Source: http://www.cnitpm.com/pm/27569.html
The above is a hierarchical network design model diagram. A hierarchical network design includes the following 3 layers:
■ Core layer - provides optimal interval transmission
■ Convergence layer - provides policy-based connections
■ Access layer - provides multi-service applications and other network applications to provide user access to the network
The functions of each layer are described below
1. The function of the core layer The
core layer is a high-speed switching backbone. His design goal is to minimize the time spent exchanging packets for presentation. Like Area 0 in Open Shortest Path First (OSPF), Core and Backbone are synonymous. This layer of the campus network should not do any processing of packets/frames, such as processing access lists and filtering, as this would slow down packet switching. The current common practice is to fully adopt a Layer 3 switching environment at the core layer, which means that VLANs and VLAN trunks do not appear in the core layer. This also means that spanning tree loops in the core layer can often be avoided as well. The main function of the core layer is to provide high-speed connections between various aggregation layer devices in the campus network.
2. Functions of the aggregation layer
In the campus network, the aggregation layer is the demarcation point between the core layer and the access layer. It helps define and differentiate the core layers. The function of the aggregation layer is to define the boundaries of the network. Processing of packets/frames should be done at this layer. In a campus network environment, the aggregation layer can include some of the following functions:
■ Aggregation of addresses or areas;
■ Connecting department or workgroup access to the backbone;
■ Definition of broadcast/multicast domains;
■ Inter-VLAN (Inter-VLAN) Routing;
■ Media Conversion;
■ Security Policy.
In a non-campus environment, the aggregation layer handles the redistribution of information between routing domains and is often the demarcation point between static and dynamic routing protocols. The aggregation layer can also be the point of access for remote sites to access the corporate network. Aggregation layers can be aggregated into layers that provide policy-based connectivity. Packet processing, filtering, route summarization, route filtering, route redistribution, inter-VLAN routing, policy routing, and security policies are some of the main functions of the aggregation layer.
3. Access layer and other functions
The access layer is the point at which local end users are permitted to access the network. This layer may also use access lists or filters to meet the needs of a specific set of users, such as those who frequently participate in videoconferencing. Typically, Layer 2 switches play a very important role in the access layer. In the access layer, switches are called edge devices because they are located at the boundaries of the network. In a campus network environment, the access layer includes the following functions:
■ Shared bandwidth;
■ Switched bandwidth;
■ MAC layer filtering;
■ Micro-segmentation.
In a non-campus environment, the access layer can connect remote sites to the corporate network through wide-area technologies such as Plain Old Telephone System (POTS), Frame Relay, ISDN, xDSL, and leased lines.
Some people mistakenly believe that the 3 layers (core, aggregation and access) must exist in the network as well-defined physical entities, but this is not the case. The layers are defined to successfully implement the network design and represent the functions that must exist in the network. Instances of each layer can be individual routers and switches, represented by physical media, combined into a single device, or omitted entirely. How each layer is implemented needs to be determined according to the goals of network design. However, in order for the network to work optimally, hierarchies are necessary.