4.1 VLAN Basics
4.1.4 VLANs and broadcast distribution
Practically every network protocol creates broadcast traffic for one reason or another. For example, consider the amount of broadcast traffic AppleTalk generates. AppleTalk routers generate routing updates in the form of broadcast frames every ten seconds! Broadcasts go to all devices in the broadcast domain and must be processed by the receiving devices. Further, many multimedia applications create broadcast and multicast frames that get distributed across the broadcast domain.

So why do network administrators dislike broadcast traffic? Broadcasts are necessary to support protocol operations and therefore are overhead frames in the network. With the exception of multimedia-based traffic, broadcast frames rarely transport user data. Since broadcasts tend not to carry user data, they consume bandwidth in the network, resulting in a reduction of the bandwidth for productive traffic.

Broadcasts also have a profound effect on the performance of workstations. Any broadcast received by a workstation interrupts the CPU and prevents it from working on user applications. As the number of broadcasts per second increases at the interface, effective CPU utilization diminishes. The actual level of degradation depends upon the applications running in the workstation, the type of network interface card and drivers, the operating system, and the workstation platform.

If broadcasts are creating problems in the network, creating smaller broadcast domains can mitigate the negative effects. In VLANs, this means creating additional VLANs and attaching fewer devices to each one. The effectiveness of this action depends upon the source of the broadcast. If your broadcasts come from a local server, you might simply need to isolate the server in another domain. If your broadcasts come from end stations, creating multiple domains might help to reduce the number of broadcasts in each domain.