Three-Layer Hierarchical Design Model

Three-Layer Hierarchical Design Model
You’ll likely use the Cisco three-layer hierarchical design model as a guideline when designing
a network using the three-layer network design method. It consists of the Access, Distribution,
and Core layers. The Access layer is where end users gain access to the network. The Distribution
layer is a point where multiple Access layer devices are aggregated to the next layer. The
last layer—the Core layer—is where all distribution devices are connected, and it also connects
to other Core layer devices.
These are not hard and fast rules. You can even have a more hierarchical design in your network
by hanging another Access layer below the first Access layer. This way, the higher Access
layer takes on a partial role of the Distribution layer for the lower level Access layer, which is
why it is sometimes called an N-tier design approach. The advantages to using this design model
are improved fault isolation, ease of understanding for new engineers, easy network growth,
and the cost savings that come from a more efficient network.
Because this is a design guideline, you can also think of the three-layer hierarchy from a WAN or
campus network perspective. Both have three layers and both perform aggregation at each level, but
there are some distinct differences. In the WAN, the three layers are all routers, and the aggregation
is mainly for routes and secondarily for bandwidth. The campus network is usually a LAN where
the three layers are a combination of routers or layer 3 switches and layer 2 switches. It is used
mainly for bandwidth aggregation and secondarily for route summarization. Figure 11.1 shows a
three-layer WAN hierarchy, and Figure 11.2 shows a three-layer campus