General Network Design

Notes on the Cisco three layer campus hierarchy, listing what the core, distribution, and access layers should and should not handle, plus VLAN tips.

Cisco three-layered hierarchy

Core Layer (Also called the Backbone)

• What the Core should do/have:
  • High speed Layer 3 transport
  • Low latency for traffic
  • QoS
  • Redundancy/fault tolerance
  • Limit diameter
  • Simplicity
  • Efficiency
• What the Core should NOT do/have:
  • Packet classification for QoS: Voice, Video, Data
  • Inter-VLAN routing: This is for distribution layer
  • Packet filtering/security features
  • Complicated packet manipulation (e.g. PBR)

Distribution Layer (between Access and Core layers)

• What the Distribution should do/have:
  • Broadcast domain perimeter
  • Inter-VLAN routing
  • WAN connectivity (including Internet)
  • Aggregation of Access Layer switches (e.g., floor switches)
  • QoS/Security mechanisms
  • IP Routing
• What the Distribution should NOT do/have:
  • Direct connectivity to end users
  • Layer 2 switching
  • Extensive STP operation
  • You should not see voice VLANs

Access Layer (End-User Connectivity)

• What the Access should do/have:
  • Direct connectivity to end users
  • Layer 2 switching
  • STP operations
  • Voice VLANs
  • PoE
  • Trust classification
  • Port-based security
  • QoS packet classification
  • DAI
• What the Access should NOT do/have:
  • Inter-VLAN routing
  • Direct connectivity to other Access Layer switches
  • Single uplinks to the Distribution Layer
  • Extensive security filtering
  • WAN connectivity

Some VLAN design notes

• If your broadcast traffic is more than 25% of your traffic, you should split off into the VLANs
• Do not use VLAN 1 (shutdown!)