Cisco CCNA: Enabling Communication Between VLANs

How to Allow Communication Between VLANs

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On a corporate network, hosts on the same VLAN can communicate with each other, but they are unable to communicate with hosts on different VLANs. What is needed to allow communication between the VLANs?



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A. B. C. D.


Different VLANs can't communicate with each other, they can communicate with the help of Layer3 router. Hence, it is needed to connect a router to a switch, then make the sub-interface on the router to connect to the switch, establishing Trunking links to achieve communications of devices which belong to different VLANs.

When using VLANs in networks that have multiple interconnected switches, you need to use VLAN trunking between the switches. With VLAN trunking, the switches tag each frame sent between switches so that the receiving switch knows to what VLAN the frame belongs. End user devices connect to switch ports that provide simple connectivity to a single VLAN each. The attached devices are unaware of any VLAN structure.

By default, only hosts that are members of the same VLAN can communicate. To change this and allow inter-VLAN communication, you need a router or a layer 3 switch. Here is the example of configuring the router for inter-vlan communication RouterA(config)#int f0/0.1

RouterA(config-subif)#encapsulation ?

dot1Q IEEE 802.1Q Virtual LAN

RouterA(config-subif)#encapsulation dot1Q or isl VLAN ID RouterA(config-subif)# ip address x.x.x.x y.y.y.y

The correct answer is A, a router with subinterfaces configured on the physical interface that is connected to the switch.

A VLAN (Virtual Local Area Network) is a logical grouping of network devices that are configured to communicate with each other as if they were on the same physical network, even though they may be located on different physical segments. When hosts are on the same VLAN, they can communicate with each other directly, without the need for a router.

However, if hosts are on different VLANs, they are separated by a layer 3 boundary and cannot communicate with each other directly. To enable communication between VLANs, a layer 3 device such as a router is required.

Option B, a router with an IP address on the physical interface connected to the switch, is not sufficient because the router would need to have separate physical interfaces or subinterfaces for each VLAN in order to route traffic between them.

Option C, a switch with an access link that is configured between the switches, is also not sufficient because an access link only allows traffic to flow between a switch and a single VLAN.

Option D, a switch with a trunk link that is configured between the switches, allows multiple VLANs to pass over the same link, but it does not provide the layer 3 routing necessary to enable communication between the VLANs.

Therefore, the correct answer is A, a router with subinterfaces configured on the physical interface that is connected to the switch. Subinterfaces allow the router to connect to multiple VLANs on the same physical interface and route traffic between them.