Problem 2 Broadcast Distribution

Unfortunately, many, if not all, protocols create broadcasts. Some protocols create more than others. I happen to be particularly fond of Macintosh computers. Network administrators, however, despise AppleTalk due to the amount of broadcast traffic it generates. Every ten seconds, AppleTalk routers send routing updates that are broadcast frames in the network. Broadcasts go to all devices in the broadcast domain and must be processed by the receiving devices.

Other protocols share in the guilt. NetBEUI creates many broadcast frames, even when stations perform few network activities. TCP/IP stations create broadcasts for routing updates, ARP, and other processes. IPX generates broadcasts for SAP and GNS frames.

To add to the mix, many multimedia applications create broadcast and multicast frames that get distributed within a broadcast domain.

Why are broadcasts bad? Broadcasts are necessary to support protocol operations and are, therefore, overhead frames in the network. Broadcast frames rarely transport user data. (The exception might be for multimedia applications.) Because they carry no user data, they consume bandwidth in the network, reducing the effective available bandwidth for productive transfers.

Broadcasts also affect the performance of workstations. Any broadcast received by a workstation interrupts the CPU preventing 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 a problem in your network, you might mitigate the effect by creating smaller broadcast domains. This was described in Chapter 2. In VLANs, this means creating additional VLANs and attaching fewer devices to each. The effectiveness of this action depends upon the source of the broadcast. If your broadcasts come from a localized server, you might simply need to isolate the server in another domain. If your broadcasts come from stations, creating multiple domains might help to reduce the number of broadcasts in each domain.

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