Cisco Hierarchical Model

Cisco Hierarchical Model

• Defined by Cisco to simplify the design, implementation, and maintenance of responsive, scalable, reliable, and cost-effective networks.

• The 3 layers are logical and not physical – there may be many devices in a single layer, or a single device may perform the functions of 2 layers, eg: core and distribution.

The Cisco Hierarchical Model

The 3 layers in the Cisco Hierarchical Model:

Core layer:

Also referred to as the backbone layer. It is responsible for transferring large amounts of traffic reliably and quickly – switches traffic as fast as possible. A failure in the core can affect many users; hence fault tolerance is the main concern in this layer. The core layer should be designed for high reliability, high availability, high redundancy, high speed, and low convergence. Do not support workgroup access, implement access lists, VLAN routing, and packet filtering which can introduce latency to this layer.

Distribution layer:

Also referred to as the workgroup layer. Its primary functions are routing, Inter-VLAN routing, defining or segmenting broadcast and multicast domains, network security and filtering with firewalls and access lists, WAN access, and determining (or filtering) how packets access across the core layer.

Access layer

Also referred to as the desktop layer. Here is where end systems gain access to the network. The access layer (switches) handles traffic for local services (within a network) whereas the distribution layer (routers) handles traffic for remote services. It mainly creates separate collision domains. It also defines the access control policies for accessing the access and distribution layers.

• In a hierarchical network, traffic on a lower layer is only allowed to be forwarded to the upper layer after it meets some clearly defined criteria. Filtering rules and operations restrict unnecessary traffic from traversing the entire network, which results in a more responsive (lower network congestion), scalable (easy to grow), and reliable (higher availability) network. 
• A clear understanding of the traffic flow patterns of an organization helps to ensure the placement of network devices and end systems within the organization.