vlan note

1. What is a VLAN?
A virtuallocalareanetwork(VLAN) isa group ofhostswith a
common setof requirementsthatcommunicate asiftheywere
attachedtothesame broadcast domain regardlessof their
physical location.
TraditionalLAN:
A traditionalLANwould require allusersof thesame requirements
andsame IPsubnet (broadcast domain) be connected to thesame
equipment.
LAN-based LAN:
By utilizingVLANs, thesame userscan be spread outovervarious
geographical locationsandstillremain in theirsame IP subnet
(broadcast domain).

2. How VLANs work?
* VLANs areidentifiedby a number.
* Validranges1-4094.
* On a VLAN-capable switch, you assign portswith the
appropriate VLAN number.
* The switch then onlyallowsdata to be sent betweenports with
thesame VLAN.
* Forexample, if therewere 6hostson each switch on 6 different
vlans,you wouldneed6 portson each switch toconnect the
switchestogether. This wouldmean thatif you had24 different
vlansyou could onlyhave24hostson a 48 port switch.

3. Benefits of VLANs
 Geographicallyseparatedusers on thesame IP subnet
(broadcast domain).
 Limit thesize of broadcast domains andlimit broadcast
activity.
 Security benefitsby keep hostsseparatedby VLAN and
limitingwhatdevices can talktothose hosts.
 Cost savingsasyou don’tneedadditional hardware and
cabling.
 Operational benefitsbecause changing a user’sIP subnet
(Broadcast Domain)isin software.
Drawbacks of VLANs
 VLANs wereused toallow grouping ofusers togetherwiththe
same requirements andallowingthemto be in separate
locations
 The majordrawback isthatthegeneral idea istomake allusers
thatneedthesame requirementsbe in the same VLAN

4.  VLANs workat Layer2 andthatlayerdoesn’t handle
redundancy in an efficientmanner.
VLAN types
There are threebasic VLAN memberships fordeterminingand
controllinghow apacket getsassigned: -
 Port-basedVLANs
 MAC address based
 Protocolbased VLANs
The frame headersare encapsulatedormodified toreflect a
VLAN ID before theframe issent overthelinkbetween
switches.
Before forwardingto thedestinationdevice, theframe header
ischanged back to theoriginal format.
Types
Port-basedVLANs
MAC address based VLANs
Protocolbased VLANs

5. An important consideration in definingthesize of theswitch
andthe number ofVLANs istheIP addressing scheme.
Because a one-to-onecorrespondence betweenVLANs andIP
subnetsisstrongly recommended, therecan be no more than
254devices inany one VLAN.
It isfurtherrecommended thatVLANs shouldnot extend
outside of theLayer2 domain of the distribution switch.
Advantages of VLAN
Performance. As mentioned above,routers thatforward
datainsoftwarebecomea
bottleneckas LANdatarates increase.Doingaway withthe

6. routers removes this
bottleneck.
Formation of virtual workgroups. Because
workstations canbemoved fromone
VLAN to another just bychanging theconfigurationon
switches,itisrelativelyeasy toput
allthe people workingtogetheron aparticularproject all
into asingle VLAN.They can
thenmore easilyshare filesand resources witheachother.
To behonest, though, virtualworkgroups sound likea good
ideaintheory, butoftendo
not work wellinpractice.Itturnsout thatusers are usually
more interestedinaccessing
company-wideresources (fileservers, printers,etc.)than
filesoneach other’s' PCs.
Greater flexibility. Ifusers move theirdesks,or just move
around theplace withtheir
laptops, then,iftheVLANs are setup therightway,they can

7. plug theirPC inatthenew
location,and stillbewithinthesameVLAN. This ismuch
harder whena network is
physically dividedupby routers.
Ease of partitioning off resources. Ifthereareservers
or other equipmentto which
thenetwork administratorwishes to limitaccess,thenthey
canbeput offintotheirown
VLAN. Thenusers inother VLANs canbegivenaccess
selectively.