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| 2.1 |  | VLAN Basics |
| 2.1.4 |  | VLANs and broadcast distribution |

Practically every network protocol creates broadcast traffic. AppleTalk hosts, Microsoft hosts, and Novell servers create broadcasts to announce or request services. Further, multimedia applications create broadcast and multicast frames that are distributed across the broadcast domain.

Broadcasts are necessary to support protocol operations, and are therefore acceptable overhead in the network. So why do network administrators dislike broadcast traffic? Broadcasts go to all devices in the broadcast domain and must be processed by the receiving devices. With the exception of multimedia-based traffic, broadcast frames rarely transport user data. Since switches flood broadcasts, they consume bandwidth in the LAN, 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.

VLANs form broadcast domains. Broadcasts and multicasts are not propagated between VLANs unless the intermediary network devices are specifically configured to do so. If broadcasts and multicasts are creating problems in the network, creating smaller broadcast domains can mitigate the negative effects. 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 a local server is sending broadcast traffic, then that server should be isolated in another broadcast domain. If the broadcasts and multicasts come from end stations, creating additional VLANs will help to reduce the number of broadcasts in each domain.