Figure 314 An example of a collapsed backbone

An important consideration in collapsed backbone topologies is that user communities are seldom conveniently distributed throughout a building. Instead, the users are scattered far and wide, which means that there is a good chance that they will be found on different LANs interconnected via a collapsed backbone router. Subsequently, simple network tasks among the members of a workgroup are likely to traverse the router. As fast as routers may be, they are still software driven. Therefore, in comparison to purely hardware devices, such as hubs and switches, routers are slow. Consequently, collapsed backbones might actually introduce a performance penalty not present with Layer 2-only LAN backbone solutions.

Today, Layer 3 LAN switches are available that duplicate the functionality of the router in a collapsed backbone without duplicating its slow performance. In other words, the IP switch performs inter-LAN routing at wire speeds!

Care should be taken when designing collapsed backbone LANs to absolutely minimize the amount of traffic that must cross the router. Use it as a traffic aggregator for LAN-level resources, like WAN facilities, and not indiscriminately, like a bridge.

Collapsed backbones, like the one shown in Figure 3-15, have another flaw: They introduce a single point of failure in the LAN. This is not necessarily a fatal flaw. In fact, many of the other LAN backbone topologies also introduce a single point of failure into the LAN. Nevertheless, that weakness must be considered when planning a network topology.

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