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VLAN AND IT’S IMPLEMENTATION

Submitted in

Partial fulfillment of the requirement for the award of

the degree

Bachelor of Computer Application

By

MOHIT KUMAR

Under Guidance of

MR. MOHD. ASAD SIDDIQUI

Centre of Computer Education

Institute of Professional Studies

University Of Allahabad

Allahabad

2013

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CONTENT

1. Acknowledgement

2. Certificate

3. Certificate of Approval

4. Problem Definition-Defining the problem.

5. Introduction

6. Network and It‘s Types

7. Actual Topic- VLAN

8. VLAN Membership

9. VTP

10. Implementation

11. Conclusion

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ACKNOWLEDGEMENT

I would like to express my heartfelt gratitude to my project guide ‗Mr. MOHD ASAD

SIDDIQUI‘ department of Computer Science, University Of Allahabad for his guidance,

support and timely advice. I could not have completed this project without his

encouragement and valuable suggestions.

My heartfelt debt and thanks goes to my teachers Mr. AMIT KUMAR SINGH and Mr.

R. K. Pandey , Centre of Computer Education, Institute of Professional Studies,

University of Allahabad for their advice and encouragement during the past years.

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CERTIFICATE

It is certified that Mr. MOHIT KUMAR of Bachelor of Computer Application, Centre of

Computer Education, Institute of Professional Studies, University of Allahabad has

carried out the project work on ‗VLAN AND IT’S IMPLEMENTATION‘ under my

guidance. The student has tried to understand the involved concepts. To the best of my

knowledge a similar work has not been submitted at any other institution for the award of

any degree or diploma.

MR. MOHD. ASAD SIDDIQUI

Resource Person

Centre of Computer Education

Institute of Professional Studies

University Of Allahabad

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CERITIFICATE OF APPROVAL

This is to certify that the project entitled ‗VLAN AND IT’S IMPLEMENTATION‘

submitted by:

MOHIT KUMAR

is in the partial fulfillment of the requirement for the award of the degree of Bachelor of

Computer Application awarded by the University of Allahabad, Allahabad.

Internal Examiner External Examiner

Course Coordinator

Centre of Computer Education

Institute of Professional Studies

University Of Allahabad

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PROBLEM DEFINITION

A station is considered part of a LAN if it physically belongs to that LAN. The

criterion of membership is geographic. What happens if we need a virtual connection

between two stations belonging to two different physical LANs? We can roughly define a

virtual local area network (VLAN) as a local area network configured by software, not by

physical wiring using VTP(VLAN Trunk Protocol).

NOW THE QUESTION OCCURES WHAT IS :

NETWORK

TYPES OF NETWORK

LAN (LOCAL AREA NETWORK)

WAN(WIDE AREA NETWORK)

VLAN

WORKING OF VLAN

CREATION OF VLAN

In this project, you‘re going to learn, in detail, exactly what a VLAN is and how

VLAN is created and how VLAN memberships are used in a switched network. Also,

I‘m going to tell you all about how VLAN Trunk Protocol (VTP) is used with VLAN

information and how trunking is used to send information from all VLANs across a

single link.

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Introduction

Network: A network consists of two or more computers that are linked in order to

share resources (such as printers and CDs), exchange files, or allow electronic

communications. The computers on a network may be linked through cables,

telephone lines, radio waves, satellites, or infrared light beams.

A computer network can also be defined as: A computer network or data

network is a telecommunications network that allows computers to exchange data.

In computer networks, networked computing devices pass data to each other along

data connections.

TYPES OF NETWORK:

LAN- Local Area Network

WLAN - Wireless Local Area Network

WAN- Wide Area Network

MAN- Metropolitan Area Network

VLAN-VIRTUAL LAN

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LAN (LOCAL AREA NETWORK): A LAN connects network devices over a

relatively short distance. A networked office building, school, or home usually

contains a single LAN, though sometimes one building will contain a few small

LANs (perhaps one per room), and occasionally a LAN will span a group of

nearby buildings.

A local area network (LAN) is a group of computers and associated devices that

share a common communications line or wireless link. Typically, connected

devices share the resources of a single processor or server within a small

geographic area.

The Figure shows a switch connecting three LANs.

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WLAN(WIRELESS LAN): A LAN based on Wi-Fi wireless network

technology. It inherits all the properties of lan istead of it is not hard wired

connected.

A Wireless Local Area Network (WLAN) links two or more devices using some

wireless distribution method (typically spread-spectrum or OFDMradio), and

usually providing a connection through an access point to the wider Internet. This

gives users the ability to move around within a local coverage area and still be

connected to the network. Most modern WLANs are based on IEEE standards,

marketed under the Wi-Fi brand name.

Wireless LANs have become popular in the home due to ease of installation, and in

commercial complexes offering wireless access to their customers; often for free.

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The figure shows a Wi-Fi range.

WAN(WIDE AREA NETWORK):

As the term implies, a WAN spans a large physical distance. The Internet is

the largest WAN, spanning the Earth. A WAN is a geographically-dispersed

collection of LANs. A network device called a router connects LANs to a WAN.

In IP networking, the router maintains both a LAN address and a WAN address.

A WAN differs from a LAN in several important ways. Most WANs (like

the Internet) are not owned by any one organization but rather exist under

collective or distributed ownership and management. WANs tend to use

technology like ATM, Frame Relay connectivity over the longer distances.

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MAN (Metropolitan Area Network):

A network spanning a physical area larger than a LAN but smaller than a

WAN, such as a city. A MAN is typically owned an operated by a single entity

such as a government body or large corporation.

A Metropolitan Area Network (MAN) is a computer network in which

two or more computers or communicating devices or networks which are

geographically separated but in same metropolitan city and are connected to each

other are said to be connected on MAN. Metropolitan limits are determined by

local municipal corporations; the larger the city, the bigger the MAN, the smaller a

metro city, smaller the MAN.

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The problem here starts to become evident as we populate the network with more

switches and workstations. Since most workstations tend to be loaded with the

Windows operating system, this will result in unavoidable broadcasts being sent

occasionally on the network wire - something we certainly want to avoid.

Another major concern is security. In the above network, all users are able to see

all devices. In a much larger network containing critical file servers, databases and

other confidential information, this would mean that everyone would have network

access to these servers and naturally, they would be more susceptible to an attack.

To effectively protect such systems from your network you would need to restrict

access at the network level by segmenting the existing network or simply placing a

firewall in front of each critical system, but the cost and complexity will surely

make most administrators think twice about it. .

WHAT IS VLAN?

Welcome to the wonderful world of VLANs!

All the above problems, and a lot more, can be forgotten with the creation of

VLANs...well, to some extent at least.

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.

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.

Short for virtual LAN, A network of computers that behave as if they are

connected to the same wire even though they may actually be physically located on

different segments of a LAN. 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.

In computer networking, a single layer-2 network may be partitioned to create

multiple distinct broadcast domains, which are mutually isolated so that packets

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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.

Computer networks can be segmented into local area networks (LAN) and wide

area networks (WAN). Network devices such as switches, hubs, bridges,

workstations and servers connected to each other in the same network at a specific

location are generally known as LANs. A LAN is also considered a broadcast

domain.

A VLAN allows several networks to work virtually as a 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. Furthermore, traffic patterns can easily be controlled

by using VLANs.

The original information will need to be resent after waiting for the collision to be

resolved, thereby incurring a significant wastage of time and resources.

To prevent collisions from traveling through all the workstations in the

network, a bridge or a switch can be used. These devices will not forward

collisions, but will allow broadcasts and multicasts to pass through.

A router may be used to prevent broadcasts and multicasts from traveling

through the network.

The workstations, hubs, and repeaters together form a LAN segment.

A LAN segment is also known as a collision domain since collisions remain

within the segment.

The area within which broadcasts and multicasts are confined is called a

broadcast domain or LAN. To define broadcast and collision domains in a LAN

depends on how the workstations, hubs, switches,

and routers are physically connected together by LAN that is located in the same

area.

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The figure shows a switch using VLAN software.

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NEED OF VLAN(VIRTUAL LAN):

These are few reasons why we should use VLANs –

1. To create more flexible designs that group users by department, or by groups

that work together instead of by physical location.

2. To segment devices into smaller LANs (Broadcast Domains) to reduce

overhead caused to each host in the LAN.

3. To reduce the workload for the Spanning Tree Protocol (STP) by limiting a

VLAN to a single access switch.

4. To ensure better security by keeping hosts that work with sensitive data on a

separate VLAN.

5. To separate traffic sent by a IP phone from traffic sent by PCs connected to

the phones.

6. A proper VLAN design can ensure that only devices that have that VLAN

defined on it will receive and forward packets intended as source or

destination of the network flow.

7. VLAN's can be used to create broadcast domains which eliminate the need

for expensive routers.

8. We can break apart our network as needed without having to go and move

cables around; if we used different switches for each group then we would

have many more switches and much more cabling in place to support them.

9. VLANs allow QoS measures to be taken on devices normally fighting for

shared bandwidth.

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TYPES OF VLAN

STATIC VLAN

DYNAMIC VLAN

STATIC VLAN: Creating static VLANs is the most common way to create a VLAN, and one of the

reasons for that is because static VLANs are the most secure. This security stems

from the fact that any switch port you‘ve assigned a VLAN association to will

always maintain it unless you change the port assignment manually.

Static VLAN configuration is pretty easy to set up and supervise, and it works

really well in a networking environment where any user movement within the

network needs to be controlled. It can be helpful to use network management

software to configure the ports, but you don‘t have to use it if you don‘t want to.

DYNAMIC VLAN: A dynamic VLAN determines a node‘s VLAN assignment automatically.

Using intelligent management software, you can enable hardware (Media Access

Control [MAC]) addresses, protocols, or even applications to create dynamic

VLANs; it‘s up to you.

For example, suppose MAC addresses have been entered into a centralized VLAN

management application. If a node is then attached to an unassigned switch port,

the VLAN management database can

look up the hardware address and assign and configure the switch port to the

correct VLAN.

This is very cool—it makes management and configuration easier because if a user

moves, the

switch will assign them to the correct VLAN automatically.

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Other VLAN classification criteria

Up until now, we have been thinking just of port-based VLANs. However, there

are other ways of defining VLAN membership. In this section, we will consider

two examples of these other types of VLAN:

Protocol-based VLANs

Subnet-based VLANs

Protocol-based VLANs With this method, different protocol types are assigned to different VLANs. For

example, IP defines one VLAN; IPX defines another VLAN, Netbeui yet another

VLAN, etc.

Subnet-based VLANs With this method, the VLAN membership is defined by the subnet to which a

workstation's IP address belongs.

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Advantages of VLAN

There are several advantages to using VLANs.

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.

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.

Simplified Administration 70% network costs are a result of adds, moves, and changes of users in the

network. Every time a user is moved in a LAN, 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.

VLAN's is a tool that created which can allow network managers to drag and

drop users into different VLAN's or to set up aliases.

Cost and Time Reduction VLANs can reduce the migration cost of stations going from one group to another.

Physical reconfiguration takes time and is costly. Instead of physically moving one

station to another segment or even to another switch, it is much easier and quicker

to move it by using software.

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Broadcast Control Broadcasts are required for the normal function of a network. Many protocols and

applications depend on broadcast communication to function properly. A layer 2

switched network is in a single broadcast domain and the broadcasts can reach the

network segments which are so far where a particular broadcast has no scope and

consume available network bandwidth. A layer 3 device (typically a Router) is

used to segment a broadcast domain.

Creating Virtual Work Groups VLANs can be used to create virtual work groups. For example, in a campus

environment, professors working on the same project can send broadcast messages

to one another without the necessity of belonging to the same department. This can

reduce traffic if the multicasting capability of IP was previously used.

Security VLANs provide an extra measure of security. People belonging to the same group

can send broadcast messages with the guaranteed assurance that users in other

groups will not receive these messages.

Physical Layer Transparency VLANs are transparent on the physical topology and medium over which the

network is connected.

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How VLANs work

When a LAN bridge receives data from a workstation, it tags the data with a

VLAN identifier indicating the VLAN from which the data came. This is called

explicit tagging.

It is also possible to determine to which VLAN the data received belongs using

implicit tagging. In implicit tagging the data is not tagged, but the VLAN from

which the data came is determined based on other information like the port on

which the data arrived.

Tagging can be based on the port from which it came, the source Media Access

Control (MAC) field, the source network address, or some other field or

combination of fields.

VLAN's are classified based on the method used. To be able to do the tagging

of data using any of the methods, the bridge would have to keep an updated

database containing a mapping between VLAN's and whichever field is used for

tagging.

Example: If tagging is by port, the database should indicate which ports belong to

which VLAN. This database is called a filtering database. Bridges would have to

be able to maintain this database and also to make sure that all the bridges on the

LAN have the same information in each of their databases. The bridge determines

where the data is to go next based on normal LAN operations. Once the bridge

determines where the data is to go, it now needs to determine whether the VLAN

identifier should be added to the data and sent.

If the data is to go to a device that knows about VLAN implementation

(VLAN-aware), the VLAN identifier is added to the data.

If it is to go to a device that has no knowledge of VLAN implementation

(VLAN-unaware), the bridge sends the data without the VLAN identifier.

In order to understand how VLAN's work, we need to look at the types of VLAN's,

the types of connections between devices on VLAN's, the filtering database which

is used to send traffic to the correct VLAN, and tagging, a process used to identify

the VLAN originating the data.

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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. 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.

Trunk link between two VLAN-aware bridges.

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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.

Access link between a VLAN-aware bridge and a VLAN-unaware device.

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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.

Hybrid link containing both VLAN-aware and VLAN-unaware devices.

It must also be noted that the network can have a combination of all three types

of links.

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Here’s a short list of ways VLANs simplify network management:

Network adds, moves, and changes are achieved with ease by just

configuring a port into the appropriate VLAN.

A group of users that need an unusually high level of security can be put into

its own VLAN so that users outside of the VLAN can‘t communicate with

them.

As a logical grouping of users by function, VLANs can be considered

independent from their physical or geographic locations.

VLANs greatly enhance network security.

VLANs increase the number of broadcast domains while decreasing their

size.

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Identifying VLANs

Know that switch ports are layer 2–only interfaces that are associated with a

physical port. A switch port can belong to only one VLAN if it is an access port or

all VLANs if it is a trunk port. You can manually configure a port as an access or

trunk port, or you can let the Dynamic Trunking Protocol (DTP) operate on a per-

port basis to set the switchport mode. DTP does this by negotiating with the port

on the other end of the link.

Switches are definitely pretty busy devices. As frames are switched throughout the

network, they‘ve got to be able to keep track of all the different types plus

understand what to do with them depending on the hardware address. And

remember—frames are handled differently according to the type of link they‘re

traversing.

There are two different types of links in a switched environment:

Access ports An access port belongs to and carries the traffic of only one

VLAN. Traffic is both received and sent in native formats with no VLAN tagging

whatsoever. Anything arriving on an access port is simply assumed to belong to

the VLAN assigned to the port. So, what do you think will happen if an access port

receives a tagged packet, like IEEE 802.1Q tagged? Right— that packet would

simply be dropped. But why? Well, because an access port doesn‘t look at the

source address, so tagged traffic can be forwarded and received only on trunk

ports.

Trunk Ports An access link, this can be referred to as the configured VLAN of the

port. Any device attached to an access link is unaware of a VLAN membership—

the device just assumes it‘s part of the same broadcast domain, but it doesn‘t have

the big picture, so it doesn‘t understand the physical network topology at all.

Inter-Switch Link (ISL)

Inter-Switch Link (ISL) is a way of explicitly tagging VLAN information onto an

Ethernet frame. This tagging information allows VLANs to be multiplexed over a

trunk link through an external encapsulation method (ISL), which allows the

switch to identify the VLAN membership of a frame over the trunked link.

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Membership

What characteristic can be used to group stations in a VLAN? Vendors use

different characteristics such as port numbers, MAC addresses, IP addresses, IP

multicast addresses, or a combination of two or more of these.

Port Numbers Some VLAN vendors use switch port numbers as a membership characteristic. For

example, the administrator can define that stations connecting to ports 1, 2, 3, and

7 belong to VLAN 1; stations connecting to ports 4, 10, and 12 belong to VLAN 2;

and so on.

MAC Addresses Some VLAN vendors use the 48-bit MAC address as a membership characteristic.

For

example, the administrator can stipulate that stations having MAC addresses

E21342A12334

and F2A123BCD341belong to VLAN 1.

IP Addresses Some VLAN vendors use the 32-bit IP address (see Chapter 19) as a membership

characteristic.

For example, the administrator can stipulate that stations having IP addresses

181.34.23.67, 181.34.23.72, 181.34.23.98, and 181.34.23.112 belong to VLAN 1.

Multicast IP Addresses Some VLAN vendors use the multicast IP address (see Chapter 19) as a

membership characteristic. Multicasting at the IP layer is now translated to

multicasting at the data link layer.

Combination Recently, the software available from some vendors allows all these characteristics

to be combined. The administrator can choose one or more characteristics when

installing the software. In addition, the software can be reconfigured to change the

settings.

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Configuration How are the stations grouped into different VLANs? Stations are configured in one

of three ways: Manual, Semi-Automatic, and Automatic.

Manual Configuration:

In a manual configuration, the network administrator uses the VLAN software to

manually

assign the stations into different VLANs at setup. Later migration from one

VLAN to another is also done manually. Note that this is not a physical

configuration;

it is a logical configuration. The term manually here means that the administrator

types the port numbers, the IP addresses, or other characteristics, using the VLAN

software.

Automatic Configuration:

In an automatic configuration, the stations are automatically connected or

disconnected from a VLAN using criteria defined by the administrator. For

example, the administrator can define the project number as the criterion for being

a member of a group. When a user changes the project, he or she automatically

migrates to a new VLAN.

Semiautomatic Configuration:

A semiautomatic configuration is somewhere between a manual configuration and

an

automatic configuration. Usually, the initializing is done manually, with migrations

done automatically.

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USE OF SWITCHES

Generally, In implementation of VLAN one have to use

Switches,because it provides better network services than hubs & also

provides the following advantages:

Broadcast Control-Since switches have become more affordable lately,

a lot of companies are replacing their flat hub networks with pure switched

network and VLAN environments. All devices within a VLAN are members

of the same broadcast domain and receive all broadcasts. By default, these

broadcasts are filtered from all ports on a switch that aren‘t members of the

same VLAN

Security- A flat internetwork‘s security used to be tackled by connecting hubs and switches together with routers. So it was

basically the router‘s job to maintain security.

Flexibility- Layer 2 switches only read frames for filtering—they don‘t

look at the Network layer protocol. And by default, switches forward all

broadcasts. But if you create and implement VLANs, you‘re essentially

creating smaller broadcast domains at layer 2.

Scalability- Another advantage is that when a VLAN gets too big, you

can create more VLANs to keep the broadcasts from consuming too much

bandwidth—the fewer users in a VLAN, the fewer users affected by

broadcasts.

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Communication Between Switches

In a multiswitched backbone, each switch must know not only which station

belongs to which VLAN, but also the membership of stations connected to other

switches. For example, in Figure 15.17, switch A must know the membership

status of stations connected to switch B, and switch B must know the same about

switch A. Three methods have been devised for this purpose: table maintenance,

frame tagging, and time-division multiplexing.

Table Maintenance

In this method, when a station sends a broadcast frame to its group members, the

switch creates an entry in a table and records station membership. The switches

send their tables to one another periodically for updating.

Frame Tagging

In this method, when a frame is traveling between switches, an extra header is

added to

the MAC frame to define the destination VLAN. The frame tag is used by the

receiving

switches to determine the VLANs to be receiving the broadcast message.

Time-Division Multiplexing (TDM)

In this method, the connection (trunk) between switches is divided into timeshared

channels (see TDM in Chapter 6). For example, if the total number of VLANs in a

backbone is five, each trunk is divided into five channels. The traffic destined for

VLAN 1 travels in channell, the traffic destined for VLAN 2 travels in channel 2,

and so on. The receiving switch determines the destination VLAN by checking the

channel from which the frame arrived.

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Two switches in a backbone using VLAN software.

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VLAN TRUNKING PROTOCOL(VTP)

Cisco created this one too. The basic goals of VLAN Trunking Protocol (VTP) are

to manage all configured VLANs across a switched internetwork and to maintain

consistency throughout that network VTP allows you to add, delete, and rename

VLANs—information that is then propagated to all other switches in the VTP

domain.

Here‘s a list of some of the cool features VTP has to offer:

Consistent VLAN configuration across all switches in the network.

VLAN trunking over mixed networks, such as Ethernet to ATM LANE or

even FDDI.

Accurate tracking and monitoring of VLANs

Dynamic reporting of added VLANs to all switches in the VTP domain.

Plug and Play VLAN adding

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Very nice, but before you can get VTP to manage your VLANs across the network,

you have to create a VTP server.

All servers that need to share VLAN information must use the same domain name,

and a switch can be in only one domain at a time.

So basically, this means that a switch can only share VTP domain information

with other switches if they‘re configured into the same VTP domain. You can use a

VTP domain if you have more than one switch connected in a network, but if

you‘ve got all your switches in only one VLAN, you just don‘t need to use

VTP.

Do keep in mind that VTP information is sent between switches only via a trunk

port.

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ROUTING BETWEEN VLAN

Hosts in a VLAN live in their own broadcast domain and can communicate freely.

VLANs create network partitioning and traffic separation at layer 2 of the OSI, and

as I said when I told you why we still need routers, if you want hosts or any other

IP-addressable device to communicate between VLANs, you just have to have a

layer 3 device—period.

For this, you can use a router that has an interface for each VLAN or a router that

supports ISL or 802.1Q routing. The least expensive router that supports ISL or

802.1Q routing is the 2600 series router. (You‘d have to buy that from a used-

equipment reseller, because they are end of life, or EOL.) The 1600, 1700, and

2500 series don‘t support ISL or 802.1Q routing.

I‘d recommend at least a 2800 as a bare minimum, and that only supports

802.1Q—Cisco is really moving away from ISL, so you probably should only be

using 802.1Q anyway. (Some IOSs on the 2800 may support both ISL and

802.1Q—I just have never seen it supported.)

The figure shows a routers with individual VLAN association.

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Implementing VLANs

Port-based VLANs

In the previous section, we simply stated that the network is split up into sets of

virtual LANs. It is one thing to say this; it is quite another thing to understand how

this is actually achieved. Fundamentally, the act of creating a VLAN on a switch

involves defining a set of ports, and defining the criteria for VLAN membership

for workstations connected to those ports.

With port-based VLANs, the ports of a switch are simply assigned to VLANs, with

no extra criteria.

All devices connected to a given port automatically become members of the

VLAN to which that port was assigned.

Distributing a single VLAN across multiple switches The ABOVE FIGURE shows an example of a VLAN based network. It shows

some of VLAN A connected to one switch, and some more of VLAN A connected

to another switch. You may be asking ―Are these both part of the same VLAN A,

or separate VLANs that all happen to be called VLAN A?‖ The answer is that

they are all parts of the same VLAN—there is a single VLAN A that is

Spread across two switches.

How is this achieved? How does one switch know that when it receives a broadcast

packet that it associates to VLAN A that it must also forward that broadcast to

other switches?

This can be done in a number of different ways, and in the early days of VLANs,

just about every one of these ways was tried. Some vendors had their switches use

a proprietary protocol to inform each other of their VLAN tables; some vendors

used time-divided multiplexing in which different timeslots were allocated to

different VLANs; other vendors used frame tagging. In the end, frame tagging

became the accepted standard. As we will see, in most respects this is a simple and

elegant solution. However, it initially had one big downside: it required a

fundamental change to format of the Ethernet header. This split the world‘s

Ethernet devices into those that recognized tagged headers and those that did not

recognize tagged headers. In other words, a lot of Ethernet equipment was

rendered obsolete.

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The key benefits of implementing VLANs includes:

Allow network administrators to apply additional security to network

communication.

Make expansion and relocation of a network or a network device easier.

Provide flexibility because administrators are able to configure in a

centralized environment while the devices might be located in different

geographical locations.

Decrease the latency and traffic load on the network and the network

devices, offering increased performance

37

Conclusion

In computer networking, virtual local area network, virtual LAN or VLAN is a

concept of partitioning a physical network, so that distinct broadcast domains are

created. This is usually achieved on switch or router devices.

The basic reason for splitting a network into VLANs is to reduce congestion on a

large LAN. To understand this problem, we need to look briefly at how LANs have

developed over the years.

Initially LANs were very flat—all the workstations were connected to a single

piece of coaxial cable, or to sets of chained hubs. In a flat LAN, every packet that

any device puts onto the wire gets sent to every other device on the LAN.