LAN Architectures

Campus LAN Models

3-Tier (Traditional)

Layer	Function
Access	End hosts, QoS marking, PoE
Distribution / Aggregation	Aggregates access switches; policy, routing, QoS
Core	High-speed transport between distribution layers

2-Tier (Collapsed Core)

Layer	Function
Access	End hosts
Distribution	Aggregation + core combined

SOHO

All functions in a single multipurpose home router / wireless router.

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Traffic Patterns

Pattern	Direction	Description
North-South	Vertical	Traffic entering/leaving the data center
East-West	Horizontal	Traffic between servers inside the data center

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Topologies

Topology	Diagram
Star
Full Mesh
Partial Mesh

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Spine-Leaf (Clos Architecture)

Optimized for East-West traffic in data centers.

Three Golden Rules of Spine-Leaf

1. Every Leaf connects to every Spine
2. Spines never connect to other Spines
3. Leafs never connect to other Leafs

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VXLAN (Virtual Extensible LAN)

Network virtualization technology to overcome VLAN limits in massive data centers.

In simple terms: VXLAN stretches a Layer 2 network over a Layer 3 routed fabric.

Why it’s needed

In Spine-Leaf, all links are routed (L3) to avoid STP. But what if a VM on Leaf 1 needs the same subnet (L2) as a VM on Leaf 4? Traditional VLANs can’t cross routers.

How it works

1. Original L2 Ethernet frame is encapsulated in a UDP packet (port 4789)
2. Routed across the Spine-Leaf L3 backbone
3. Destination Leaf decapsulates — servers think they’re on the same local switch

Key Concepts

Term	Definition
VTEP (VXLAN Tunnel Endpoint)	Device (usually Leaf switch) that encapsulates/decapsulates VXLAN packets
VNI (VXLAN Network Identifier)	24-bit ID — the VXLAN equivalent of a VLAN ID (16M+ segments vs 4K)
Encapsulation	MAC-in-UDP (Outer: IP/UDP/VXLAN; Inner: original L2 frame)