4.1 VLAN Basics
4.1.5 VLANs and bandwidth utilization
When users attach to the same shared segment, all of them share the bandwidth of the segment. Every additional user attached to the shared medium means there is less average bandwidth available for each user. If the sharing becomes too great, user application performance will begin to suffer. The network administrator will begin to suffer as well because users will begin complaining and asking for more bandwidth. VLANs, which are usually created with LAN switch equipment, can offer more bandwidth to users than is inherent in a shared network.

Remember that each interface on a switch behaves like a port on a legacy bridge. Bridges filter traffic that does not need to go to segments other than the source. If a frame needs to cross the bridge, the bridge forwards the frame to the correct interface and to no others. If the bridge or switch does not know where the destination resides, it floods the frame to all ports in the broadcast domain (VLAN) except the "source port."

In a switched environment, a station will usually see only traffic destined specifically for it. The switch will filter most of the other background traffic in the network. This allows the workstation to have full, dedicated bandwidth for sending or receiving interesting traffic. Unlike a shared-hub system where only one station can transmit at a time, the switched network in the Figure allows many concurrent transmissions within a broadcast domain without directly affecting other stations inside or outside of the broadcast domain. Station pairs A/B, C/D, and E/F can all communicate without affecting the other station pairs.