8. Cisco Hierarchical Model

There are three layers to the Cisco hierarchical model

• The core (backbone) layer provides optimal transport between sites.
• The distribution layer provides policy-based connectivity.
• The local-access layer provides workgroup/user access to the network.

 Core Layer

• Responsible for transporting large amounts of traffic reliably and quickly.
• Only purpose is to switch traffic as fast as possible (speed and latency are factors).
• Failure at the Core layer can affect every user, design for fault tolerance at this level.
• Design specifications

Don't Do at this layer

• Don't use access lists, packet filtering, or VLAN Routing.
• Don't support workgroup access here.
• Don't expand (i.e. more routers), upgrade devices instead (faster with more capacity).

Do at this layer 

• Design for high reliability (FDDI, Fast Ethernet with redundant links, or ATM).
• Design for speed and low latency.
• Use routing protocols with low convergence times.

 Distribution Layer

• Also called workgroup layer, this is the communication point between the access and core layers.
• Primary functions include routing, filtering, WAN access, and determining how packets can access the Core layer if necessary.
• Determines fastest/best path and sends request to the Core layer. Core layer will then quickly transport the request to the correct service.
• Place to implement network policies.

 Distribution Layer Functions 

• Access lists, packet filtering, queuing.
• Security and network policies such as address translation and firewalling.
• Re-distribution between routing protocols including static routing.
• Routing between VLANs and other workgroup support functions.
• Departmental or workgroup access.
• Definition of broadcast and multicast domains.
• Any media transitions that need to occur.

 Access Layer

• Controls local end user access to internetwork resources.
• Also called desktop layer.
• The resources most users need will be available locally.
• Distribution layer handles traffic for remote services.
• Continued use of access lists and filters.
• Creation of separate collision domains (segmentation).
• Workgroup connectivity at Distribution layer.
• Technologies such as DDR and Ethernet switching are seen in the Access layer.
• Static routing is here.