The Enterprise Composite Network model provides a modular framework
for designing networks. This modularity allows flexibility in network design
and facilitates ease of implementation and troubleshooting. The hierarchical
model divides networks into the Building Access, Building Distribution, and
Building Core layers, as follows:

-
Building Access layer – The Building Access layer is used to grant
user access to network devices. In a network campus, the Building Access layer
generally incorporates switched LAN devices with ports that provide
connectivity to workstations and servers. In the WAN environment, the Building
Access layer at remote sites may provide access to the corporate network across
WAN technology.
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Building Distribution layer – The Building Distribution layer
aggregates the wiring closets and uses switches to segment workgroups and
isolate network problems. Routing and packet manipulation occur in the Building
Distribution layer.
-
Building Core layer – The Building Core layer is a high-speed
backbone and is designed to switch packets as fast as possible. Because the
core is critical for connectivity, it must provide a high level of availability
and adapt to changes very quickly. Routing and packet manipulation above Layer
2 should be avoided in the Core, if possible.
The Enterprise Composite Model divides the enterprise network into
physical, logical and functional boundaries. These boundaries allow network
designers and engineers to associate and to configure specific network
functionality on equipment based upon its placement and function in the model.
Enterprise Composite Model Functional Areas
The Enterprise
Composite Network model introduces modularity by dividing the network into
functional areas that ease design, implementation and troubleshooting tasks. An
Enterprise Campus is defined as one or more buildings, with multiple virtual
and physical networks, connected across a high-performance, multilayer-switched
backbone. 
The Enterprise Composite Network model contains these three major functional
areas:
-
Enterprise Campus – The Enterprise Campus functional area contains
the modules required to build a hierarchical, highly robust campus network that
offers performance, scalability, and availability. This area contains the
network elements required for independent operation within a single campus,
such as access from all locations to central servers. The Enterprise Campus
functional area does not offer remote connections or Internet access.
-
Enterprise Edge – The Enterprise Edge aggregates connectivity from
the various resources external to the enterprise network. As traffic comes into
the campus, this area filters traffic from the external resources and routes it
into the Enterprise Campus functional area. It contains all of the network
elements for efficient and secure communication between the Enterprise Campus
and remote locations, remote users, and the Internet. The Enterprise Edge would
replace the "DMZ" area of most networks.
-
Service Provider Edge – This functional area represents connections
to resources external to the campus. This area facilitates communication to WAN
and Internet Service Providers’ (ISPs) technologies.