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LAN Backbones


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Routers and LANs



LAN Domains



LAN Segmentation



LAN Backbones



WAN Gateway




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IP Routing Fundamentals

From: IP Routing Fundamentals
Author: Mark Sportack
Publisher: Cisco Press (53)
More Information

LAN Backbones

A LAN backbone is any mechanism or facility that interconnects all the LAN's hubs. There are many different ways to construct a LAN backbone. Some of these are clever and highly functional. Others are simplistic and shortsighted. Some are easy to scale, and some are not. Regardless of its components or topology, a LAN backbone unifies the disparate mini-LANs that would exist if the hubs were not interconnected.

The simplest of all LAN backbones is a hub that interconnects other hubs. As discussed earlier in the chapter, this creates large, flat LANs with singular media access and MAC broadcast domains. Although this may be, in fact, the most economical backbone for small LAN environments, it does not scale very well. Adding new users may require the addition of hubs. It doesn't take a great imagination to see that all the ports available on the backbone hub can quickly be consumed by connections to other hubs. When this happens, the solution, typically, is to just keep adding hubs to the backbone. This is known as a serial, or daisy-chained, backbone. Daisy-chained hubs can quickly become an administrative nightmare; it is difficult to maintain accurate records of the LAN's topology and wiring schemes over time.

Routers can be used to form a highly scalable LAN backbone in one of two main ways:

  • Collapsed backbones

  • Parallel backbones

Collapsed Backbones

A collapsed backbone topology features a single, centralized router that interconnects all the LAN segments in a building. The router effectively creates multiple media access and broadcast domains, and thereby increases the performance of each of the LAN segments.

The simplest, and most typical, collapsed backbone topology segments a building according to its physical wire distribution. That is, all the computing and/or communications devices whose wiring is physically terminated in a single telephone closet become a segment. A single physical connection to this telephone closet (and its hubs and their computers, printers, and so on) runs back to a single port on a router. Therefore, they form a self-contained LAN segment, complete with their own media access domain, MAC broadcast domain, and IP addressing. A LAN segmented with a router using a collapsed backbone topology is illustrated in Figure 3-14.

Figure 3-14. 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.

Parallel Backbones

In some of the cases where collapsed backbones are untenable, a modified version may prove ideal. This modification is known as the parallel backbone. The reasons for installing a parallel backbone are many. User communities may be widely dispersed throughout a building; some groups and/or applications may have stringent network security requirements; or high network availability may be required. Regardless of the reason, running parallel connections from a building's collapsed backbone router to the same telephone closet enables supporting multiple segments to be run from each closet, as shown in Figure 3-15.

Figure 3-15. An example of parallel backbone topology.

The parallel backbone topology is a modification of the collapsed backbone. Much like the collapsed backbone, this backbone topology can create multiple media access and MAC broadcast domains. The key distinction is that this topology can achieve a finer degree of segmentation than a collapsed backbone can provide.

This approach marginally increases the cost of the network, but can increase the performance of each segment and satisfy additional network criteria, such as security and fault tolerance.


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