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The Cisco Hierarchical Model

Core Layer

The core layer is literally the core of the network. At the top of the hierarchy,
the core layer is responsible for transporting large amounts of traffic both
reliably and quickly. The only purpose of the core layer of the network is to
switch traffic as quickly as possible. The traffic transported across the core
is common to a majority of users. However, remember that user data is pro-
cessed at the distribution layer, and the distribution layer forwards the
requests to the core, if needed.

If there is a failure in the core, every single user can be affected. Therefore,

fault tolerance at this layer is an issue. The core is likely to see large volumes
of traffic, so speed and latency are driving concerns here. Given the function
of the core, we can now look at some design specifics to consider. Let's start
with some things you know you don't want to do:

Don't do anything to slow down traffic. This includes using access
lists, routing between virtual local area networks (VLANs), and
packet filtering.

Don't support workgroup access here.

Avoid expanding the core when the internetwork grows (i.e., adding
routers). If performance becomes an issue in the core, give preference
to upgrades over expansion.

Now, there are a few things that you want to make sure to get done as you

design the core:

Design the core for high reliability. Consider data-link technologies
that facilitate both speed and redundancy, such as FDDI, FastEthernet
(with redundant links), or even ATM.

Design with speed in mind. The core should have very little latency.

Select routing protocols with lower convergence times. Fast and
redundant data-link connectivity is no help if your routing tables
are shot!

Distribution Layer

The distribution layer is sometimes referred to as the workgroup layer and
is the communication point between the access layer and the core. The pri-
mary function of the distribution layer is to provide routing, filtering, and

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