Controlling Broadcast Domains with VLANs

Network without VLANs

In the default configuration, when a switch receives a broadcast frame on the ingress ports of the switch it forwards the frame out all ports excluding the port where the broadcast frame was received. Because the whole network is configured in the same subnet and no VLANs are configured therefore all ports are working in the same broadcast domain. As shown in the figure; when the host 1 sends out a broadcast frame, switch S2 receives the broadcast frame and sends that broadcast frame out all of its ports except ingress port. Ultimately the whole network receives the broadcast for the reason that the network is one broadcast domain.

Network with VLANs

Figure 2 illustrates the segmented network with VLANs; the network has been segmented using two VLAN, VLAN 10 and VLAN 20. The IT department is assigned VLAN 10 and the admin department has assigned VLAN 20. When a broadcast is sent from the IT department computer; Host-1, Host-2, and Host-5  to switch S2; the switch forwards that broadcast frame only to those switch ports configured to support VLAN 10 and Trunk port. Same as when computer from admin department sent a broadcast frame; the switch forwards the frame to port that is configured for VLAN-20 and 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. When S1 and S2 receive the broadcast frame on a port from VLAN-10. The switch forwards that broadcast frame out of the only other port configured to support VLAN 10; which is trunk port. When VLANs are configured on a switch, the transmission of unicast; multicast, and broadcast traffic from a host in a particular VLAN is limited to the devices that are in that VLAN. The VLAN create multiple broadcast domains in the switch so the broadcast can be controlled with creating multiple broadcast domains.