Network Design Fundamentals
Domain 1 - Network Design Fundamentals tests how you turn business requirements into a structured, resilient, addressable design: hierarchical models, redundancy, addressing and summarization, and routing-protocol selection.
Structured engineering principles
Cisco designs around four principles - hierarchy, modularity, resiliency, flexibility - delivered through the PPDIOO lifecycle (prepare, plan, design, implement, operate, optimize). In the hierarchical campus model each layer has one job: access aggregates users and enforces the trust boundary; distribution is the policy layer (summarization, ACLs, QoS, first-hop redundancy); core is simple, fast transport that should carry no complex policy. Modules (campus, WAN, data center, internet edge) contain failure domains so a fault in one does not cascade.
Resiliency is more than two of everything. You need device redundancy (dual supervisors, dual power), path redundancy (triangle or square topologies), and designed convergence (deterministic timers, fast hellos, BFD).
Addressing and summarization
Allocate one aggregate for the enterprise and carve it top-down so every tier can summarize: region, site, building, VLAN. Use /31 on point-to-point links, reserve a block for out-of-band management, and keep growth room at each tier. For IPv6, request a /48 (or /56) per site, use /64 per subnet, and decide SLAAC versus DHCPv6 up front.
Routing-protocol selection
Choose by scale, policy need, and vendor mix: OSPF or EIGRP inside the enterprise, BGP where policy, multihoming, or very large scale dominates, with summarization at area and region boundaries.
Worked example
A retailer needs 3 regions x 12 sites x 5 VLANs and expects 30% growth. Start from 10.0.0.0/12 (1,048,576 addresses). Give each region a /16, each site a /20 (4,096 addresses, i.e. 16 /24s, enough for 5 VLANs plus growth), and each VLAN a /24. The site router advertises one /20 and the region one /16, so three summaries reach the core instead of 180 subnets. Adding a VLAN later costs nothing in routing-table size.
Traps
- Calling two links resilient when they share a conduit, power feed, or line card.
- Subnetting sequentially so the region boundary cannot summarize.
- Sizing WAN or core bandwidth from user headcount instead of measured flows and oversubscription ratio.
- Deferring QoS and multicast design until after addressing is frozen.