Hierarchical internetworking model

The Hierarchical internetworking model, or three-layer model, is a network design model first proposed by Cisco. The three-layer model divides enterprise networks into three layers: core, distribution, and access layer. Each layer provides different services to end-stations and servers.

Access layer

End-stations and servers connect to the enterprise at the access layer. Access layer devices are usually commodity switching platforms, and may or may not provide layer 3 switching services. The traditional focus at the access layer is minimizing "cost-per-port": the amount of investment the enterprise must make for each provisioned Ethernet port. As the name its self is “access” basically this layer supplies traffic to the network and performs network entry control. End users access network resources by way of the access layer. Acting as the front door to a network, the access layer employs access lists designed to prevent unauthorized users from gaining entry. The access layer can also give remote sites access to the network by way of a wide-area technology, such as Frame Relay, ISDN, or leased lines.

Distribution layer

The distribution layer is the "smart" layer in the three-layer model. Routing, filtering, and QoS policies are managed at the distribution layer. Distribution layer devices also often manage individual branch-office WAN connections. The distribution layer is located between the access and core layers and helps differentiate the core from the rest of the network. The purpose of this layer is to provide boundary definition using access lists and other filters to limit what gets into the core. Therefore, this layer defines policy for the network. A policy is an approach to handling certain kinds of traffic, including the following:  Routing updates  Route summaries  VLAN traffic  Address aggregation

Core layer

The core network provides high-speed, highly-redundant forwarding services to move packets between distribution-layer devices in different regions of the network. Core switches and routers are usually the most powerful, in terms of raw forwarding power, in the enterprise; core network devices manage the highest-speed connections, such as 10 Gigabit Ethernet.

See also

• Multi-tier architecture
• Service layer

References

• Khalid Raza, Mark Turner (2002), Cisco Network Topology and Design, Cisco Press
• High Availability Campus Network Design (PDF), retrieved 2010-08-23