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Cisco Hierarchical Model

There are three layers to the Cisco hierarchical model

  1. The core (backbone) layer provides optimal transport between sites.
  2. The distribution layer provides policy-based connectivity.
  3. 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.
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