Zyxel OLT2412 [89/326] Introduction to vlans

OLT2412 User’s Guide 89

CHAPTER 9

VLAN

9.1 Introduction to VLANs

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 on the same network.

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.

Note: VLAN is unidirectional; it only governs outgoing traffic.

9.2 Introduction to IEEE 802.1Q Tagged VLANs

A 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 for the TPID (Tag Protocol Identifier, residing within the type/length field of the Ethernet

frame) and two bytes for the TCI (Tag Control Information, starting 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

frame with VID (VLAN Identifier) of null (0) is called a priority frame, meaning that only the priority

level is significant and the default VID of the ingress port is given as the VID of the frame. Of the

4096 possible VIDs, a VID of 0 is used to identify priority frames and the value 4095 (FFF) is

reserved, so the maximum possible number of VLAN configurations is 4,094.

TPID

2 Bytes

User Priority

3 Bits

CFI

1 Bit

VLAN ID

12 bits