Explanation/Reference:
Section: Design Objectives Explanation
Explanation:
Of the available choices, the modularity network architecture principle is most likely to facilitate troubleshooting in large, scalable networks. The modularity and hierarchy principles are complementary components of network architecture. The modularity principle is used to implement an amount of isolation among network components. This ensures that changes to any given component have little to no effect on the rest of the network. Modularity also simplifies the troubleshooting process by limiting the task of isolating the problem to the affected module.
The modularity principle typically consists of two building blocks: the access-distribution block and the services block. The access-distribution block contains the bottom two layers of a three-tier hierarchical network design. The services block, which is a newer building block, typically contains services like routing policies, wireless access, tunnel termination, and Cisco Unified Communications services.
The hierarchy principle is the structured manner in which both the physical and logical functions of the network are arranged. A typical hierarchical network consists of three layers: the core layer, the distribution layer, and the access layer. The modules between these layers are connected to each other in a fashion that facilitates high availability. However, each layer is responsible for specific network functions that are independent from the other layers.
The core layer provides fast transport services between buildings and the data center. The distribution layer provides link aggregation between layers. Because the distribution layer is the intermediary between the access layer and the campus core layer, the distribution layer is the ideal place to enforce security policies, provide load balancing, provide Quality of Service (QoS), and perform tasks that involve packet manipulation, such as routing. The access layer, which typically comprises Open Systems Interconnection (OSI) Layer 2 switches, serves as a media termination point for devices, such as servers and workstations.
Because access layer devices provide access to the network, the access layer is the ideal place to perform user authentication and to institute port security. High availability, broadcast suppression, and rate limiting are also characteristics of access layer devices.
Top-down and bottom-up are both network design models, not network architecture principles. The top- down network design approach is typically used to ensure that the eventual network build will properly support the needs of the network's use cases. For example, a dedicated customer service call center might first evaluate communications and knowledgebase requirements prior to designing and building out the call center's network infrastructure. In other words, a top-down design approach typically begins at the Application layer, or Layer 7, of the OSI reference model and works down the model to the Physical layer, or Layer 1.
In contrast to the top-down approach, the bottom-up approach begins at the bottom of the OSI reference model. Decisions about network infrastructure are made first, and application requirements are considered last. This approach to network design can often lead to frequent network redesigns to account for requirements that have not been met by the initial infrastructure.
Reference:
CCDA 200-310 Official Cert Guide, Chapter 2, Cisco Enterprise Architecture Model, pp. 49-50 Cisco: Enterprise Campus 3.0 Architecture: Overview and Framework: Modularity