Distribution layer is the 2nd layer of Cisco hierarchical model. At the distribution layer you have to make a decision regarding the layer-2 and layer-3 switches, this is also related to the cost because layer-3 switches are costly. So it is important that you figure out your requirement correctly in order to minimize the project cost. In some cases you may not need the layer-3 switches because of small network size and less number of users. But if you have to design a large network with high availability and redundancy then in such case Cisco 3550 or 3560 series layer-3 switches are great options for you.
Intelligence services are the part of distribution layer. This is the layer where you can implement security, policies and quality of service. If you have these types of services in your network then with layer-3 switches you handle these services easily.
Another important thing that you keep in mind while designing the core distribution layer with layer-3 infrastructure you have the Cisco professionals or professionals having knowledge of CCNP for handling and maintain the network.
If want to design a distribution layer for small network with layer-2 switches then you should have to configure STP (spanning tree protocol) or RSTP for better management and performance.
Designing the Campus Backbone:
Campus backbone design occurs very early in design of network. First thing that you have gathered from customer is the number of buildings or campuses. If you have 2 or more buildings connected through an enterprise campus infrastructure that you can use the optical fiber between them. Campus backbone is the key part of a network and it has multilayer design with layer-3 switches or routers. These routers models may vary according to size and services of network.
Network core must have high availability and also have redundancy in form of redundant hardware and redundant physical links between your key routers. If you have distant remote offices you can use the microwave/wireless media alone physical media for redundancy. But if your company doesn’t have the budget for redundancy you can go without this. But in such design you don’t have the fail-over.