When discussing network design, it is useful to categorize networks based on the number of devices are using
- Small network – Provides services for up to 200 devices.
- Medium-sized network – Provides services for 200 to 1000 devices.
- Large network – Provides services for 1000+ devices.
Network designs vary depending on the size and requirements of the organizations. For example, the networking infrastructure needs of a small organization with fewer devices will be less complex than the infrastructure of a large organization with a significant number of devices and connections.
There are many variables to consider when designing a network. For instance, consider the example in the figure. The sample high-level topology diagram is for a large enterprise network that consists of a main campus site connecting small, medium, and large sites.
Network design is an expanding area and requires a great deal of knowledge and experience. The intent of this section is to introduce commonly accepted network design concepts.
Regardless of network size or requirements, a critical factor for the successful implementation of any network design is to follow good structured engineering principles. These principles include:
Hierarchy – A hierarchical network model is a useful high-level tool for designing a reliable network infrastructure. It breaks the complex problem of network design into smaller and more manageable areas.
Modularity – By separating the various functions that exist on a network into modules, the network is easier to design. Cisco has identified several modules, including the enterprise campus, services block, data center, and Internet edge.
Resiliency – The network must remain available for use under both normal and abnormal conditions. Normal conditions include normal or expected traffic flows and traffic patterns, as well as scheduled events such as maintenance windows. Abnormal conditions include hardware or software failures, extreme traffic loads, unusual traffic patterns, Denial-of-Service (DoS) events, whether intentional or unintentional, and other unplanned events.
Flexibility – The ability to modify portions of the network, add new services, or increase capacity without going through a major fork-lift upgrade (i.e., replacing major hardware devices).
To meet these fundamental design goals, a network must be built on a hierarchical network architecture that allows for both flexibility and growth.