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.
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