Data Center and WAN/Edge Design
Data center topologies
Expect questions on spine-leaf (Clos) fabrics: why leaf switches connect only to spines, how ECMP provides active-active paths, and why a consistent hop count simplifies troubleshooting. Understand VXLAN overlay with an EVPN control plane, the role of VTEPs, and how overlay independence from the physical underlay enables workload mobility. Know multichassis aggregation (MLAG/VSX) and its failure modes, and the difference between a Layer 2 stretch and a routed DCI.
WAN and edge
Know the shift from hub-and-spoke MPLS to hybrid WAN with internet and SD-WAN: application-aware routing, per-application policy, encryption overlays, and local internet breakout. Understand when to centralize versus distribute security inspection, and how overlay tunnels interact with underlay SLA. Edge topics include NAT, high availability pairs, and routing protocol choice at the edge (BGP for multihoming, with prefix filtering and AS-path considerations).
Traps
Common traps: assuming ECMP works without consistent hashing inputs; forgetting that VXLAN adds 50 bytes of overhead and therefore requires a larger underlay MTU; treating an active/standby firewall pair as if it scales throughput; and assuming an SD-WAN overlay fixes a poor underlay rather than reacting to it.
Worked rule of thumb
For any fabric, compute bisection bandwidth: with N leaves each uplinked at L Gbps to every spine, total leaf-to-spine capacity is N × L per spine tier. If oversubscription at the leaf exceeds the design target (commonly 3:1 or better for general workloads, closer to 1:1 for storage-heavy), add uplinks or spines before adding leaves.