In computer networking, a single layer-2 network may be partitioned to create multiple distinct broadcast domains, which are mutually isolated so that packets can only pass between them via one or more routers; such a domain is referred to as a virtual local area network, virtual LAN or VLAN. A virtual local area network (VLAN) is a logical group of workstations, servers and network devices that appear to be on the same LAN despite their geographical distribution. A VLAN allows a network of computers and users to communicate in a simulated environment as if they exist in a single LAN and are sharing a single broadcast and multicast domain.

1. VLAN (Virtual Local Area Network)
In computer networking, a single layer-2 network may be partitioned to create multiple distinct
broadcast domains, which are mutually isolated so that packets can only pass between them via one or
more routers; such a domain is referred to as a virtual local area network, virtual LAN or VLAN.
A virtual local area network (VLAN) is a logical group of workstations, servers and network devices that
appear to be on the same LAN despite their geographical distribution. A VLAN allows a network of
computers and users to communicate in a simulated environment as if they exist in a single LAN and are
sharing a single broadcast and multicast domain. VLANs are implemented to achieve scalability, security
and ease of network management and can quickly adapt to change in network requirements and
relocation of workstations and server nodes.
Higher-end switches allow the functionality and implementation of VLANs. The purpose of
implementing a VLAN is to improve the performance of a network or apply appropriate security
features.
A VLAN allows several networks to work virtually as an LAN. One of the most beneficial elements of a
VLAN is that it removes latency in the network, which saves network resources and increases network
efficiency. In addition, VLANs are created to provide segmentation and assist in issues like security,
network management and scalability. Traffic patterns can also easily be controlled by using VLANs.
VLANs are configured through software rather than hardware, which makes them extremely flexible.
One of the biggest advantages of VLANs is that when a computer is physically moved to another
location, it can stay on the same VLAN without any hardware reconfiguration.
Figure 1 Physical View

2. VLAN (Virtual Local Area Network)
Physical n Logical view of a VLAN
VLAN's also allow broadcast domains to be defined without using routers. Bridging software is used
instead to define which workstations are to be included in the broadcast domain. Routers would only
have to be used to communicate between two VLAN's
VLANs can be used to partition a local network into several distinctive segments, for example:
1. Production
2. Voice over IP
3. Network management
4. Storage area network (SAN)
5. Guest network
6. Demilitarized zone (DMZ)
7. Client separation (ISP)
In a common infrastructure shared across VLAN trunks can provide a very high level of security with
great flexibility for a comparatively low cost. Quality of Service schemes can optimize traffic on trunk
links for realtime (VoIP) or low-latency requirements (Storage Area Network).
History?
After successful experiments with Voice over Ethernet from 1981 to 1984, Dr. W. David Sincoskie joined
Bellcore and began addressing the problem of scaling up Ethernet networks. At 10 Mbit/s, Ethernet was
faster than most alternatives at the time; however, Ethernet was a broadcast network and there was no
good way of connecting multiple Ethernet networks together. This limited the total bandwidth of an
Ethernet network to 10 Mbit/s and the maximum distance between any two nodes to a few hundred
feet.
Why use VLAN's?
VLAN's offer a number of advantages over traditional LAN's. They are:
Figure 2 Logical View

3. VLAN (Virtual Local Area Network)
1) Performance
In networks where traffic consists of a high percentage of broadcasts and multicasts, VLAN's can reduce
the need to send such traffic to unnecessary destinations. For example, in a broadcast domain consisting
of 10 users, if the broadcast traffic is intended only for 5 of the users, then placing those 5 users on a
separate VLAN can reduce traffic.
Compared to switches, routers require more processing of incoming traffic. As the volume of traffic
passing through the routers increases, so does the latency in the routers, which results in reduced
performance. The use of VLAN's reduces the number of routers needed, since VLAN's create broadcast
domains using switches instead of routers.
2) Formation of Virtual Workgroups
Nowadays, it is common to find cross-functional product development teams with members from
different departments such as marketing, sales, accounting, and research. These workgroups are usually
formed for a short period of time. During this period, communication between members of the
workgroup will be high. To contain broadcasts and multicasts within the workgroup, a VLAN can be set
up for them. With VLAN's it is easier to place members of a workgroup together. Without VLAN's, the
only way this would be possible is to physically move all the members of the workgroup closer together.
However, virtual workgroups do not come without problems. Consider the situation where one user of
the workgroup is on the fourth floor of a building, and the other workgroup members are on the second
floor. Resources such as a printer would be located on the second floor, which would be inconvenient
for the lone fourth floor user.
Another problem with setting up virtual workgroups is the implementation of centralized server farms,
which are essentially collections of servers and major resources for operating a network at a central
location. The advantages here are numerous, since it is more efficient and cost-effective to provide
better security, uninterrupted power supply, consolidated backup, and a proper operating environment
in a single area than if the major resources were scattered in a building. Centralized server farms can
cause problems when setting up virtual workgroups if servers cannot be placed on more than one VLAN.
In such a case, the server would be placed on a single VLAN and all other VLAN's trying to access the
server would have to go through a router; this can reduce performance.
3) Simplified Administration
Seventy percent of network costs are a result of adds, moves, and changes of users in the network [
Buerger]. Every time a user is moved in a LAN, recabling, new station addressing, and reconfiguration of
hubs and routers becomes necessary. Some of these tasks can be simplified with the use of VLAN's. If a
user is moved within a VLAN, reconfiguration of routers is unnecessary. In addition, depending on the
type of VLAN, other administrative work can be reduced or eliminated [ Cisco white paper]. However

4. VLAN (Virtual Local Area Network)
the full power of VLAN's will only really be felt when good management tools are created which can
allow network managers to drag and drop users into different VLAN's or to set up aliases.
Despite this saving, VLAN's add a layer of administrative complexity, since it now becomes necessary to
manage virtual workgroups.
4) Reduced Cost
VLAN's can be used to create broadcast domains which eliminate the need for expensive routers.
5) Security
Periodically, sensitive data may be broadcast on a network. In such cases, placing only those users who
can have access to that data on a VLAN can reduce the chances of an outsider gaining access to the data.
VLAN's can also be used to control broadcast domains, set up firewalls, restrict access, and inform the
network manager of an intrusion.
VLANs also have some disadvantages and limitations as listed below:
1. High risk of virus issues because one infected system may spread a virus through the whole
logical network.
2. Equipment limitations in very large networks because additional routers might be needed to
control the workload.
3. More effective at controlling latency than a WAN but less efficient than a LAN.
Establishing VLAN Memberships?
The two common approaches to assigning VLAN membership are as follows:
 Static VLANs
 Dynamic VLANs
Static VLANs are also referred to as port-based VLANs. Static VLAN assignments are created by assigning
ports to a VLAN. As a device enters the network, the device automatically assumes the VLAN of the port.
If the user changes ports and needs access to the same VLAN, the network administrator must manually
make a port-to-VLAN assignment for the new connection.
Dynamic VLANs are created using software. With a VLAN Management Policy Server (VMPS), an
administrator can assign switch ports to VLANs dynamically based on information such as the source
MAC address of the device connected to the port or the username used to log onto that device. As a
device enters the network, the switch queries a database for the VLAN membership of the port that
device is connected to.

5. VLAN (Virtual Local Area Network)
Types of VLAN's
VLAN membership can be classified by port, MAC address, and protocol type:
1) Layer 1 VLAN: Membership by Port
Membership in a VLAN can be defined based on the ports that belong to the VLAN. For example, in a
bridge with four ports, ports 1, 2, and 4 belong to VLAN 1 and port 3 belongs to VLAN 2.
Port VLAN
1 1
2 1
3 2
4 1
Assignment of ports to different VLAN's
The main disadvantage of this method is that it does not allow for user mobility. If a user moves to a
different location away from the assigned bridge, the network manager must reconfigure the VLAN.
2) Layer 2 VLAN: Membership by MAC Address
Here, membership in a VLAN is based on the MAC address of the workstation. The switch tracks the
MAC addresses which belong to each VLAN (see Figure4). Since MAC addresses form a part of the
workstation's network interface card, when a workstation is moved, no reconfiguration is needed to
allow the workstation to remain in the same VLAN. This is unlike Layer 1 VLAN's where membership
tables must be reconfigured.
MAC Address VLAN
1212354145121 1
2389234873743 2
3045834758445 2
5483573475843 1
Assignment of MAC addresses to different VLAN's
The main problem with this method is that VLAN membership must be assigned initially. In networks
with thousands of users, this is no easy task. Also, in environments where notebook PC's are used, the
MAC address is associated with the docking station and not with the notebook PC. Consequently, when
a notebook PC is moved to a different docking station, its VLAN membership must be reconfigured.
3) Layer 2 VLAN: Membership by Protocol Type
VLAN membership for Layer 2 VLAN's can also be based on the protocol type field found in the Layer 2
header.

6. VLAN (Virtual Local Area Network)
Protocol VLAN
IP 1
IPX 2
Assignment of protocols to different VLAN's
4) Layer 3 VLAN: Membership by IP Subnet Address
Membership is based on the Layer 3 header. The network IP subnet address can be used to classify
VLAN membership.
IP Subnet VLAN
23.2.24 1
26.21.35 2
Assignment of IP subnet addresses to different VLAN's
Although VLAN membership is based on Layer 3 information, this has nothing to do with network
routing and should not be confused with router functions. In this method, IP addresses are used only as
a mapping to determine membership in VLAN's. No other processing of IP addresses is done.
In Layer 3 VLAN's, users can move their workstations without reconfiguring their network addresses.
The only problem is that it generally takes longer to forward packets using Layer 3 information than
using MAC addresses.
5) Higher Layer VLAN's
It is also possible to define VLAN membership based on applications or service, or any combination
thereof. For example, file transfer protocol (FTP) applications can be executed on one VLAN and telnet
applications on another VLAN.
The 802.1Q draft standard defines Layer 1 and Layer 2 VLAN's only. Protocol type based VLAN's and
higher layer VLAN's have been allowed for, but are not defined in this standard. As a result, these
VLAN's will remain proprietary.
Types of Connections
Devices on a VLAN can be connected in three ways based on whether the connected devices are VLAN-
aware or VLAN-unaware. Recall that a VLAN-aware device is one which understands VLAN memberships
(i.e. which users belong to a VLAN) and VLAN formats.
1) Trunk Link
All the devices connected to a trunk link, including workstations, must be VLAN-aware. All frames on a
trunk link must have a special header attached. These special frames are called tagged frames.

7. VLAN (Virtual Local Area Network)
2) Access Link
An access link connects a VLAN-unaware device to
the port of a VLAN-aware bridge. All frames on
access links must be implicitly tagged (untagged).
The VLAN-unaware device can be a LAN segment
with VLAN-unaware workstations or it can be a
number of LAN segments containing VLAN-unaware
devices (legacy LAN).
3) Hybrid Link
This is a combination of the previous two links. This is a link where both VLAN-aware and VLAN-unaware
devices are attached. A hybrid link can have both tagged and untagged frames, but all the frames for a
specific VLAN must be either tagged or untagged.
It must also be noted that the network can have a combination of all three types of links.
Figure 3 Trunk link between two VLAN-aware bridges.
Figure 4 Access link between a VLAN-aware
bridge and a VLAN-unaware device.
Figure 5 Hybrid link containing both VLAN-
aware and VLAN-unaware devices.

8. VLAN (Virtual Local Area Network)
Default Ethernet VLAN Configuration
The switch module supports only Ethernet interfaces. Table shows the default configuration for
Ethernet VLANs.
Parameter Default Range
VLAN ID 1 1 to 4094.
Note Extended-range VLANs (VLAN IDs 1006 to
4094) are not saved in the VLAN database.
VLAN name VLANxxxx , where xxxx represents four
numeric digits (including leading zeros)
equal to the VLAN ID number
No range
802.10 SAID 100001 (100000 plus the VLAN ID) 1 to 4294967294
MTU size 1500 1500 to 9198
Translational
bridge 1
0 0 to1005
Translational
bridge 2
0 0 to1005
VLAN state active active, suspend
Remote SPAN disabled enabled, disabled
Private VLANs none configured 2 to 1001, 1006 to 4094.
UNI-ENI VLAN UNI-ENI isolated VLAN 2 to 1001, 1006 to 4094.
VLAN 1 is always a UNI-ENI isolated VLAN.
Protocol-based VLANs?
In a switch that supports protocol-based VLANs, traffic is handled on the basis of its protocol.
Essentially, this segregates or forwards traffic from a port depending on the particular protocol of that
traffic; traffic of any other protocol is not forwarded on the port.
For example, it is possible to connect the following to a given switch:
1. A host generating ARP traffic to port 10
2. A network with IPX traffic to port 20
3. A router forwarding IP traffic to port 30

9. VLAN (Virtual Local Area Network)
If a protocol-based VLAN is created that supports IP and contains all three ports, this prevents IPX traffic
from being forwarded to ports 10 and 30, and ARP traffic from being forwarded to ports 20 and 30,
while still allowing IP traffic to be forwarded on all three ports.
Protocols and design?
1) IEEE 802.1Q
2) Cisco VLAN Trunking Protocol (VTP)
3) Multiple VLAN Registration Protocol
4) Shortest Path Bridging
Creating or Modifying an Ethernet VLAN
To access VLAN configuration mode, enter the vlan global configuration command with a VLAN ID. Enter
a new VLAN ID to create a VLAN, or enter an existing VLAN ID to modify that VLAN. You can use the
default VLAN configuration or enter commands to configure the VLAN.
Switch# configure terminal
Switch(config)# vlan 20
Switch(config-vlan)# name test20
Switch(config-vlan)# end
Assigning Static-Access Ports to a VLAN
You can assign a static-access port to a VLAN.
Switch# configure terminal
Enter configuration commands, one per line. End with CNTL/Z.
Switch(config)# interface fastethernet0/1
Switch(config-if)# switchport mode access
Switch(config-if)# switchport access vlan 2
Switch(config-if)# end
Configuring a Trunk Port
Beginning in privileged EXEC mode, follow these steps to configure a port as an 802.1Q trunk port:
Switch# configure terminal
Enter configuration commands, one per line. End with CNTL/Z.
Switch(config)# interface fastethernet0/2
Switch(config-if)# switchport mode trunk
Switch(config-if)# switchport trunk native vlan 33
Switch(config-if)# end