Understand the concept behind the three-layer model. In order to provide some framework to the design process, Cisco has designed the three-layer model, with the built-in principles that functionality can be assigned to a specific layer. This allows easier equipment selection and configuration, as long as you remember which layer does what! The access layer is used to provide access for most users into the rest of the network. The distribution layer is used for routing, filtering, and for some access tasks. Finally, the core layer is used to link switch blocks, and nothing that slows traffic down should be run here.
Understand the reasoning behind each of the switch block types. A switch block is a collection of switching devices that provide access and distribution layer functions. Each of the block models has specific needs, and the Cisco range of equipment is designed to carry out the appropriate tasks. The result is that different switches perform optimally at different layers. Servers may benefit from duplex connectivity and larger bandwidth than clients, due to the aggregated traffic, and because SAFE planning demands that the network be protected in depth, blocks must be clearly defined.
Understand the different product lines and the individual products that Cisco has available for switching tasks. Some Cisco devices are standard layer 2 switches, and use just the MAC address for forwarding. This is simple, cheap, and pretty fast. But the limits of scalability mean that such devices can not be used throughout the network, so Cisco also manufactures switches that provide real layer 3 services. Understanding the needs of different layers assists with the selection of the correct switch and the planning of the appropriate configuration, which might be simple layer 2 switching, or possibly MLS.