The MAC Broadcast Domain

A MAC broadcast domain consists of all the devices connected to a LAN that receive framed data broadcast by a machine to all other machines on the LAN. The concept of a MAC broadcast is virtually universal throughout all IEEE-compliant LANs, regardless of their media access methodology. Consequently, this chapter examines MAC broadcast domains only in the context of Ethernet LANs.

Note FDDI is considered an IEEE-compliant LAN, even though it was not created by the IEEE. This is because the IEEE standards are passed to the American National Standards Institute (ANSI) for integration with their national standards. FDDI is an ANSI specification that complies with the ANSI equivalents of the IEEE 802.1 and 802.2 standards. Therefore, FDDI is IEEE compliant.

In essence, a MAC broadcast domain is the set of devices that can communicate directly without requiring higher-layer protocols or addressing. To better illustrate the difference between MAC broadcast and media access domains, compare Figure 3-1 and Figure 3-4.

Figure 3-4: An Ethernet MAC broadcast domain with five devices.

Figure 3-4 uses the same LAN configuration depicted in Figure 3-1, but identifies its MAC broadcast domain rather than the media access domain. The key distinction between MAC broadcast and media access domains will become obvious as the various LAN segmentation mechanisms are examined.

As with the media access domain, adding a second isolated LAN creates a second, fully separate broadcast domain. Figure 3-5 identifies the MAC broadcast domains of the LAN configuration presented in Figure 3-2.

Figure 3-5: Two separate Ethernet MAC broadcast domains.

Figure 3-5: Two separate Ethernet MAC broadcast domains.

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Interconnecting these LANs in the manner demonstrated in Figure 3-3 results in a single, but larger, MAC broadcast domain. Figure 3-6 illustrates this new broadcast domain.

Figure 3-6: Making one Ethernet LAN and MAC broadcast domain of the original two.

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This LAN's MAC broadcast domain consists of all the devices that populated the original two LANs' broadcast domains. In this scenario, any given broadcast message is now propagated across the network to twice as many devices as before. Therein lies the proverbial double-edged sword of LANs with large MAC broadcast domains: They can become quite large due to the segmentation of their media access domains, but suffer from the flatness, or lack of a hierarchy, to their MAC broadcast domain.

The Trouble with Flat LANs

LANs built with a single MAC broadcast domain are known as flat LANs. They are flat because there is no structure or hierarchy to their broadcast domains. The benefit of having a large broadcast domain is that it is extremely easy to reach all the devices that are interconnected on the LAN. The potential danger, also, is that it is extremely easy to reach all the devices on the LAN. The more devices you connect to a flat LAN, the more resources are consumed by each MAC broadcast message. Using the wrong communications protocol (that is, one that makes extensive use of MAC broadcasting) could easily compromise the performance of the network, as well as all the devices that populate it.

Note MAC broadcasts are performed by setting the destination MAC address of a frame of data to its highest possible value: FF:FF:FF:FF:FF:FF. This reserved address value, when placed in a frame's destination address field, is interpreted by all IEEE-compliant LANs as being addressed to all local machines. Therefore, it is accepted by all machines, regardless of what their actual MAC address is.

This paradox shouldn't be misinterpreted as meaning flat LANs are undesirable. On the contrary! The introduction of LAN switching led to a flattening of LANs. The larger a flat LAN is (in terms of its population), however, the more important it is to segment it. Segmentation is a technique that allows the overall size of a LAN to be expanded, by controlling the sizes of its media access and/or MAC broadcast domains.

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