WLAN Topologies
Modularity

The core layer is the central internetwork for the entire enterprise and may include LAN and WAN backbones. The primary function of this layer is to provide an optimized and reliable transport structure and to forward traffic at high speeds. In addition, the core layer is a high-speed switching backbone. Because the primary job of a device in the core of the network is to switch packets, the student should design the core layer to switch packets as fast as possible. Therefore, the core of the network should not perform any packet manipulation. Packet manipulation, such as checking access lists or filtering, would slow down the switching of packets.

Modularity is another benefit of using a hierarchical design, because changes in the internetwork are easily facilitated. Figure shows the three major layers of a hierarchical network design. In addition, modularity in network design allows the user to create design elements that can be replicated as the network grows. When a network design element requires a change, the cost and complexity of making the upgrade is constrained to a small subset of the overall network. In large flat or meshed network architectures, changes tend to impact a large number of systems.

Modular structuring of the network into small, easy-to-understand elements also simplifies fault isolation. The user can easily understand the transition points in the network, and thus identify failure points.

The Cisco three-layer hierarchical internetworking model is illustrated in Figure . Sometimes it is mistakenly thought that the core, distribution, and access layers must each exist as a clear and distinct physical entity. However, this does not have to be the case. The layers are defined to aid in successful network design and to represent functionality that must exist, in a network. Refer to Figures , , and for detailed definitions for each layer. Each layer can be in distinct routers or switches, can be combined in a single device, or can be omitted altogether. The way the layers are implemented depends on the needs of the network being designed. Note, a hierarchy must be maintained in order for a network to function optimally.


Interactive Media Activity

Interactive Activity: Cisco Three-Layer Internetwork Design Model

This is a drag and drop activity relating the concepts in each of the three layers of internetwork design.