Virtual Local Area Network


A virtual LAN, commonly known as a VLAN , is a method of creating independent logical networks within a physical network.


Basically VLAN setup that behave as single link while physically it connected with different segments of LANs(by manageable switches). VLAN configured through software rather then hardware (like in LAN).if two network in a VLAN so they could not communicate to each other, for this purpose a gateway(router) is needed.



VLAN’s allow a network manager to logically segment a LAN into different broadcast domains. Since this is a logical segmentation and not a physical one, workstations do not have to be physically located together. Users on different places can now belong to the same LAN. 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.


Physical View


When a VLAN switches 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. There are two types of tagging ISL and 802.1Q both provide trunking. 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 MAC field, the source network address, or some other field or combination of fields.



Network Add




VLAN’s are classified based on the method used. To be able to do the tagging of data using any of the methods, the Switches would have to keep an updated database containing a mapping between VLAN’s and whichever field is used for tagging. For example, if tagging is by port, the database should indicate which ports belong to which VLAN. This database is called a filtering database. Switches would have to be able to maintain this database and also to make sure that all the switches on the LAN have the same information in each of their databases. The switches determines where the data is to go next based on normal LAN operations. Once the switches 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 switches sends the data without the VLAN identifier.

Why use VLAN?

VLAN’s offer a number of advantages over LAN’s. These are..

  • Increase the number of broadcast domains but reduce the size of each broadcast domain, which in turn reduces network traffic and increases network security (both of which are hampered in cases of single large broadcast domains).
  • Reduce management effort to create subnetworks.
  • Reduce hardware requirement, as networks can be separated logically instead of physically.
  • Increase control over multiple traffic types.
  • Create multiple logical switches in a physical switch

Case Study For Single Switch Configuration



In this case study, the user starts by creating two LANs, VLAN_1 & VLAN_2 while VLAN_3 is work by default, interfaces 5 through 8 in VLAN_2, and interfaces 9 through 12 in VLAN_3.However the configuration requires the use of VLAN configuration mode as well as the normal configuration mode.

VLAN configuration mode behaves a little differently from configuration mode. These commands are..

Switch_A#valan database

Switch_ A(vlan)#vlan 2 name VLAN_2

VLAN 2 added:

Name: VLAN_2

Switch_A(vlan )#vlan 3 name VLAN_3

VLAN 3 added:

Name: VLAN_3

Swtich_A(vlan )#exit

APPLY completed.


Switch_A# config t

Enter configurwatioon commnands,one per line. End with CNTL/Z

Switch_A(config)#int range fastethernet 0/5 – 8

Switch_A(config-if)#switchport mode access

Switch_A(config-if)#switchport access VLAN 2

Switch_A(config)#int range fastethernet 0/9 – 12

Switch_A(config-if)#switchport mode access

Switch_A(config-if)#switchport access VLAN 3


Switch_A#show vlan brief

VLAN Name Status Ports

—- ——————————– ——— ——————————-

1 default active Fa0/1, Fa0/2, Fa0/3

2 vlan_2 active Fa0/5, Fa0/6, Fa0/7, Fa0/8

3 vlan_3 active Fa0/9, Fa0/10, Fa0/11, Fa0/12

1002 fddi-default active

1003 token-ring-default active

1004 fddinet-default active, Fa0/4

Case Study & Configuration for VTP

cisco witches use the proprietary VTP to exchange VLAN configuration information between switches. The VTP process begins with Vlan creation on a switch_A called a Server. The changes are distributed as a broadcast throughout the network. Both VTP clients (Switch_B) and server hear the VTP messages and update their configuration based on those messages, So VTP allows switched network solution to scale to large sizes by reducing the manual configuration needs in the network.




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