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P.G. Dept. Of Computer Sci. VLAN

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A reports on Networks technology

Virtual Local Area Network

November 2012

Submitted By: - Shivaji R. Bhodkhe (MCA-1styr)

E-mail:[email protected]

(P.G. Dept. of Computer Science, SGBAU, Amravati-444602)
mailto:[email protected]:[email protected]:[email protected]:[email protected]


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INDEXContain Page Numbers

1. Abstract

2.Introduction3.Computer Network

1

4. Local-Area Network (LAN)

4.1. Types of Local-Area Networks (LANs)

5.Metropolitan Area Ne twork (MAN)

2

5.1.Types of MAN (metropolitan Area Network)

Technologies

5.2How MAN works

5.3. Advantages of MAN (Metropolitan Area Network)

3

6. Wide area network (WAN) 4

7. Network Devices7.1 Router

5

7.1.1 Why do I need a Router?

7.1.2. Restrict Broadcasts to the LAN

7.1.3. Act as the Default Gateway

7.1.4. Move (route) Data between Networks

6

7.1.5. Learn and Advertise Loop-Free Paths

7.2. Hubs7.3. Switches

7

7.3.1. Types of Network Switch:

7.3.2. Managed Switches:

8

7.3.3. Unmanaged Network Switch:

7.3.4. Smart Switches:

7.3.5. Advantages of Network Switch:

8. Types of VLAN's

8.1. Layer 1 VLAN: Membership by Port

9

8.2. Layer 2 VLAN: Membership by MAC Address

8.3. Layer 2 VLAN: Membership by Protocol Type

8.4. Layer 3 VLAN: Membership by IP Subnet Address

10

8.5 Higher Layer VLAN's

8.6. Types of Connections

8.6.1 Trunk Link

8.6.2 Access Link

11


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8.6.3. Hybrid Link

9. Frame Processing

9.1 Filtering Database

9.2 Static Entries

9.3. Dynamic Entries

12

9.4. Tagging 1310. Summary

11. References

14

12. Abbreviations 15


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Virtual Local Area Network

1. Abstract

The Virtual Local Area Network (VLAN) technology is one of the hottest areas of

networking systems. A VLAN is a logical connection rather than physical that allows

network devices to be combined as "virtual LANs". The VLAN technology functions by

logically segmenting the network into different broadcast domains so that packets can

only delivered between ports with the same VLAN identity (group member). By using

this characteristic of VLAN there is a very flexible mechanism to group the physical ports

together. Wireless networks also need the flexibility to collect more than two devices

equipped with wireless communication and networking capability. In recent years,

wireless network has been attracting a lot of attention due to wireless devices have

enjoyed a tremendous rise in popularity. In order to communicate among some groups of

wireless devices without the convention concept of clusters we propose a novel model to

form the multiple-domain or the multiple-group as "virtual LANs" according to the

logical connection in ad hoc networks. In this paper discuss on single switch VLAN,

multiple switch VLAN and its working methods.

2. Introduction.

A Local Area Network (LAN) was originally defined as a network of computers

located within the same area. Today, Local Area Networks are defined as a single

broadcast domain. This means that if a user broadcasts information on his/her LAN, the

broadcast will be received by every other user on the LAN. Broadcasts are prevented

from leaving a LAN by using a router. The disadvantage of this method is routers usually

take more time to process incoming data compared to a bridge or a switch. More

importantly, the formation of broadcast domains depends on the physical connection of

the devices in the network. Virtual Local Area Networks (VLAN's) were developed as an

alternative solution to using routers to contain broadcast traffic.

3. Computer Network.

A computer network, or simply a network, is a collection ofcomputers and other

hardware interconnected by communication channels that allow sharing of resources and

information. Where at least one process in one device is able to send/receive data to/from

at least one process residing in a remote device, then the two devices are said to be in a

network. Simply, more than one computer interconnected through a communication

medium for information interchange is called a computer network.
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Networks may be classified according to a wide variety of characteristics, such as the

medium used to transport the data,communications protocol used, scale,topology,

benefit, and organizational scope.

Communications protocols define the rules and data formats for exchanginginformation in a computer network, and provide the basis fornetwork programming.

Well-known communications protocols include twoEthernet,a hardware andlink layer

standard that is ubiquitous inlocal area networks,and theInternet protocol suite,which

defines a set of protocols for internetworking, i.e. for data communication between

multiple networks, as well as host-to-host data transfer, and application-specific data

transmission formats.

4. Local-Area Network (LAN)

A local-area network (LAN) is acomputernetwork that spans a relatively small area.Most LANs are confined to a single building or group of buildings; however, one LAN

can be connected to other LANs over any distance via telephone lines and radio waves. A

system of LANs connected in this way is called awide-area network (WAN).

Most LANs connectworkstations andpersonal computers.Eachnode (individual

computer ) in a LAN has its ownCPU with which itexecutesprograms,but it also is able

toaccessdata anddevices anywhere on the LAN. This means that manyusers can share

expensive devices, such aslaser printers,as well as data. Users can also use the LAN to

communicate with each other, by sendinge-mail or engaging inchat sessions.

LANs are capable of transmitting data at very fast rates, much faster than data can be

transmitted over a telephone line; but the distances are limited, and there is also a limit on

the number of computers that can be attached to a single LAN.

4.1. Types of Local-Area Networks (LANs)

There are many different types of LANs, withEthernetsbeing the most common forPCs.

MostApple Macintosh networks are based onApple'sAppleTalk network system, which

is built into Macintosh computers.

The following characteristics differentiate one LAN from another:

1. Topology: The geometric arrangement of devices on the network. For example,devices can be arranged in a ring or in a straight line.

2. Protocols: The rules and encoding specifications for sending data. The protocolsalso determine whether the network uses apeer-to-peer orclient/server

architecture.

3. Media: Devices can be connected bytwisted-pair wire,coaxial cables,orfiberoptic cables. Some networks do without connecting media altogether,

communicating instead via radio waves.
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5. Metropolitan Area Network (MAN)

A large computer network which extends to a city or to a large university campus is

termed as metropolitan area network or MAN. The purpose of MAN (Metropolitan Area

Network) is to provide the link to the internet in the long run. A MAN (Metropolitan Area

Network) usually incorporates a number of LANs to form a network. This large network

MANs (Metropolitan Area Network) backbone comprises of an optical fiber set-up.

5.1. Types of MAN (metropolitan Area Network) Technologies

Most widely used technologies to develop a MAN (Metropolitan Area Network) network

are FDDI (fiber distribution data interface), ATM (Asynchronous Transfer Mode) and

SMDS (switched multi megabit data service).ATM (Asynchronous Transfer Mode) is the

most frequently used of all. ATM (Asynchronous Transfer Mode) is a digital data transfer

technology. It was developed in 1980 to improve the transportation of real time data over

a single network. ATM (Asynchronous Transfer Mode) works just like cell relay system,

where data is separated in the form of fixed equal sized packets and is transferred

overtime. The purpose of ATM (Asynchronous Transfer Mode) was to access clear audio

and video results during a video conferencing. The attributes of ATM has enabled it to

become a base of wide area data networking. ATM (Asynchronous Transfer Mode)

combines the characteristics of circuit switching and packet switching, which allows it to

transfer even the real time data. FDDI is a standard for data transfer over LAN, which can

be extended to the range of approximately 200kms. FDDI can help support the data

transmission of many thousand users. This is the reason why it is referred to as the MAN

(Metropolitan Area Network) technology. FDDI uses optical fiber for its basic

infrastructure that is why it is referred to as fiber distribution data interface. When data is


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transferred through a connectionless service we use the technology named as SMDS.

Connectionless service implies that data is transferred by storing the information in the

header and it reaches its destination independently through any network. When the data is

transferred using the technology of SMDS, it also forms small data packets just like in

ATM. However SMDS allows the transmission of data over large geographical areas inthe form of datagrams(the data packets of an unreliable data service provider).

Nowadays MAN (Metropolitan Area Network) links are established using infrared and

microwave signals.

5.2How MAN works

MAN (Metropolitan Area Network) usually falls between LAN and WAN. It is generally

applied to connect geographically dispersed LANs. Therefore the goal of MAN is to

develop a communication link between two independent LAN nodes. A MAN

(Metropolitan Area Network) is usually established using optical fiber. The network is

established using routers and switches. A switch is a port which is active in handling the

filtration of data usually coming in the form of frames. Any switch acts as a dual port, at

one end it is handling filtration of data and at the other end managing connections. Router

is another device for facilitating the networks connection. Router helps the data packets to

identify the path to be taken. Hence in other words it keeps an eye on the data transfer.

MAN (Metropolitan Area Network) is usually operated over an area of up to 50kms.

5.3. Advantages of MAN (Metropolitan Area Network)

MAN (Metropolitan Area Network) falls in between the LAN and WAN. It therefore

increases the efficiency of handling data while at the same time saves the cost attached to

establish a wide area network. MAN (Metropolitan Area Network) offers centralized

management of data. It enables you to connect many fast LANs together. Telephone

companies worldwide have facilitated the transfer of data with the help of an underground

optical fiber network. These optical fibers increase the efficiency and speed of data

transfer. The optical fibers enable you to access a speed of almost 1000mbps. If you

develop a WAN of 1.45 mbps its cost is more than what it gives you. Whereas when you

establish metropolitan area network it offers you the speed of 1000mbps as a whole with

the lowest cost involved.


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6. Wide area network (WAN)

A wide area network (WAN) is a computer network that covers up a broad area

(i.e., any network whose communications links traverse metropolitan, regional, or

national boundaries. Unlike personal area networks (PANs), local area networks (LANs),

campus area networks (CANs), or metropolitan area networks (MANs) are usually

limited to a room, building, campus or particular metropolitan area (e.g. a town)

correspondingly.

WANs are used to connect LANs and other kinds of networks together, so that

users and computers in one place can communicate with users and computers in other

places. There are many WANs that are available to one particular organization working

privately. Others built by ISPs provide connections from LAN of an organization to the

Internet.

There are several options of WAN connectivity that are discussed below: Circuit

Switching Circuit switching is another WAN option. A circuit path is devoted at the end

paths. Dialup connections are perfect example of circuit switching, and it is less

expensive then leased lines. The speed of this network is 28-144 kbps and protocols are

PPP, ISDN.

Circuit Switching It is another option available in (WAN). A devoted circuit path

is created between end points. An excellent example is dialup connections. It is relatively

less expensive. The broadband range of this option is 28 - 144 kbps. Available protocols

include PPP, ISDN.


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Call relay is comparable to packet switching, but employs fixed length cells as an

alternative to changeable length packets. Statistics is confused into fixed-length cells, and

after that, it is elated to fundamental routes. It is finest for instantaneous exercise of voice

and information, the visual projection of call relay can be extensive, a paradigm of

protocol in the advertisement places is ATM.

Generally, WANs are frequently assembled by means of leased lines. At every

periphery of the leased line, a router is attached to the LAN on one side, and a centre

within the WAN on the other. Despite the fact that there has been utilization of leased

lines, WANs can in addition be ascertained by means of a modestly expensive circuit

switching, or packet switching mode. Network protocols comprise TCP/IP that conveys

transfer, and directing errands. Service providers to compel the connections that are

exercised in WANs regularly operate protocols together with Packet over SONET/SDH,

MPLs, ATM, and frame relay. X.25 was the chief untimely WAN protocol, and is

repeatedly acknowledged as the grandfather of frame relay as several of the essential

protocols, and utility of X.25 are offered in this contemporary period by frame relay.

Companies use these to minimize cost by cutting the need for travel, and

expensive long distance phone calls.These can diminish the charges of companies by

diminishing the necessity of journey, and also of high-priced extended distance phone

calls. In (WAN), you are able to contribute to data as well as video conferencing.

In (WAN), you can share data as well do as video conferencing. When

communication is being shared between computers, each operator can have the right to

use the same information at the same time. The use of a WAN also improves employee

output by increasing and accelerating work applications as well as dipping replication

across the WAN.


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7. Network Devices

Networking hardware or networking equipment typically refers to devices

facilitating the use of a computer network. Typically, this includes gateways, routers,

network bridges, switches, hubs, and repeaters. Also, hybrid network devices such as

multilayer switches, protocol converters, bridge routers, proxy servers, firewalls, network

address translators, multiplexers, network interface controllers, wireless network interface

controllers, modems, ISDN terminal adapters, line drivers, wireless access points,

networking cables and other related hardware

7.1 Router

A router is specializedcomputer connected to more than one network running

software that allows the router to move data from onenetwork to another. Routers operateat the network layer (OSI Model'slayer 3). The primary function of a router is to connect

networks together and keep certain kinds of broadcast traffic under control. There are

several companies that make routers:Cisco (Linksys),Juniper,Nortel (Bay Networks),

Redback,Lucent, 3Com,andHPjust to name a few.

7.1.1 Why do I need a Router?

Routers used in networks perform the following functions:

1. Restrict broadcasts to the LAN
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2. Act as the default gateway.

3. Move (route) data between networks

4. Learn and advertise loop free paths

7.1.2. Restrict Broadcasts to the LAN

Networks (especially Ethernet networks) use broadcast communication at the

physical, datalinkand network layer.Network layerbroadcasts are transmissions sent to

all hosts using thenetwork layerprotocol (usuallyInternet Protocol [IP] or IPX).Network

broadcast communication is used to communicate certain kinds of information that makes

the network function (ARP,RARP,DHCP,IPX-SAP broadcasts etc.). Since several

devices could attempt to transmit simultaneously and cause collisions, it is preferable to

separate large sets of hosts into different broadcast domains using aswitch,or router.

As the number of hosts on thenetwork increases, the amount of broadcast traffic

increases. If enough broadcast traffic is present on thenetwork,then ordinary

communication across thenetworkbecomes difficult.

To reduce broadcasts, a network administrator can break up anetwork with a large

number of hosts into two smallernetworks.Broadcasts are then restricted to each

network, and the router performs as the 'default gateway' to reach the hosts on the other

networks.

7.1.3. Act as the Default Gateway

Especially in today'snetworks,people are connecting to theInternet.When yourcomputer wants to talk to acomputer on anothernetwork,it does so by sending your data

to thedefault gateway (your local router). The router receives your data, looks for the

remote address of that far-offcomputer makes a routing decision and forwards your data

out a different interface that is closer to that remotecomputer.There could be several

routers between you and the remotecomputer,so several routers will take part in handing

off thepacket,much like a fireman's bucket brigade.

7.1.4. Move (route) Data between Networks

Routers have the capability to move data from onenetwork to another. This

allows two networks managed by different organizations to exchange data. They create a

network between them and exchange data between the routers on that network. Because a

router can accept traffic from any kind of network it is attached to, and forward it to any

other network, it can also allow networks that could not normally communicate with each

other to exchange data. In technical terms, a token ringnetwork and an Ethernet network

can communicate over a serial network. Routers make all this possible.

A router can take in anEthernet frame, strip the Ethernet data off, and then drop

theIP data into a frame of another type such as SDH/SONET, PDH/T1, ATM, and FDDI.

In this way a router can also perform 'protocol conversion', provided it has the appropriate
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hardware and software to support such a function. The whole point, however, is to

forward the data from the interface it receives data on, to another interface that

retransmits the received data onto another interface serving another network.

7.1.5. Learn and Advertise Loop-Free Paths

Routers can only learn and advertise routes dynamically if they are using a routing

protocol such as RIP, OSPF, EIGRP, IS-IS or BGP. Otherwise, a human has to configure

the routes by hand, which is called static routing.

Routing moves data on a hop-by-hop basis, what is often called 'hot potato'

routing. If a set of routers ends up passing the data around in a circle, without reaching

the destination, it's called a 'routing loop'. Packets get tossed around the loop until they

die of old age: their 'Time to Live' counter in the IP datagram is decremented as it passes

through each router and eventually it reaches zero and is discarded.

7.2. Hubs

On 10BaseT and 100BaseTX Ethernet networks larger than two computers, each

computer or printer (or other networked device) is connected to a hub. The hub is a small

box that gathers the

Signals from each individual device, optionally amplifies each signal, and then sends the

signal out to all other connected devices. Amplification helps to ensure that devices on

the network receive reliable information. You can think of an Ethernet hub like the hub of

a wheel, at the center of the spokes that connect each individual computer or printer.

Hubs are also called

Concentrators or repeaters. Hubs come in various sizes, the most common being 12-port

or 24port (meaning they can connect to 12 or 24 computers/printers/hubs). All of the

clients, servers, and peripherals connected to a hub (or to a set of interconnected hubs)

share the bandwidth (data delivery capacity) of your network. Technically, they form a

single

collision domainan area of an Ethernet network in which data sent to or from a device

may potentially collide with the data from other devices. As you add more clients,

servers, and peripherals to an Ethernet network, the number of

collisions increases and the performance of your network degrades. You can improve


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performance by isolating network traffic into many smaller collision domains.

Unfortunately, hubs cannot divide a network in this fashion; they simply repeat every

signal all to all connected devices. Instead, to divide networks into multiple collision

domains you can deploy switches, bridges, or routers. Each switch port, bridge port, or

router port forms a new collision domain.

7.3. Switches

Like a hub, an Ethernet switch is a device that gathers the signals from devices

that are connected to it, and then regenerates a new copy of each signal. Switches,

however, are more powerful than hubs and can substantially increase your networkperformance. In order to understand how they perform this magic, it is necessary to

understand first how they work.

Most common switches operate by learning the MAC addresses of all connected clients,

servers, and peripherals, and associating each address with one of its ports. When a

switch receives an incoming signal, it creates a temporary circuit between the sender and

receiver. The temporary circuit provides two important benefits.

First, the circuit allows the sender and receiver momentarily

First, the circuit allows the sender and receiver momentarily to exchange

information without intrusion from other devices on the network. That is, each pair of

communicating devices utilizes the full bandwidth (data carrying capacity) of the

network instead of sharing that bandwidth, as they do in unswitched Ethernet networks.

To say this another way, each switch port defines a collision domain containing only a

small number of devices and thereby helps provide maximum performance for Ethernet

networks.

Second, the circuit ensures that information travels directly between the communicating

computers.

This behavior differs markedly from unswitched Ethernet networks. In

unswitched networks, data from a transmitting computer is sent by the nearest hub to all

connected devices (not just to the recipient) and therefore congests parts of the network

needlessly.Like all network equipment, switches benefit your network only if they are

deployed in the proper manner. If your network is congested and if traffic pools in certain

areas, then you can improve network performance by replacing hubs with switches, or by

connecting hubs to switches in a hierarchical manner. (You can see a diagram of a school

network that uses a hierarchy of switches and hubs at

http://www.3com.com/nsc/500612c.html . The switches are gray boxes and the hubs are

black boxes labeled with numbers to indicate how many ports they have.) For the pools of


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heavy traffic, switches increase bandwidth while segregating the traffic from the rest of

the network. However, if your network is not congested or if your traffic patterns do not

create pools of congestion, then switches may actually cause your network performance

to deteriorate. This performance degradation occurs because switches examine the

information inside each signal on your network (to determine the addresses of the senderand receiver) and therefore process network information more slowly than hubs.

Recently, manufacturers have begun to offer switches that examine OSI level 3

(network routing) information such as that contained in the IP portions (rather than the

data link portions) of a network signal. Later in this chapter, you will discover that routers

also examine this information. Level 3 switches blur the distinction between switches and

routers. Level 3 switches can replace routers within your network or between your

network and the Internet (while level 2 switches can replace hubs, but not routers).

7.3.1. Types of Network Switch:

There are different types of networking switch based upon the form and the

configuration. On the basis of their form they are categorized into rack mounted, chassis

or catalyst switch etc. And on the basis of configuration they are differentiated into

managed, unmanaged, smart or enterprise managed switches.

7.3.2. Managed Switches:

A type of network switch in which different types of methods are used to manage

the different parts of the network and can able to upgrade the working and the

performance of the switch with the help of common methods of management is called as

the managed network switch.

7.3.3. Unmanaged Network Switch:Basically these networking switches are designed for those customers that are not

able to spend more money because those are less expensive. A type of network switch in

which interface is not involved is called as unmanaged network switches. They are

designed for the direct use.

7.3.4. Smart Switches:

Basically the smart network switches are the important types of managed switches

in which the specific management features are discussed. Typically these switches reused
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for the networking devices such as VLANs. They also increase the working ability of the

parts connected by the switches.

7.3.5. Advantages of Network Switch:

Due to the reliable and the easy working of the network switches to manage the

network by joining the different segments of the network. Some of the particular

advantages of the network switching are given below.

1. Network switches are very beneficial for the expenditure of the network and canalso helpful in decreasing the load from the systems individually

2. They are also helpful for the in maintaining and enhancing the performance of thenetwork using switches

3. In the networking data is transmitted in the form of the data packets and in thesecases there are more chances of collision between the packets but network

switches are also able to avoid the collision between the data grams

8. Types of VLAN's

VLAN membership can be classified by port, MAC address, and protocol type.

8.1. 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 (seeFigure).

Port VLAN

1 1

2 1

3 2

4 1

Figure: Assignment of ports to different VLAN's.


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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.

8.2. 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 (seeFigure). 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

Figure: 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.

8.3. 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 (seeFigure).

Protocol VLAN

IP 1

IPX 2

Figure: Assignment of protocols to different VLAN's.


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8.4. 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 (seeFigure).

IP Subnet VLAN

23.2.24 1

26.21.35 2

Figure: 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 theirnetwork addresses. The only problem is that it generally takes longer to forward packets

using Layer 3 information than using MAC addresses.

8.5 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.

8.6. 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.


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8.6.1 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 (seeFigure).

Figure: Trunk link between two VLAN-aware bridges.

8.6.2 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) (seeFigure8).

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).

Figure 8: Access link between a VLAN-aware bridge and a VLAN-unaware device.

8.6.3. 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 (seeFigure9). A hybrid link can have

both tagged and untagged frames, but all the frames for a specific VLAN must be either

tagged or untagged.


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Figure9: 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.

9. Frame Processing

A bridge on receiving data determines to which VLAN the data belongs either by

implicit or explicit tagging. In explicit tagging a tag header is added to the data. The

bridge also keeps track of VLAN members in a filtering database which it uses to

determine where the data is to be sent. Following is an explanation of the contents of the

filtering database and the format and purpose of the tag header .

9.1 Filtering Database

Membership information for a VLAN is stored in a filtering database. The

filtering database consists of the following types of entries:

9.2 Static Entries

Static information is added, modified, and deleted by management only. Entries

are not automatically removed after some time (ageing), but must be explicitly removed

by management. There are two types of static entries:

a) Static Filtering Entries: which specify for every port whether frames to be sent to a

specific MAC address or group address and on a specific VLAN should be forwarded or

discarded, or should follow the dynamic entry, and

b) Static Registration Entries: which specify whether frames to be sent to a specific

VLAN are to be tagged or untagged and which ports are registered for that VLAN.

9.3. Dynamic Entries

Dynamic entries are learned by the bridge and cannot be created or updated by

management. The learning process observes the port from which a frame, with a given


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source address and VLAN ID (VID), is received, and updates the filtering database. The

entry is updated only if all the following three conditions are satisfied:

a) this port allows learning,

b) the source address is a workstation address and not a group address, and

c) there is space available in the database.

Entries are removed from the database by the ageing out process where, after a

certain amount of time specified by management (10 sec --- 1000000 sec), entries allow

automatic reconfiguration of the filtering database if the topology of the network changes.

There are three types of dynamic entries:

a) Dynamic Filtering Entries: which specify whether frames to be sent to a specific MAC

address and on a certain VLAN should be forwarded or discarded.

b) Group Registration Entries: which indicate for each port whether frames to be sent to a

group MAC address and on a certain VLAN should be filtered or discarded. These entries

are added and deleted using Group Multicast Registration Protocol (GMRP). This allows

multicasts to be sent on a single VLAN without affecting other VLAN's.

c) Dynamic Registration Entries: which specify which ports are registered for a specific

VLAN. Entries are added and deleted using GARP VLAN Registration Protocol (GVRP),

where GARP is the Generic Attribute Registration Protocol.

GVRP is used not only to update dynamic registration entries, but also tocommunicate the information to other VLAN-aware bridges.

In order for VLAN's to forward information to the correct destination, all the

bridges in the VLAN should contain the same information in their respective filtering

databases. GVRP allows both VLAN-aware workstations and bridges to issue and revoke

VLAN memberships. VLAN-aware bridges register and propagate VLAN membership to

all ports that are a part of the active topology of the VLAN. The active topology of a

network is determined when the bridges are turned on or when a change in the state of the

current topology is perceived. The active topology is determined using a spanning tree

algorithm which prevents the formation of loops in the network by disabling ports. Once

an active topology for the network (which may contain several VLAN's) is obtained, the

bridges determine an active topology for each VLAN. This may result in a different

topology for each VLAN or a common one for several VLAN's. In either case, the VLAN

topology will be a subset of the active topology of the network (seeFigure 10).


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Figure: Active topology of network and VLAN A using spanning tree algorithm.

9.4. Tagging

When frames are sent across the network, there needs to be a way of indicating to

which VLAN the frame belongs, so that the bridge will forward the frames only to those

ports that belong to that VLAN, instead of to all output ports as would normally have

been done. This information is added to the frame in the form of a tag header. In addition,

the tag header:

i) Allows User Priority Information To Be Specified,

ii) Allows Source Routing Control Information To Be Specified, And

iii) Indicates The Format Of Mac Addresses.

Frames in which a tag header has been added are called tagged frames. Tagged

frames convey the VLAN information across the network. The tagged frames that are sent

across hybrid and trunk links contain a tag header. There are two formats of the tag

header:

i) Ethernet Frame Tag Header: The ethernet frame tag header (see Figure11) consists of a

tag protocol identifier (TPID) and tag control information (TCI).


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Figure11: Ethernet frame tag header.

ii) Token Ring and Fiber Distributed Data Interface (FDDI) tag header: The tag headers

for both token ring and FDDI networks consist of a SNAP-encoded TPID and TCI.

Figure12: Token ring and FDDI tag header.

TPID is the tag protocol identifier which indicates that a tag header is following and TCI

(seeFigure 13) contains the user priority, canonical format indicator (CFI), and the

VLAN ID.

Figure13: Tag control information (TCI).

User priority is a 3 bit field which allows priority information to be encoded in the

frame. Eight levels of priority are allowed, where zero is the lowest priority and seven is

the highest priority. How this field is used is described in the supplement 802.1p.

The CFI bit is used to indicate that all MAC addresses present in the MAC data

field are in canonical format. This field is interpreted differently depending on whether it

is an ethernet-encoded tag header or a SNAP-encoded tag header. In SNAP-encoded

TPID the field indicates the presence or absence of the canonical format of addresses. In

ethernet-encoded TPID, it indicates the presence of the Source-Routing Information (RIF)

field after the length field. The RIF field indicates routing on Ethernet frames.

The VID field is used to uniquely identify the VLAN to which the frame belongs.

There can be a maximum of (212- 1) VLAN's. Zero is used to indicate no VLAN ID, but

that user priority information is present. This allows priority to be encoded in non-priority

LAN's.


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10. Conclusion

As we have seen there are significant advances in the field of networks in the form of

VLAN's which allow the formation of virtual workgroups, better security, improved

performance, simplified administration, and reduced costs. VLAN's are formed by thelogical segmentation of a network and can be classified into Layer1, 2, 3 and higher

layers. Only Layer 1 and 2 are specified in the draft standard 802.1Q. Tagging and the

filtering database allow a bridge to determine the source and destination VLAN for

received data. VLAN's if implemented effectively, show considerable promise in future

networking solutions.


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11. References

1) David Passmore, John Freeman, ``The Virtual LAN Technology Report,'' March 7,1997,http://www.3com.com/nsc/200374.html

A very good overview of VLAN's, their strengths, weaknesses, and implementation

problems.

2) IEEE, ``Draft Standard for Virtual Bridge Local Area Networks,'' P802.1Q/D1, May

16, 1997,

This is the draft standard for VLAN's which covers implementation issues of Layer 1 and

2 VLAN's.

3) Mathias Hein, David Griffiths, Orna Berry, ``Switching Technology in the Local

Network: From LAN to Switched LAN to Virtual LAN,'' February 1997,Textbook explanation of what VLAN's are and their types.

7) Susan Biagi, "Virtual LANs," Network VAR v4 n1 p. 10-12, January 1996,

An Overview of VLAN's, advantages, and disadvantages.

8) David J. Buerger, ``Virtual LAN cost savings will stay virtual until networking's next

era,'' Network World, March 1995,

A short summary on VLAN's.

9) IEEE, ``Traffic Class Expediting and Dynamic Multicast Filtering,'' 802.1p/D6, April

1997,

This is the standard for implementing priority and dynamic multicasts. Implementation of

priority in VLAN's is based on this standard.
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12. Abbreviations

CFI - Canonical Format Indicator FDDI - Fiber Distributed Data Interface FTP - File Transfer Protocol GARP - Generic Attribute Registration Protocol GMRP - Group Multicast Registration Protocol GVRP - GARP VLAN Registration Protocol IEEE - Institute of Electrical and Electronic Engineers LAN - Local Area Network MAC - Media Access Control RIF - Source-Routing Information TCI - Tag Control Information TPID - Tag Protocol Identifier VID - VLAN ID VLAN - Virtual Local Area Network

Documents

Reports on Vlan