1Q Tagged VLAN uses both explicit and implicit tagging.
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Explicit Tagging
A VLAN identifier is added to the frame header that identifies the source VLAN.
2
Implicit Tagging
The MAC (Media Access Control) number, the port or other information is used to identify the source
of a VLAN frame.
Tagged VLAN uses an explicit tag (VLAN ID) in the MAC header to identify the VLAN membership of
a frame across bridges - they are not confined to the switch on which they were created. The VLANs
can be created statically by hand or dynamically through GVRP. The VLAN ID associates a frame
with a specific VLAN and provides the information that switches need to process the frame across
the network. A tagged frame is four bytes longer than an untagged frame and contains two bytes of
TPID (Tag Protocol Identifier, residing within the type/length field of the Ethernet frame) and two
bytes of TCI (Tag Control Information, starts after the source address field of the Ethernet frame).
The CFI (Canonical Format Indicator) is a single-bit flag, always set to zero for Ethernet switches. If
a frame received at an Ethernet port has a CFI set to 1, then that frame should not be forwarded as
it is to an untagged port. The remaining twelve bits define the VLAN ID, giving a possible maximum
number of 4,096 VLANs. Note that user priority and VLAN ID are independent of each other. A