Campus Wired Network Design
The three-tier and collapsed-core models
Campus wired design is built on a hierarchy: access (user ports, PoE, VLAN edge), distribution (policy, inter-VLAN routing, aggregation), and core (high-speed transport). Small campuses collapse distribution and core into a collapsed core. Modern designs often use a two-tier spine-leaf in the data center and a routed-access or layer-3-to-the-edge approach in the campus to shrink failure domains.
What to design
Key concepts: oversubscription ratios (access-to-uplink bandwidth), link aggregation (LACP) for uplink resilience, Spanning Tree (RSTP/MSTP) as a loop guard where L2 remains, first-hop redundancy (VRRP/VSX/MLAG) so gateways survive a link or device failure, and PoE budget planning for APs, phones, and cameras. Know the difference between blocking (STP) and routing (ECMP) as redundancy mechanisms - routed designs converge faster and scale better.
Traps to watch
- Forgetting PoE class and total power budget when sizing access switches for Wi-Fi 6/6E APs.
- Assuming a redundant link alone provides redundancy - if both links share one power supply or one closet, it does not.
- Confusing oversubscription (a ratio) with utilization (a percentage).
- Ignoring cable and physical constraints: fiber distance, Cat6A for 10G to the edge, conduit capacity.
Rule of thumb
Size uplinks so peak access demand fits within the oversubscription target. Example: 48 access ports at 1 Gbps = 48 Gbps potential; two 10G uplinks = 20 Gbps, an oversubscription of 2.4:1 - acceptable for general access, too high for dense server or AP aggregation.