Data Center Networking Fundamentals
Core concepts
A data center fabric is judged by predictable latency, high bisection bandwidth, and fast failure convergence. Understand the two dominant architectures: the classic three-tier (core/aggregation/access with STP) and the spine-leaf Clos fabric, where every leaf connects to every spine and no leaf-to-leaf links exist. Know why Clos wins: equal-cost multipathing (ECMP), horizontal scale, and a bounded, predictable hop count. Be able to explain Layer 2 vs Layer 3 at the leaf, VXLAN as a MAC-in-UDP overlay with a 24-bit VNI, and the role of an underlay (routed, IP) versus overlay (tunnels).
Concepts to master
Oversubscription ratios, east-west vs north-south traffic, MLAG for dual-homing servers without STP, and the control-plane/data-plane split. Understand jumbo MTU requirements for VXLAN (extra 50 bytes of encapsulation) and why MTU mismatches silently break tunnels.
Traps
Questions love to conflate VXLAN with a routing protocol (it is encapsulation, not routing) or to claim MLAG requires STP. Watch for "all leafs connect to each other" - false in a Clos. Also distinguish a VNI (24-bit, ~16M) from a VLAN ID (12-bit, 4094).
Rule of thumb
For a VXLAN underlay, budget MTU = 1500 payload + 50 bytes overhead = 1550 minimum; configure 9214 on fabric links on Arista gear. If a tunnel pings small but drops large frames, suspect MTU before anything else.