Where excessive broadcasts are observed, it is important to identify the
source of the broadcasts so that appropriate steps can be taken. When analyzing
traffic, keep in mind that switches are responding to broadcast frames, while
traffic analyzers are usually more concerned about the Layer 3 packets.
Generally this is not a problem as a broadcast packet is carried by broadcast
frame but different tools will report broadcasts differently depending on
whether they have a Layer 2 or Layer 3 focus.
Generally, excessive broadcasts result from one of the following situations:
- Poorly programmed or configured applications.
- Large Layer 2 broadcast domains.
- Underlying network problems such as STP loops or route flapping.
Using a protocol analyzer, the source of the problem can be readily
ascertained. Each of the above situations dictates a different solution.
Some applications such as Symantec’s Ghost server and streaming video
servers use broadcast and multicast traffic. This is an attempt to reduce the
number of streams of traffic sent, thereby minimizing bandwidth usage and to
improving performance. If the network requires applications that are heavy
producers of broadcast and multicast traffic, consider the following techniques
to reduce the impact on other devices in the network:
- Create a separate VLAN for devices and hosts that are broadcast intensive.
- Configure switches to be multicast aware.
- Consider using scheduling for distribution services such as Ghost, so that
broadcast traffic can be limited to off peak times.
Large Layer 2 broadcast domains can sometimes occur where the hosts
themselves do not generate much traffic and their perceived network needs are
not great. It is tempting in these situations to avoid the expense of a Layer 3
hierarchy and simply adopt a flat, switched, structure. Unfortunately, modern
operating systems use broadcasts extensively to discover network services and
other hosts. Large, lightly loaded, Layer 2 networks will quickly find that the
majority of their traffic is made up of broadcasts.