Data Center Networking

Modern DC fabrics are routed, overlayed and built for server-to-server (east-west) traffic.

// spine-leaf fabric

Every leaf connects to every spine — any server is exactly two hops from any other (predictable east-west latency).

Three-tier

core / aggregation / access

North-south optimized; STP blocks redundant links (half idle)

Spine-leaf (Clos)

two tiers — every leaf ↔ every spine

East-west optimized; ECMP keeps all links active; any host 2 hops away

// fabric roles

Leaf	Access / top-of-rack; acts as VTEP; servers & storage attach here
Spine	High-speed backbone; connects every leaf; often the BGP route-reflector
Super-spine	Interconnects multiple pods in a 5-stage Clos (hyperscale)
Border leaf	Gateway out of the fabric to WAN / external networks

// overlay & EVPN

Underlay	Routed IP fabric (OSPF/BGP) with ECMP; switches peer over loopbacks
Overlay	VXLAN tunnels carry tenant L2/L3 on top of the underlay
VXLAN	MAC-in-UDP encapsulation, port 4789, 24-bit VNI ≈ 16M segments
VTEP	VXLAN tunnel endpoint — encapsulates/decapsulates, usually the leaf
EVPN	BGP control plane: Type-2 = MAC/IP, Type-5 = IP prefix
IRB	Distributed anycast gateway for routing between VNIs

// resilience — Uptime Institute tiers

Tier	Availability	Max downtime	Topology
Tier I	99.671%	~28.8 h/yr	Single path, no redundancy
Tier II	99.741%	~22 h/yr	Redundant components (partial N+1)
Tier III	99.982%	~1.6 h/yr	Concurrently maintainable (N+1)
Tier IV	99.995%	~26 min/yr	Fault tolerant (2N / 2N+1)