Cisco Enterprise Networks - Design

Network design is unavoidable. Even when expanding an existing network to a new branch, you are doing design — the existing network constrains your choices, which can make design feel trivial. But when you must design from scratch, or when you need to justify or change an existing design, a solid understanding of theory is essential.

The most important insight is that networking is inter-process communication. The primary purpose of any network is to allow applications running on different systems to communicate. Every design decision should ultimately serve that goal.

Most "design work" in practice is copying what already exists. That is not inherently wrong, but it means most engineers never deeply understand why a design works — or why it breaks under different conditions.