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Campus LAN Design Fundamentals

1. Network Topologies: Flat vs. Hierarchical

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Feature Flat Network Architecture Hierarchical Network Architecture
Structure All devices connect via L2 switches on a single subnet. Multi-tiered design dividing network functions into distinct layers.
Broadcast Domains Single large broadcast domain; every packet consumes CPU across all end hosts. L3 devices (routers/L3 switches) bound and isolate broadcast domains.
Scalability Poor performance as host count increases. Highly scalable; easy to add blocks/buildings without redesigning.

2. The 3-Tier Enterprise Campus Model

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Core Layer (The Backbone)

  • Primary Function: High-speed, low-latency transport across the enterprise.

  • Key Requirements: High availability, fast convergence, modular scalability, and minimal packet processing overhead.

Distribution Layer

  • Primary Function: Aggregates wiring closets (access layer) and enforces policy-based control.

  • Key Responsibilities: Workgroup segmentation, security policy enforcement, problem isolation, and campus WAN aggregation.

Access Layer (Client Access)

  • Primary Function: Provides end-user devices, workstations, servers, and teleworkers direct access to network communication services.

3. Access Layer Architectures: L2 Switched vs. L3 Routed

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Option 1: Traditional Layer 2 Access (Switched)

  • L2/L3 Boundary: VLANs extend through access switches and terminate at the Distribution layer.

  • Spanning Tree Protocol (STP): Required to prevent loops, which results in blocking ~50% of available redundant uplinks.

  • VLAN Flexibility: VLANs can theoretically span multiple access switches (though discouraged in modern design).

  • Pros/Cons: Cheaper and simpler initial deployment, but wastes bandwidth due to STP blocking.

Option 2: Routed Layer 3 Access (Routed)

  • L2/L3 Boundary: VLANs terminate locally on the Access layer switch. Links between Access and Distribution are L3 routed links.

  • Uplink Utilization: Eliminates STP blocking on uplinks; all links actively forward traffic via Layer 3 Equal-Cost Multi-Pathing (ECMP) routing.

  • VLAN Locality: VLANs are strictly local to a single access switch and cannot span across switches.

  • Design Challenges: Requires extra planning for traffic isolation (e.g., separating guest traffic from internal corporate traffic using VRFs).

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