The hierarchical network design model divides network functionality into the access layer, the distribution layer, and the core layer. The Cisco Enterprise Architecture further divides the network into functional components.
A well-designed network controls traffic and limits the size of failure domains. Routers and multilayer switches can be deployed in pairs so that the failure of a single device does not cause service disruptions.
A network design should include an IP addressing strategy, scalable and fast-converging routing protocols, appropriate Layer 2 protocols, and modular or clustered devices that can be easily upgraded to increase capacity.
A mission-critical server should have a connection to two different access layer switches. It should have redundant modules when possible and a power backup source. It might be appropriate to provide multiple connections to one or more ISPs.
Security monitoring systems and IP telephony systems must have high availability and often have special design considerations.
The network designer should specify a router from the appropriate category: branch router, network edge router, or service provider router. It is important to also deploy the appropriate type of switches for a given set of requirements, switch features and specifications, and expected traffic flow.