Broadcast Frames

Not all frames contain unicast destination addresses. Some have broadcast or multicast destination addresses. Stations treat broadcast and multicast frames differently than they do unicast frames. Stations view broadcast frames as public service announcements. When a station receives a broadcast, it means, "Pay attention! I might have an important message for you!" A broadcast frame has a destination MAC address of FF-FF-FF-FF-FF-FF (all binary 1s). Like unicast frames, all stations receive a frame with a broadcast destination address. When the interface compares its own MAC address against the destination address, they don't match. Normally, a station discards the frame because the destination address does not match its own hardware address. But broadcast frames are treated differently. Even though the destination and built-in address don't match, the interface module is designed so that it still passes the broadcast frame to the processor. This is intentional because designers and users want to receive the broadcast frame as it might have an important request or information. Unfortunately, probably only one or at most a few stations really need to receive the broadcast message. For example, an IP ARP request creates a broadcast frame even though it intends for only one station to respond. The source sends the request as a broadcast because it does not know the destination MAC address and is attempting to acquire it. The only thing the source knows for sure when it creates the ARP request is the destination's IP address. That is not enough, however, to address the station on a LAN. The frame must also contain the MAC address.

Routing protocols sometimes use broadcast MAC addresses when they announce their routing tables. For example, by default, routers send IP RIP updates every 30 seconds. The router transmits the update in a broadcast frame. The router does not necessarily know all of the routers on the network. By sending a broadcast message, the router is sure that all routers attached to the network will receive the message. There is a downside to this, however. All devices on the LAN receive and process the broadcast frame, even though only a few devices really needed the updates. This consumes CPU cycles in every device. If the number of broadcasts in the network becomes excessive, workstations cannot do the things they need to do, such as run word processors or flight simulators. The station is too busy processing useless (for them) broadcast frames.

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