Cisco has developed a three-layer hierarchical model to help you design campus networks. Cisco uses this model to simplify designing, implementing, and managing large-scale networks. With traditional network designs, it was common practice to place the networking services at the center of the network and the users at the periphery.
However, many things in networking have changed over the past decade, including advancements in applications, developments in graphical user interfaces (GUIs), the proliferation of multimedia applications, the explosion of the Internet, and fast-paced changes in your users traffic patterns. Cisco developed the three-layer model to accommodate these rapid changes.
Cisco’s hierarchical model contains three layers: core, distribution, and access. A well-designed network typically follows this topology.
Core Layer
The core layer, as its name suggests, is the backbone of the network. It provides a high speed connection between the different distribution layer devices. Because of the need for high-speed connections, the core consists of high-speed switches and will not, typically, perform any type of packet or frame manipulations, such as filtering or Quality of Service.
Because switches are used at the core, the core is referred to as a layer-2 core. The traffic that traverses the core is typically to access enterprise corporate resources: connections to the Internet, gateways, e-mail servers, and corporate applications.
Distribution Layer
Of the three layers, the distribution layer performs most of the connectivity tasks. In larger networks, routers are used at the distribution layer to connect the access layers to the core. For smaller networks, sometimes switches are used. The responsibilities of the distribution layer include the following:
- Containing broadcasts between the layers
- Securing traffic between the layers
- Providing a hierarchy through layer-3 logical addressing and route summarization
- Translating between different media types
As mentioned in the section (Network Layer), routers give you by far the most flexibility in enforcing your company’s networking policies, since routers deal with logical addresses. And because routers are used at the distribution layer, the implementation of your policies, at least most of them, is done here.
Access Layer
The bottom layer of the three-layer hierarchical model is the access layer. Actually, the access layer is at the periphery of your campus network, separated from the core layer by the distribution layer. The main function of the access layer is to provide the user’s initial connection to your network.
Typically, this connection is provided by a switch, or sometimes, a hub. Sometimes if the user works at a small branch office or home office, this device can also be a router. But in most cases, the connection is provided by a switch.
However, many things in networking have changed over the past decade, including advancements in applications, developments in graphical user interfaces (GUIs), the proliferation of multimedia applications, the explosion of the Internet, and fast-paced changes in your users traffic patterns. Cisco developed the three-layer model to accommodate these rapid changes.
Cisco’s hierarchical model contains three layers: core, distribution, and access. A well-designed network typically follows this topology.
Core Layer
The core layer, as its name suggests, is the backbone of the network. It provides a high speed connection between the different distribution layer devices. Because of the need for high-speed connections, the core consists of high-speed switches and will not, typically, perform any type of packet or frame manipulations, such as filtering or Quality of Service.
Because switches are used at the core, the core is referred to as a layer-2 core. The traffic that traverses the core is typically to access enterprise corporate resources: connections to the Internet, gateways, e-mail servers, and corporate applications.
Distribution Layer
Of the three layers, the distribution layer performs most of the connectivity tasks. In larger networks, routers are used at the distribution layer to connect the access layers to the core. For smaller networks, sometimes switches are used. The responsibilities of the distribution layer include the following:
- Containing broadcasts between the layers
- Securing traffic between the layers
- Providing a hierarchy through layer-3 logical addressing and route summarization
- Translating between different media types
As mentioned in the section (Network Layer), routers give you by far the most flexibility in enforcing your company’s networking policies, since routers deal with logical addresses. And because routers are used at the distribution layer, the implementation of your policies, at least most of them, is done here.
Access Layer
The bottom layer of the three-layer hierarchical model is the access layer. Actually, the access layer is at the periphery of your campus network, separated from the core layer by the distribution layer. The main function of the access layer is to provide the user’s initial connection to your network.
Typically, this connection is provided by a switch, or sometimes, a hub. Sometimes if the user works at a small branch office or home office, this device can also be a router. But in most cases, the connection is provided by a switch.
