Network without VLANs
Controlling Broadcast Domains is essential for optimizing network performance, especially in environments where broadcast traffic can overwhelm resources. In the default configuration, when a switch receives a broadcast frame on its ingress ports, it forwards the frame out all ports, excluding the port where the frame was received. Because the whole network is configured in the same subnet and no VLANs are configured, all ports operate within the same broadcast domain, leading to inefficiencies that can be mitigated with proper segmentation techniques.
As shown in the figure below, when the PC-1 sends out a broadcast frame, switch S-1 receives the broadcast frame and sends that broadcast frame out all of its ports except the ingress port. Ultimately, the whole network receives the broadcast for the reason that the network is one broadcast domain. As a result, the entire network receives the broadcast due to the single broadcast domain.
Network with VLANs
Figure 2 illustrates the segmented network with VLANs; the network has been segmented using two VLANs, VLAN 10 and VLAN 20. The IT department has assigned VLAN 10, and the admin department has assigned VLAN 20. When a broadcast is sent from any computer of the IT department (PC-2, PC-3, and PC-4), the switch (S1, S2) forwards that broadcast frame only to switch ports that are assigned to VLAN-10 and also to the Trunk port. Likewise, when a computer from the admin department sends a broadcast frame, the switch forwards it to ports configured for VLAN-20 and the trunk port.
The ports that connect both switches S1 and S2 are trunks and have been configured to support all the VLANs in the network. These trunk ports use 802.1Q tagging to carry traffic from multiple VLANs, ensuring frames are identified by their VLAN ID for proper forwarding. When S1 and S2 receive the broadcast frames on a port from VLAN-10. The switch forwards the broadcast frame out of the only other port configured to support VLAN 10, including the trunk port.
Conclusion – Controlling Broadcast Domains
In summary, the implementation of VLANs transforms network management by addressing the inefficiencies of a single broadcast domain inherent in networks without VLANs. Without VLANs, broadcast traffic floods across all ports, straining network resources and performance, as seen in the default configuration of switches like S-1 and S-2. By contrast, VLANs—such as VLAN 10 for IT and VLAN 20 for admin departments—segment the network into controlled broadcast domains, leveraging trunk ports with 802.1Q tagging to maintain efficient traffic flow.
This segmentation not only enhances security and scalability but also aligns with CCNA and CCNP objectives, equipping network professionals with the skills to optimize modern network designs as of July 12, 2025. Embracing VLANs is a critical step toward building robust, future-ready networks.
FAQs
What is a broadcast domain and why is it important to control?
A broadcast domain is a network segment where all devices receive broadcast traffic, which can overwhelm resources if uncontrolled. Managing it with VLANs reduces unnecessary traffic, improving performance and security for efficient network operation.
How do VLANs help in controlling broadcast domains?
VLANs segment a network into multiple broadcast domains, limiting broadcast traffic to specific VLANs like VLAN-10 or VLAN-20. This isolation, supported by trunk ports, enhances efficiency and prevents network congestion.
What are the benefits of controlling broadcast domain with VLANs?
VLANs improve network performance by controlling broadcast domains, enhance security by isolating traffic, and simplify management by grouping devices logically. They are essential for scalable, modern network designs as of July 2025.
How can trunk ports assist in controlling broadcast domains?
Trunk ports connect switches like S-1 and S-2, carrying traffic from multiple VLANs using tagging to maintain separation. This ensures broadcast frames stay within their designated VLAN, optimizing network flow.
