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The Cisco Three-Layer Hierarchical Model

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. The pri-
mary functions of the distribution layer are 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 network service
requests are handled--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 then quickly transports 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 operation.
There are several actions that generally should be done at the distribution
layer. They include the following:

Implementation of tools such as access lists, of packet filtering, and of
queuing

Implementation of security and network policies, including address
translation and firewalls

Redistribution between routing protocols, including static routing

Routing between VLANs and other workgroup support functions

Definitions of 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.

The 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 most users need will be available locally. The distribution
layer handles any traffic for remote services. The following are some of the

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