Enterprise Campus: Modularity
By applying the hierarchical design model across the multiple functional blocks of the enterprise campus network, a more scalable and modular campus architecture (commonly referred to as building blocks) can be achieved. This modular enterprise campus architecture offers a high level of design flexibility that makes it more responsive to evolving business needs. As highlighted earlier in this book, modular design makes the network more scalable and manageable by promoting fault domain isolation and more deterministic traffic patterns. As a result, network changes and upgrades can be performed in a controlled and staged manner, allowing greater stability and flexibility in the maintenance and operation of the campus network. Figure 3-3 depicts a typical campus network along with the different functional modules as part of the modular enterprise architecture design.
Figure 3-3 Typical Modular Enterprise Campus Architecture
When Is the Core Block Required?
A separate core provides the capability to scale the size of the enterprise campus network in a structured fashion that minimizes overall complexity when the size of the network grows (multiple campus distribution blocks) and the number of interconnections tying the multiple enterprise campus functional blocks increases significantly (typically leads to physical and control plane complexities), as exemplified in Figure 3-4. In other words, not every design requires a separate core.
Figure 3-4 Network Connectivity Without Core Versus With Core
Besides the previously mentioned technical considerations, as a network designer you should always aim to provide a business-driven network design with a future vision based on the principle “build today with tomorrow in mind.” Taking this principle into account, one of the primary influencing factors with regard to selecting two-tier versus three-tier network architecture is the type of site or network (remote branch, regional HQ, secondary or main campus), which will help you, to a certain extent, identify the nature of the site and its potential future scale (from a network design point of view). For instance, it is rare that a typical (small to medium-size) remote site requires a three-tier architecture even when future growth is considered. In contrast, a regional HQ site or a secondary campus network of an enterprise can have a high potential to grow significantly in size (number of users and number of distribution blocks). Therefore, a core layer or three-tier architecture can be a feasible option here. This is from a hypothetical design point of view; the actual answer must always align with the business goals and plans (for example if the enterprise is planning to merge or acquire any new business); it can also derive from the projected percentage of the yearly organic business growth. Again, as a network designer, you can decide based on the current size and the projected growth, taking into account the type of the targeted site, business nature, priorities, and design constraints such as cost. For example, if the business priority is to expand without spending extra on buying additional network hardware platforms (reduce capital expenditure [capex]), in this case the cost savings is going to be a design constraint and a business priority, and the network designer in this type of scenario must find an alternative design solution such as the collapsed architecture (two-tier model) even though technically it might not be the optimal solution.
That being said, sometimes (when possible) you need to gain the support from the business first, to drive the design in the right direction. By highlighting and explaining to the IT leaders of the organization the extra cost and challenges of operating a network that was either not designed optimally with regard to their projected business expansion plans, or the network was designed for yesterday’s requirements and it will not be capable enough to handle today’s requirements. Consequently, this may help to influence the business decision as the additional cost needed to consider three-tier architecture will be justified to the business in this case (long-term operating expenditure [opex] versus short-term capex). In other words, sometimes businesses focus only on the solution capex without considering that opex can probably cost them more on the long run if the solution was not architected and designed properly to meet their current and future requirements