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The Cisco Three-Layer Model

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 we want to make sure to do as we design

the core. They include:

Design the core for high reliability. Consider data-link technologies
that facilitate both speed and redundancy, such as FDDI, Fast Ether-
net (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!

The 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 layer. The
primary function of the Distribution layer is to provide routing, filtering, and
WAN access and to determine how packets can access the core, if needed.
The Distribution layer must determine the fastest way that user requests are
serviced, for example, how a file request is forwarded to a server. After the
Distribution layer determines the best path, it forwards the request to the
Core layer. The Core layer is then responsible for quickly transporting the
request to the correct service.

The Distribution layer is the place to implement policies for the network.

Here, you can exercise considerable flexibility in defining network opera-
tion. There are several items that generally should be done at the Distribution
layer. They include

Implementing tools such as access lists, packet filtering, and queuing

Implementing security and network policies, including address trans-
lation and firewalls

Redistribution between routing protocols, including static routing

Routing between VLANs and other workgroup support functions

Broadcast and multicast domain definition

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