Hierarchical Network Architecture

Hierarchical LAN Design

When building a network it is a great advantage to follow a structured, hierarchical approach. The Hierarchical LAN Design breaks a complex networks into several layers that can provide improved resiliency, fault isolation and simplified maintenance.
The Hierarchical LAN Design comprises of 3 layers: Access, Distribution and Core.
At each aggregation layer the complexity is reduced from N*(N-1) connections needed to mesh connect N devices down to 2*N (when using individual links to a redundant aggregation devices) or even N (when using link aggregation to the aggregation devices).
Hierarchical LAN Design

Access Layer

  • Provides access to the network for endpoints and users. This means it also should support various technologies to accomodate different type of devices and network needs - for example, PoE (Power over Ethernet.
  • It is the network boundary and therefore it acts as security, QoS and policy trust boundary. The access layer must protect the network from human errors and malicious attacks
  • Traditionally access layer devices operate as L2 only devices but more modern takes on the design of the Access Layer will support devices operating at L3.

Distribution Layer

  • aggregates the access layer and provides connectivity from one access layer device to another, or to the WAN or other services.
  • Traditionally access layer has devices operating at both L2 (towards the Access Layer) and L3 (towards the Core Layer).

Core Layer

  • is used to aggregate multiple distributions. Without a core layer the distribution layer devices would need to be fully mashed.
  • It must be designed to provide packet swithing as fast as possible.
  • Devices of the Core Layer operate almost always as L3 devices.

Hierarchical Design Options

The 3 Tier Design

The 3 Tier design contains all three layers and makes use of the Core Layer to interconnect multiple distributions.

Collapsed Core - The 2 Tier Design

This design usually works for smaller deployments where the effort of adding and maintaining a Core Layer to connect the distributions would not reduce the complexity enough. In a collapsed core design, the Core Layer functionality is provided by the distribution layer.
Using the complexity formula to figure out when a collapsed core would be more beneficial than The 3 Tier Design, N*(N-1) is higher than 2*N when N is higher than 3.
Collapsed Core Design

Access Layer Design Options

  • Traditional Layer 2 Access should be used with a Distribution Layer that makes use of a FHRP protocol to provide gateway redundancy for a VLAN.
    • Looped Design
      • In the looped design STP must be used to break the loops. This will block traffic on some of the links thus making resource utilization sub-optimal
    • Loop Free Design
      • Is less common because it requires that each access switch supports a single VLAN which limits the flexibility of the network
  • Layer 3 Access
    • This option moves the L2 boundry to the access switch, effectively avoiding the L2 loops and making use of all possible paths. However not all applications can work in a L3 only network and still require L2 connectivity between hosts.

Simplified Distribution Layer

This design is called simpliefied because it makes use of technologies such as VSS (Virtual Switch System) or Stackwise.

Using VSS

  • VSS is usually used at the Distribution Layer and makes the (usually redundant) distribution devices work as a single logical device.
  • The Distribution Layer devices transfer control layer information over a Virtual Switch Link.
  • The design makes use of MEC (Multichassis EtherChannel) connections which bundles multiple physical links into a single logical link.
  • Since the VSS switch works as a single logical device there is no need for FHRP protocols for gateway redundancy.

Using Stackwise

  • Stackwise can be used at the Access Layer and can connect multiple devices into a single logical link
  • The devices that are part of a stack exchange control plane information using the Stackwise Interconnect ring.
  • The design makes use fo standard EtherChannel since the bundled links are between the same two devices.
Traditional vs VSS vs Stackwise
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Hierarchical LAN Design
Access Layer
Distribution Layer
Core Layer
Hierarchical Design Options
The 3 Tier Design
Collapsed Core - The 2 Tier Design
Access Layer Design Options
Simplified Distribution Layer