Many networks do not require the complexity of a full three-layer hierarchy. Simpler hierarchies may be used.

An enterprise with several relatively small branches that require minimal inter-branch traffic may choose a one-layer design. Historically this has not been popular because of the length of the leased lines. Frame Relay, where charges are not distance related, is now making this a feasible design solution.

If there is a need for some geographical concentration, a two-level design is appropriate. This produces a “star of stars” pattern. Again, the pattern chosen based on leased line technology will be considerably different from the pattern based on Frame Relay technology.

When planning simpler networks, the three-layer model should still be considered as it may provide for better network scalability. The hub at the center of a two-layer model is also a core, but with no other core routers connected to it. Likewise, in a single-layer solution the area hub serves as the regional hub and the core hub. This allows easy and rapid future growth as the basic design can be replicated to add new service areas.