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The typical network installation will have a single main distribution point for the network. This location would terminate all the vertical runs and all the telecommunications services from outside the building.
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Network Design in the Real World: Cabling I inherited a network years ago that had chronic problems. User connections would degrade or fail at seemingly random intervals. The tools available to us showed huge jumps in error rates, although no new stations had been added to the network. Both Token Ring and Ethernet were affected. Eventually, we learned that copper cables had been run next to the freight elevator shaft, and the elevator motor and systems played havoc with the data. When fiber was installed along with shielding (for copper-only services), the problem was resolved. A sharp electrician found the problem. |
| The distribution room is typically in the basement or on the first floor of the building, although the designer should consider the risk of flooding and other disasters before allocating facilities. Usually, the room will need to align with the wiring closets on the other floors. |
Figure 2.4 illustrates a typical building installation. This design is called a distributed backbone—routers on each floor connect to the backbone, typically via FDDI. No end stations are placed on the backbone.
The actual design shown in Figure 2.4 is uncommon in modern designs. This is primarily due to the expense of having routers on each floor. This design would likely have used hubs in the place of switches.
Figure 2.4 also has similarities with legacy Token-Ring installations. Consider Figure 2.5, which illustrates a common Token-Ring installation. All rings operate at 16Mbps. It should be clear that a bottleneck will appear at the backbone or on the server ring—four user rings at 25 percent utilization would equal the entire backbone capacity. The use of the 80/20 rule (where 80 percent of traffic remains local) would provide more growth room. However, many Token Ring installations were installed for mainframe (offsubnet) access. FastEthernet or FDDI was often used to resolve this over-subscription problem. Another popular technique was to create multiple backbone rings, typically divided on a per-protocol basis.
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FIGURE 2.4 LAN intra-building installation
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FIGURE 2.5 LAN intra-building installation with Token Ring
As routing technology advanced and port density increased, the LAN model migrated toward the collapsed backbone. This design would place a single router in the main telecommunications room and connect it to hubs in the wiring closets. This configuration would frequently incorporate switches. Figure 2.6 illustrates the collapsed backbone design. Note that the vertical links would likely use fiber connections. FDDI is still extremely popular today among many Fortune 500 companies due to its fully redundant design capability.
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FIGURE 2.6 LAN intra-building installation with collapsed backbone
Current network design models strive to eliminate spanning-tree issues. As a result, switches and routers must work together to create a redundant, loop-free topology without relying on the Spanning-Tree Protocol or Layer 2 redundancy. As switch technology has advanced, this option has been made more available.
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Network Design in the Real World: The Future of Token Ring While only time will tell, it appears fairly inevitable that Token Ring will depart from the landscape. As of this writing, the 802.5 committee (responsible for Token Ring standards) had diminished substantially and was discussing its options—including a hibernation phase for the group. Whatever happens, it seems clear that efforts to migrate to and install Ethernet will be more prevalent in the future. |
Please note that this section is beyond the scope of the exam, but it is likely that Cisco will include this material in future exam revisions. A practical application of this material necessitates its inclusion here.
Consider the design illustrated in Figure 2.7. A complete loop has been created at Layer 2, but spanning tree is configured to block a port on the access-layer switch. Routers are not displayed in order to emphasize the Layer 2 facets of this installation.
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FIGURE 2.7 Layer 2 switch design
Consider the change to the network that is illustrated in Figure 2.8. The link between the two distribution layer switches has been removed for the VLAN that services the access layer. HSRP has also been deployed. While this design is shown in Figure 2.8 with external routers, the connections could also be provided by a route module in the switch.
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FIGURE 2.8 Layer 3 switch design
 | Figure 2.8 shows the use of external routers, which may lead to a split subnet or black hole problem, as discussed in Chapter 13. This design works best when using RSM or internal Layer 3 logic in the switch, as the link failure from the distribution switch to the access switch will down the router interface, preventing this problem. |
In making this change, the designer has eliminated the slower spanning-tree process and potentially eliminated the need for BPDUs (Bridge Protocol Data Units) altogether—although there is still a risk of the users creating bridging loops. The design is redundant and quite scalable. In addition, with routers and switches working together in multilayer switching configurations, the latency often associated with routers is reduced as well. A typical installation using this design model would place a single transit VLAN between the switches. Such a design would still avoid a Layer 2 loop while maintaining a through switch connection. Designers should consider the expected network behavior during both normal and failed scenarios when architecting any configuration.
 | Designers should not disable the Spanning-Tree Protocol unless they can ensure a loop-free topology. |
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