The core layer is the central internetwork for the entire enterprise and may
include LAN and WAN backbones. The primary function of this layer is to provide
an optimized and reliable transport structure and to forward traffic at high
speeds. In addition, the core layer is a high-speed switching backbone. Because
the primary job of a device in the core of the network is to switch packets,
the student should design the core layer to switch packets as fast as possible.
Therefore, the core of the network should not perform any packet manipulation.
Packet manipulation, such as checking access lists or filtering, would slow
down the switching of packets.
Modularity is another benefit of using a hierarchical design, because
changes in the internetwork are easily facilitated. Figure
shows the three
major layers of a hierarchical network design. In addition, modularity in
network design allows the user to create design elements that can be replicated
as the network grows. When a network design element requires a change, the cost
and complexity of making the upgrade is constrained to a small subset of the
overall network. In large flat or meshed network architectures, changes tend to
impact a large number of systems.
Modular structuring of the network
into small, easy-to-understand elements also simplifies fault isolation. The
user can easily understand the transition points in the network, and thus
identify failure points.
The Cisco three-layer hierarchical
internetworking model is illustrated in Figure
.
Sometimes it is mistakenly thought that the core, distribution, and access
layers must each exist as a clear and distinct physical entity. However, this
does not have to be the case. The layers are defined to aid in successful
network design and to represent functionality that must exist, in a network.
Refer to Figures
,
, and
for detailed
definitions for each layer. Each layer can be in distinct routers or switches,
can be combined in a single device, or can be omitted altogether. The way the
layers are implemented depends on the needs of the network being designed.
Note, a hierarchy must be maintained in order for a network to function
optimally.