Distribution-to-Core Layer Design Principles

Author Ermias Teffera

The Distribution-to-Core layer in a hierarchical network is critical because it connects the distribution layer (aggregation of access switches) to the backbone (core) of the network. Proper design ensures high availability, scalability, and performance.

1. Role of the Distribution Layer

• Aggregates multiple access switches from floors, buildings, or campuses.

• Enforces policies, security (ACLs), and QoS.

• Performs routing between VLANs for inter-VLAN communication.

• Connects upstream to the core layer.

  
  

2. Core Layer Responsibilities

• Provides high-speed backbone for all traffic between distribution blocks.

• Minimal processing: No access control or QoS enforcement, just fast packet forwarding.

• Ensures redundancy and resiliency across the network.

  
  

3. Design Principles Between Distribution and Core

a) Oversubscription Ratios

• Cisco recommends a Distribution-to-Core ratio of 8:1.

• This means up to 8 distribution uplinks can share 1 core uplink in high-level planning.

• The goal is to balance cost with sufficient bandwidth.

  
  

b) Redundant Links

• Use dual uplinks from distribution to core for high availability.

• Combine with EtherChannel to aggregate bandwidth.

  
  

c) Layer 3 Routed Links

• Typically, distribution-to-core links are routed (Layer 3) to avoid loops.

• Using Layer 3 eliminates the need for spanning tree, allowing faster convergence.

  
  

d) ECMP (Equal-Cost Multi-Path)

• Use ECMP to load-balance traffic across multiple core uplinks.

• Ensures that traffic doesn’t congest a single path.

  
  

e) Minimal Core Services

• The core should not perform security checks, VLAN tagging, or QoS—these are handled in the distribution.

• Core switches focus solely on high-speed forwarding.

4. Diagram: Distribution-to-Core Layer

• Each distribution switch connects to multiple core switches for redundancy.

• ECMP or EtherChannel ensures load balancing and bandwidth aggregation.

5. Best Practices

• Use Layer 3 links to the core to prevent spanning tree issues.

• Dual-core connections for high availability.

• Aggregate links with EtherChannel to increase bandwidth.

• Monitor utilization and adjust oversubscription ratios if needed.

• Keep the core simple and fast, avoiding unnecessary services.

  
  

6. Summary

• The distribution layer aggregates access networks and enforces policies.

• The core layer provides a high-speed backbone with redundancy.

• Use Layer 3 routing, dual links, ECMP, and EtherChannel to ensure scalable, resilient, and high-performance network design.