Summary

Network topology design can make the rest of the design process either significantly easier or more difficult. Cisco recommends using a hierarchical network design, which offers many benefits, including predictability, scalability, efficiency, cost control, and security.

Furthermore, Cisco recommends that small- to medium-sized businesses use a three-layered approach to hierarchy consisting of these layers: core, distribution, and access. Each layer has clearly defined functions, and once the network is established, it can scale significantly before it needs to be re-engineered. The core layer at the top of the hierarchy is for transporting large amounts of traffic reliably and quickly. Fault tolerance should be designed within the core layer, and high speed and low latency should be the driving concerns. The distribution layer should be designed for network traffic policies; control mechanisms such as access lists, packet filters, address translation, and redistribution; DDR; static routing; and firewall rules. The access layer at the bottom of the hierarchy is where users connect and should be where collision domains are defined.

In practice, you can break the real-world enterprise down into simpler modules using the Enterprise Composite Network Model. It segments the network into three functional areas, each of which contains multiple modules. Those functional areas are Enterprise Campus, Enterprise Edge, and Service Provider Edge. This is not a replacement for hierarchical design; rather, it is a systematic and scalable way to implement the hierarchical design principles.

Topologies that enhance network fault tolerance are often required. IP features such as HSRP and proxy ARP can improve fault tolerance in the workstation-to-router communication. Redundant WAN links can provide additional fault tolerance and can be used inside of hierarchical designs. Technologies such as DDR provide for backup links. When redundant links are used, design consideration should be given to load balancing. Issues such as pinhole congestion should be identified and avoided.