VLAN??a href=”http://www.pass4sure.com/cisco-index.html”>Virtual LAN
As the network is gaining in popularity, the demand on the network in some enterprises is increasing sharply, and the scare of their internal corporate networks become larger and larger. Along with this situation the attendant security, network performance problems are the troubles for network administrators. However, VLAN technology, has solved the series of problems.
Firstly??t the first, people used HUB to linkmany clients to the network, and the HUB provided a public cable for the clients, as more and more users were accessed to the network, the broad band for each user was scale-down. For example : To link 10 HUB users with 10M Ethernet, then each user has to share bandwidth 10M/10-1M. Obviously, the way with sharing hub has been unable to meet the increasing demand for surge of network scale.People began to use switches to replace the hub. Switches access each user to provide a dedicated data channel, which is no longer similar to the HUB as a whole to share bandwidth.Although the bandwidth solved the distribution problem, but as more and more users access the network, people have found a new problem: Any arbitrary news will be broadcasting for all users in the network, the greater the size of network, the greater broadcasting on the scope, the performance of the entire network has become so intolerable to the customers because of broadcasting.At this time, network administrators use the router (which can be isolated broadcasting domain) to divide the bloated network into a number of smaller networks, the problem seems to have been resolved, but because the router only has few interface links to up the expense, so make the sub-tasks to the high cost on this network.
The switch has a number of interfaces, and transmit the data faster, but could not prevent the spread of the broadcasting;

Routers can prevent the broadcasting domain, but only a few interfaces for large networks subparagraph, the costs are too high, and because the speed delay to extend the time, it may reduce the network performance in a certain.
If there was certain technology or equipment to resolve this conflict on the issue then we can improve network performance greatly.
Secondly??t the first, a company with the Ministry of Finance, technical and sales departmets links the three office floors through computer switches together into the company network.We can envision a situation in such an interconnection, there is no security of the network environment, the three departments of office PCs can visit each other arbitrarily,The results will be that the monthly financial statements are passed round for perusal in all the company, sales of customer lists flow into the hands of those evil-intentions employees, the core technology information about the company’s products are showed to all staff… this is a very terrible thing! Then how to solve this kind of problem?
1. These three departments use an independent computer network, and not to link to the company network?
2. To separate the three departments networks with Routers, then control the visit with the visit list?

VLAN technology can solve the security and network performance problems.
VLAN technology allows two-story equipment with a three-tier broadcasting equipment isolation functions, and cam also blcok different VLAN members to exchange of visits.

Take catalyst switches to Cisco as an example. When the switch defined a VLAN and then adds its interface, then those specified interface will become a member of the VLAN, as the following graphs:
In a CISCO 1900 catalyst switches, it was divided two VLANs (left is the accounting VLAN, the right is management VLAN)

The switches’ port 1 to port12 are distributed to the accounting VLAN, port13 to port 24 are assigned to the Management VLAN. Since the switch does not allow broadcasting between the VLANs, so it means that the switches divided the network in the graph into sectors logically.

workstation A send a broadcasting to the network, then all workstations in the accounting VLAN will receive this broadcasting, but the switch will not send this broadcasting to any port on the management VLAN.In fact, the second-level switch will not send a frame to another VLAN. If indeed there is such a need, they can consider using routers or multilayer switches to communicate with each other between different VLAN.
For different VLAN, we can recards them as two independent network segments to each other on both logical and physical, on these two network segments, if need be through transmition then the best solution is to address through a third layer to enable them to exchange.
Pay attention to following problems when divide the VLAN::
1.Different vendors VLAN breakdown of the equipment may not be compatible;
2.After defined the VLAN, remember to add the members to their corresponding interfaces into the VLAN, otherwise VLAN will be invalid.
Problems must be attentionsed when one VLAN is used to numbers of swiches:
1.Trunk must be opened
2.VLAN must be corresponded
3.The interface of trunk must be achieving 100M Full Duplex
problems in doing different VLANs with one-arm Router to communicate:
1.Router’s trunk must be 100M Full Duplex
2.Do not configure IP to Router’s trunk, and do remain “no shutdown”.
3.The sub-interfaces which corresponds to the Routers, must match the VLAN.
Refer to following two samples:
Refer to the exhibit. How many broadcast domains exist in the exhibited topology?

A: one
B: two
C: three
D: four
Correct Answers: C
The topic test the CISCO candidates’ ability on division of the broadcasting and VLAN domain and understanding on basic functions. How many broadcasting domains are there in following graph? Careless candidates will choose A in a hurry, because the switch is not broadcasting in isolation, all the switches connected clients are in jurisdictions with the same broadcasting domain.In this question, network administrators divided the same VLAN on both of the switches - 3 VLANs across two switches. Each VLAN is a broadcasting domain, aso the answer is C.

Refer to the exhibit. The network administrator has created a new VLAN on Switch1 and added host C and host D. The administrator has properly configured switch interfaces FastEthernet0/13 through FastEthernet0/24 to be members of the new VLAN. However, after the network administrator completed the configuration, host A could communicate with host B, but host A could not communicate with host C or host D. Which commands are required to resolve this problem?

A: Router(config)# interface fastethernet 0/1.3
Router(config-if)# encapsulation dot1q 3
Router(config-if)# ip address 192.168.3.1 255.255.255.0
B: Router(config)# router rip
Router(config-router)# network 192.168.1.0
Router(config-router)# network 192.168.2.0
Router(config-router)# network 192.168.3.0
C: Switch1# vlan database
Switch1(vlan)# vtp v2-mode
Switch1(vlan)# vtp domain cisco
Switch1(vlan)# vtp server
D: Switch1(config)# interface fastethernet 0/1
Switch1(config-if)# switchport mode trunk
Switch1(config-if)# switchport trunk encapsulation isl
Correct Answers: A
As the graph, network administrators add a new VLAN3 on the S1, and add the F0/14 and F0/13 of S1 as VLAN3 members. But when all configuration is completed, administrators found VLAN2 members can exchange visits, but VLAN2 and VLAN3 members can not visit each other. To solve this problem, which orders must be added to the router? Analysis : within the same VLAN members exchange visits are no additional configuration, but different VLAN visits need the support of router.
From the graph, Router1 shows that the router connects two networks: 192.168.1.0 and the network of 192.168.2.0, but the lack of subnet 192.168.3.0 routing. The problem is right here. As long as the routers add a new sub-interface, the configuration of the network 192.168.3.0 of this can be resolved by mutual visits between different VLANs. So the answer is A.