Campus Networking Fundamentals
Campus design questions test whether you can reason from traffic flow and failure domains rather than from memorized topologies. Know the classic three-tier model (access, distribution, core) and the collapsed two-tier spine-leaf variant used in modern campuses. Access switches aggregate endpoints and enforce port-level policy; distribution switches route between VLANs and summarize reachability; the core provides fast transport between distribution blocks. In a spine-leaf campus, every leaf connects to every spine, and routing happens at the leaf (L3 down to the access edge) so that any single link or spine failure is absorbed without STP convergence.
Key concepts: VLANs and 802.1Q trunking, spanning tree (MSTP is the practical default; RSTP for faster convergence), link aggregation (LACP), first-hop redundancy (VRRP), and MTU/underlay considerations. Understand broadcast domains, ARP behavior, and why large flat L2 domains are avoided.
Traps: (1) Confusing where routing should occur - in a routed-access design, the leaf/access switch is the L3 gateway, not the distribution. (2) Assuming STP is still the primary loop-prevention tool in a fully routed leaf-spine fabric; it is not. (3) Forgetting that VRRP priority and preempt settings determine which device is master. (4) Mixing up the direction of trunk allowed-VLAN lists.
Rule of thumb: if a question mentions a loop between two switches in the same VLAN, think STP; if it mentions redundant paths between leaves and spines, think routing/ECMP. A worked example: two access switches dual-homed to two distribution switches with VRRP - set the higher VRRP priority on the distribution switch that owns the STP root for that VLAN to keep the L2 and L3 paths congruent and avoid asymmetric forwarding.