Difficulty: Easy
Correct Answer: It breaks up broadcast domains in a layer 2 switch internetwork by creating separate virtual networks
Explanation:
Introduction / Context:
Virtual LANs, commonly known as VLANs, are a key feature of modern Ethernet switches. In a simple flat network, all devices connected to a switch share the same broadcast domain, which can lead to excessive broadcast traffic and limited security separation. VLANs allow network administrators to logically segment a physical switch into multiple virtual networks, each with its own broadcast domain. Understanding what a VLAN does and why it is used is essential for network design and is a frequent topic in certification and interview questions.
Given Data / Assumptions:
Concept / Approach:
A VLAN is a logical grouping of switch ports that forms its own broadcast domain, independent of physical topology. When a broadcast frame is sent by a device in one VLAN, it is forwarded only to devices in that same VLAN, not to all ports on the switch. This segmentation reduces unnecessary broadcast traffic and allows separation of departments, user groups, or security zones on the same physical infrastructure. VLANs are identified by numbers, often using the IEEE 802.1Q tagging standard to carry VLAN IDs across trunk links between switches.
Step-by-Step Solution:
Step 1: Consider a switch with multiple ports, all initially in the same default VLAN, so all connected devices are in one broadcast domain.Step 2: When one device sends an Address Resolution Protocol broadcast, the frame floods to every other port, even if the devices belong to different departments.Step 3: By configuring VLANs, the administrator assigns certain ports to VLAN 10, others to VLAN 20, and so on, effectively creating separate logical networks.Step 4: After VLAN configuration, a broadcast from a device in VLAN 10 is forwarded only to other ports in VLAN 10 and does not reach VLAN 20, which means the broadcast domain has been split.Step 5: When communication between VLANs is needed, a router or layer 3 switch performs inter VLAN routing, maintaining segmentation while allowing controlled connectivity.
Verification / Alternative check:
In a lab environment, you can verify the effect of VLANs by connecting two groups of computers to a managed switch and assigning each group to a different VLAN. Use simple tools like ping and watch how devices in the same VLAN can reach each other, while those in different VLANs cannot communicate until a router is added. Capturing broadcast frames will show that broadcasts stay within each VLAN, confirming that VLANs break up broadcast domains within the layer 2 switched network.
Why Other Options Are Wrong:
Option B talks about collision domains on a single switch port, but each switch port is already a separate collision domain. VLANs operate at the level of broadcast domains, not collision domains on a single port. Option C incorrectly suggests that a VLAN automatically increases link speed, which it does not; speed is determined by physical interface capabilities. Option D claims that VLANs convert a switch into a router, but routing requires layer 3 functions, not just VLAN configuration. These options do not accurately describe what a VLAN does.
Common Pitfalls:
A common mistake is to create VLANs without planning inter VLAN routing, leading to user complaints that systems cannot communicate. Another pitfall is misconfiguring trunk ports and VLAN tags, which causes traffic loss between switches. Administrators must also carefully manage which VLANs are allowed on trunks to avoid unnecessary broadcast propagation. Good VLAN design groups devices by function, security level, or department, and uses proper routing and access control to balance isolation with needed connectivity.
Final Answer:
Correct answer: It breaks up broadcast domains in a layer 2 switch internetwork by creating separate virtual networks
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