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The Cisco Three-Layered Hierarchical Model

By 2004

Cisco has defined a hierarchical model known as the hierarchical internetworking model. This model simplifies the task of building a reliable, scalable, and less expensive hierarchical internetwork because rather than focusing on packet construction, it focuses on the three functional areas, or layers, of your network:

Core layer: This layer is considered the backbone of the network and includes the high-end switches and high-speed cables such as fiber cables. This layer of the network does not route traffic at the LAN. In addition, no packet manipulation is done by devices in this layer. Rather, this layer is concerned with speed and ensures reliable delivery of packets.

Distribution layer: This layer includes LAN-based routers and layer 3 switches. This layer ensures that packets are properly routed between subnets and VLANs in your enterprise. This layer is also called the Workgroup layer.

Access layer: This layer includes hubs and switches. This layer is also called the desktop layer because it focuses on connecting client nodes, such as workstations to the network. This layer ensures that packets are delivered to end user computers.

Figure INT.2.1 displays the three layers of the Cisco hierarchical model.
Cisco hierarchical models

When you implement these layers, each layer might comprise more than two devices or a single device might function across multiple layers.The benefits of the Cisco hierarchical model include:
  • High Performance: You can design high performance networks, where only certain layers are susceptible to congestion.

  • Efficient management & troubleshooting: Allows you to efficiently organize network management and isolate causes of network trouble.

  • Policy creation: You can easily create policies and specify filters and rules.

  • Scalability: You can grow the network easily by dividing your network into functional areas.

  • Behavior prediction: When planning or managing a network, the model allows you determine what will happen to the network when new stresses are placed on it.

Core Layer

The core layer is responsible for fast and reliable transportation of data across a network. The core layer is often known as the backbone or foundation network because all other layers rely upon it. Its purpose is to reduce the latency time in the delivery of packets. The factors to be considered while designing devices to be used in the core layer are:
  • High data transfer rate: Speed is important at the core layer. One way that core networks enable high data transfer rates is through load sharing, where traffic can travel through multiple network connections.

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