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20 Chapter 1: Design Overview
NOTE
Business requirements govern the technical solution. For a small business, the star or ring
model could represent the ideal solution.
The Two-Tier Model
The two-tier model, shown in Figure 1-6, builds on the star model design and is beneficial in
certain settings. It allows two sites to serve as primary feeds to the remote branches. The
primary sites are also connected to each other. This solution can be effective for an organization
that has a limited budget but that still wants connectivity between its units.
Figure 1-6
Two-Tier Model
The Mesh Network
A mesh network exists in two flavors: partial mesh and full mesh. A full mesh provides full
connectivity between all systems on a WAN, and a partial mesh provides full connectivity for
some, but not all, devices connected to the WAN.
Although the full-mesh design represents the ideal solution with regard to scalability and
adaptability, it might cost too much to implement for a network that has many nodes. In an ideal
scenario, in which each device has unlimited capacity and cost is not an issue, everything could
be connected to everything.
Full mesh is a design concept that requires every device to have connectivity to every other
device (see Figure 1-7). Although the full-mesh design might be ideal for connecting two or
three devices in a small environment, it is not cost-effective to design large networks that
require all routers to behave as peers. As the number of links increases, the number of subnets
and routing peers also grows, and the complexity increases. The disadvantages of the full-mesh
topology revolve around two issues: cost and quantity. If the routers are far apart, the link costs
can quickly become prohibitively expensive because adding routers creates a geometric
explosion in links required. Also, the routers have a finite number of ports. When the number
of links surpasses a certain number, the routers will not have enough ports to support this
topology.
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87200333.book Page 20 Wednesday, August 22, 2001 2:18 PM