The Cisco Three-Layer Hierarchical Model

Unless a business is staffed only by equal partners, more than one person means that there are managers and subordinates. A hierarchy describes the relationship between managers and subordinates from the top of the company to the bottom. Except for those at the very top, owners or directors, and very bottom, entry-level, of the chain of command, employees have both a manager and subordinates

In a hierarchy, authority, responsibility and job function are clearly defined, along with the road to promotions. Communication is clear, since it travels directly from top to bottom, so everyone knows exactly what his duties are. Hierarchies organize people into departments and teams that lead to teamwork and motivation. Since decisions have to travel from top to bottom, and communication from bottom to top, hierarchies respond slowly to new information and changes and have a bureaucratic decision-making process. Departments more focused on their own goals than company goals can lead to rivalries and selfish decisions.

To stress this further more hierarchy is a way of doing things in an orderly manner, ranging from top to bottom or from button to the top. In this form, at a glance one will be able to easily identify where things are suppose to be and easily place them in accord without too many questions, this seems to be one of the key rules hierarchy plays in real life scenario, also when it comes to networking, the concepts are almost the same, things has to be well structured to ascertain network stability and obtain maximum throughput.

The following are The Three Networking Hierarchical -Layers

The Core layer

The Distribution layer

The Access layer

Each layer has specific responsibilities.

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 network’s core layer is to switch

traffic as fast as possible. The traffic transported across the core is common to

a majority of users. However, remember that user data is processed at the distribution

layer, which 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 consider some design specifics. Let’s start with some

things we 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 we want to do as we design the core.


include the following:

_ Design the core for high reliability. Consider data-link technologies

that facilitate both speed and redundancy, such as FDDI, Fast Ethernet

(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. 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 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 items 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


_ 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 contains devices that allow workgroups and users to use the services provided by the distribution and core layers. In the access layer, you have the ability to expand or contract collision domains using a repeater, hub, or standard switch. In regards to the access layer, a switch is not a high-powered device, such as those found at the core layer.

Rather, a switch is an advanced version of a hub.

A collision domain describes a portion of an Ethernet network at layer 1 of the OSI model where any communication sent by a node can be sensed by any other node on the network. This is different from a broadcast domain which describes any part of a network at layer 2 or 3 of the OSI model where a node can broadcast to any node on the network.

At the access layer, you can:
Enable MAC address filtering: It is possible to program a switch to allow only certain systems to access the connected LANs.
Create separate collision domains: A switch can create separate collision domains for each connected node to improve performance.
Share bandwidth: You can allow the same network connection to handle all data.
Handle switch bandwidth: You can move data from one network to another to perform load balancing.

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