Implementation of the Virtual LAN

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Implementation of the Virtual LAN

Virtual LANs (VLANs) are used to break up broadcast domains in a Layer 2 switched internetwork. AsVLANs promote efficient use of network resources, it is wise to beef up your knowledge of thistechnology. n this !aily !rill !own, it will e"plain how to implement the VLAN technology using#isco routers and Layer 2 switches.

A common LAN network design implemented in the last $% years or so is called a collapsed backbone.&asically, it connected all floors or rooms in a building to a network where the company's sharedser ers were located. he typical collapsed*backbone network would look something like +igure A.


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he popular solution to this dilemma was the practice of installing bridges on each floor. he newdesign looked like Figure B .

ach floor is now a separate collision domain, which really helped-for a while. &ut look again-thisnetwork is still one immense broadcast domain. As networks grew and more and more networkser ices became a ailable to users, this design became saturated, resulting in lame response time forthe users. #isco routers became more cost*effecti e. ( rior to that, they were cost prohibiti e forsmaller companies, e en though they had been a ailable.

/ith the ad ent of router affordability, the solution to the monstrous broadcast domain issue was to usea #isco router to break up both collision and broadcast domains. he new and cool network now lookedlike the one shown in Figure C . he fiber was not discarded but used in point*to*point connections fromeach floor to the router.

n this network, a single router has replaced the bridges. hat the router breaks up collision and broadcast domains and that this replaces bridges0 it doesn't 1ust add to their functionality. n fact, the bridge, if left in the network, only slowed the network down (created latency issues).

A single router connecting all the floors really worked. As long as users kept their data on the localnetwork. his type of network design was implemented worldwide, and thernet became the de factostandard that ran to each desktop.


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his type of network has been discussed, worked, and reworked. ost of the problems that typicallysurface ha e to do with physical location. n other words, for the network to work as designed, youcreate physical networks and assign subnets to these physical networks. 3sers are then placed in a

physical location by 1ob function. As long as e eryone on the same floor performed the same 1ob andshared the same network resources, the network sang. &ut flies land in the ointment en masse whenusers with disparate functions and needs are placed on the same floor. he problems created by thisscenario can include4

3sers with different 1ob functions sharing the same broadcast domain. Anomaly users (those with needs and5or functions not common to a gi en broadcast domain)

re6uired that all their data (packets) cross a Layer 7 de ice to communicate with the networkresources they needed.

&andwidth usage 6uickly became an issue because too many users were placed in the same broadcast5collision domain.

A good solution to this dilemma really didn't e"ist. here are a few solutions (workarounds) typicallyconfigured on the network4

Adding another broadcast domain by configuring another router port with another hub connectedto the floor4 his keeps the new users off the e"isting broadcast domain, but all these new usersmust still cross a Layer 7 de ice to get to the network ser ices they use.

8unning a cable from the workstations to the correct broadcast domain4 his one actually works pretty well (as long as you don't e"ceed the distance constraints), but there are dollars in ol ed inrunning the cables.

o ing the whole group to another part of the building that has enough room for e eryone4&elie e it or not, this was the most common solution.

Enter Layer 2 switching and VLANs

&ridges were the precursor to Layer 2 LAN switching. 9witches were basically designed to perform thesame function as a bridge but with more ports. A typical bridge only had two ports, although you could

buy bridges that had up to $:. A LAN switch can ha e hundreds of ports, and LAN switches are moreintelligent.

LAN switches filter the network by hardware address, break up collision domains, pro ide port security,and can create VLANs. his has changed network design $%% percent from the world of collapsed

backbones. nstead of ha ing to worry about creating networks by physical location, VLANs turned thenetwork*design world on its ear by pro iding options and fle"ibility like ne er before to fit any businessmodel. he only design constraint in this type of network is the network administrator's lack ofimagination.

Let's take a look at our pre ious network design and use VLANs instead of routers to break up ournetworks. wo VLANs were created for this e"ample (see Figure D ).


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his network is easy to maintain and create security on, and best of all, the physical location of a user iscompletely irrele ant. 8egardless of where users are located, they can be placed in any broadcastdomain (VLAN).

After studying the customer's business re6uirements by talking with both users and management, wasable to come up with a ery cool network that took only a few hours to implement. Figure F shows thenew network.

n +igure + are the names of the rooms in the building0 and named the VLANs after the rooms. hisallowed the administrators to easily identify and locate the VLANs. Also, the subnet scheme wasdesigned after the floor and room numbers, since the rooms were also numbered.

&y looking at an address on a machine, the network administrator could tell which floor, room, andVLAN this de ice.

he used of switches connects rooms of all the users and then assigned each port to a specific VLAN.

;ne 2


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placed the other 2i e your client better*than*e"pected results. 9a e time and money. #reate something the client can readily understand, control, and scale for growth (making

him5her feel competent and confident).

An important thing to understand in this e"ample is that all users need to get to VLAN $ because of ashared database. his means that the users must lea e their broadcast domain (VLAN) and getinformation from the 9er er hosting the database. o do this, we must configure a router. Luckily, the

building already had some good switches and routers?ere's the output from a 2:2$ router that shows the 9L configuration4

@output cutinterface +ast thernet%5% ip address $%.$.$.$ 2BB.2BB.2BB.%Cinterface +ast thernet%5%.$$ encapsulation isl $$

ip address $%.$.$$.$ 2BB.2BB.2BB.%Cinterface +ast thernet%5%.$2 encapsulation isl $2 ip address $%.$.$2.$ 2BB.2BB.2BB.%Cinterface +ast thernet%5%.$7 encapsulation isl $7 ip address $%.$.$7.$ 2BB.2BB.2BB.%@output cut

n this configuration, subinterfaces were used to allow all VLANs to be connected to one routerinterface. n this e"ample, the interface used is +ast thernet %5% to make the subinterfaces the samenumber as the VLAN number for easy identification. he first command under the subinterface is theencapsulation command, which is used to direct the router to the VLAN number of the subinterface andto use inter*VLAN routing.

After the encapsulation command was used to define the VLAN and inter*VLAN routing type and


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added the address assigned to the subinterface. he hosts in each VLAN would use the addressassigned to this interface as their default gateway. +or e"ample, users in VLAN $2 would be configuredto use $%.$.$2.$ as their default gateway. his allowed the users to get out of their own VLAN and toaccess company shared ser ices, as well as the nternet.

Conclusionhis helped you to understand how aluable using VLAN technology in an internetwork can be and that

you now ha e a clearer picture of how to create them. en though the largest benefit of creatingVLANs in an internetwork is that you are no longer confined to a physical location, this real*lifee"ample in ol ed creating VLANs by physical location because that was what was best for thecustomer.

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Implementation of the Virtual LAN