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Chapter 1

The Campus Network

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:

Implement tools such as access lists, packet filtering, and queuing.

Implement security and network policies, including address transla-
tion and firewalls.

Redistribute between routing protocols, including static routing.

Route between VLANs and other workgroup support functions.

Define broadcast and multicast domains.

Things to avoid at the distribution layer are limited to those functions that

exclusively belong to one of the other layers.

Access Layer

The access layer controls user and workgroup access to internetwork
resources. The access layer is sometimes referred to as the desktop layer. The
network resources that most users need will be available locally. Any traffic
for remote services is handled by the distribution layer. The following func-
tions should be included at this layer:

Continued (from distribution layer) access control and policies.

Creation of separate collision domains (segmentation).

Workgroup connectivity to the distribution layer.

Technologies such as dial-on-demand routing (DDR) and Ethernet
switching are frequently seen here in the access layer. Static routing
(instead of dynamic routing protocols) is seen here as well.

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