What is a VLAN?

• A. Logical segmentation of a L2 network; isolates broadcast domains.
• B. Physical segmentation of networks.
• C. A security mechanism at Layer 3.
• D. A type of router.

Answer: (B)

VLANs create separate, logical LAN segments within a single Layer 2 network. They let you split a switch network into multiple broadcast domains, so broadcasts from one group don’t reach devices in another. This happens without laying down new physical cables or separate switches—the same physical infrastructure can carry traffic for several VLANs by tagging frames (802.1Q) and using trunk and access ports. When devices in different VLANs need to communicate, traffic must be routed by a Layer 3 device, which adds control and policy opportunities. So the idea being tested is that a VLAN is a logical segmentation at Layer 2 that isolates broadcast domains. Descriptions of physical segmentation miss the point, because VLANs use the same physical network to create multiple logical networks. They’re not a Layer 3 security mechanism, nor a router type.