1.1 Hierarchical Network Design Model
1.1.2 Router functionality at each layer
By using a hierarchy, you can simplify tasks such as addressing and device management. Using an addressing scheme that maps to the hierarchy reduces the need to redo the network addresses as a result of growth. Knowing where devices are placed in a hierarchy enables you to configure all routers within one layer in a consistent way because they all must perform similar tasks. A hierarchical model that provides a physical topology for building internetworks is shown in the figure to the left. Because the hierarchical structure uses three distinct layers that provide unique functionality, the routers placed at each layer also have unique functionality, as described in the following list:

Core routers provide services that optimize communication among routes at different sites or in different logical groupings. In addition, core routers provide maximum availability and reliability. Core routers should be able to maintain connectivity when LAN or WAN circuits fail at this layer. A fault-tolerant network design ensures that failures do not have a major impact on network connectivity.

Distribution routers control access to resources that are available at the core layer and must, therefore, make efficient use of bandwidth. In addition, a distribution router must address the quality-of-service (QoS) needs for different protocols by implementing policy-based traffic control to isolate backbone and local environments. Policy-based traffic control enables you to prioritize traffic to ensure the best performance for the most time-critical and time-dependent applications.

Access routers control traffic by localizing broadcasts and service requests to the access media. Access routers must also provide connectivity without compromising network integrity. For example, the routers at the access point must be able to detect whether a telecommuter dialing in is legitimate, yet require minimal authentication steps by the telecommuter.