Ask the Expert: Hierarchical Network Design, Includes Core, Distribution, and Access

Question

Welcome to the Cisco® Support Community Ask the Expert conversation.  This is an opportunity to learn and ask questions about hierarchical network design. 
Recommending a network topology is required for meeting a customer's corporate network design  needs in their business and technical goals and often consists of many interrelated components. The hierarchical design made this easier like "divide and conquer" the job and develop the design in layers.
Network design experts have developed the hierarchical network design model to help to develop a topology in discrete layers. Each layer can be focused on specific functions, to select the right systems and features for the layer.
A typical hierarchical topology is
A core layer of high-end routers and switches that are optimized for availability and performance.
A distribution layer of routers and switches that implement policies.
An access layer that connects users via lower-end switches and wireless access points.
Ahmad Manzoor is a Senior Pre-Sales Engineer at AGCN, Pakistan. He has more than 10 years of experience in first-rate management, commercial and technical skills in the field of data communication and services lifecycle—from solution design through sales pitch, designing RFPs, architecture, and solution—all with the goal toward winning projects (creating win/win situations) of obsolete solutions.  Ahmad also has vast experience in designing end-to-end data centers, from building infrastructure design to data communication and network Infrastructure design. He has worked for several large companies in Pakistan and United Arab Emirates markets; for example, National Engineer, WATEEN Telecom, Emircom, Infotech, Global Solutions, NETS International, Al-Aberah, and AGCN, also known as Getronics, Pakistan.
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Answer 1

Dear Leo,
We are discussing the following without any product line, discussing the concept of hierarchical design, which will help you to take decision which model is better for you Two Layer or Three Layer hierarchical model.  
Two-Layer Hierarchy
In many networks, you need only two layers to fulfill all of the layer functions—core and aggregation
Only one zone exists within the core, and many zones are in the aggregation layer. Examine each of the layer functions to see where it occurs in a two-layer design:
Traffic forwarding—Ideally, all interzone traffic forwarding occurs in the core. Traffic flows from each zone within the aggregation layer up the hierarchy into the network core and then back down the hierarchy into other aggregation zones.
Aggregation—Aggregation occurs along the core/aggregation layer border, allowing only interzone traffic to pass between the aggregation and core layers. This also provides an edge for traffic engineering services to be deployed along.
Routing policy—Routing policy is deployed along the edge of the core and the aggregation layers, generally as routes are advertised from the aggregation layer into the core.
User attachment—User devices and servers are attached to zones within the aggregation layer. This separation of end devices into the aggregation permits the separation of traffic between traffic through a link and traffic to a link, or device. Typically, it is best not to mix transit and destination traffic in the same area of the network.
Controlling traffic admittance—Traffic admittance control always occurs where user and server devices are attached to the network, which is in the aggregation layer. You can also place traffic admittance controls at the aggregation points exiting from the aggregation layer into the core of the network, but this is not common.
You can see, then, how dividing the network into layers enables you to make each layer specialized and to hide information between the layers. For instance, the traffic admittance policy implemented along the edge of the aggregation layer is entirely hidden from the network core.
You also use the core/aggregation layer edge to hide information about the topology of routing zones from each other, through summarization. Each zone within the aggregation layer should have minimal routing information, possibly just how to make it to the network core through a default route, and no information about the topology of the network core. At the same time, the zones within the aggregation layer should summarize their reachability information into as few routing advertisements as possible at their edge with the core and hide their topology information from the network core.
Three-Layer Hierarchy
A three-layer hierarchy divides these same responsibilities through zones in three vertical network layers,
Traffic Forwarding—As with a two-layer hierarchy, all interzone traffic within a three- layer hierarchy should flow up the hierarchy, through the layers, and back down the hierarchy.
Aggregation—A three-layer hierarchy has two aggregation points:
At the edge of the access layer going into the distribution layer
At the edge of the distribution layer going into the core
At the edge of the access layer, you aggregate traffic in two places: within each access zone and flowing into the distribution layer. In the same way, you aggregate interzone traffic at the distribution layer and traffic leaving the distribution layer toward the network core. The distribution layer and core are ideal places to deploy traffic engineering within a network.
Routing policy—The routing policy is deployed within the distribution layer in a three- layer design and along the distribution/core edge. You can also deploy routing policies along the access/distribution edge, particularly route and topology summarization, to hide information from other zones that are attached to the same distribution layer zone.
User attachment—User devices and servers are attached to zones within the access layer. This separation of end devices into the access layer permits the separation of traffic between traffic through a link and traffic to a link, or device. Typically, you do not want to mix transit and destination traffic in the same area of the network.
Controlling traffic admittance—Traffic admittance control always occurs where user and server devices are attached to the network, which is in the access layer. You can also place traffic admittance controls at the aggregation points along the aggregation/core edge.
As you can see, the concepts that are applied to two- and three-layer designs are similar, but you have more application points in a three-layer design.
Now the confusion takes place in our minds where do we use Two Layer and where the Three layer hierarchical model.
Now we are discussing that How Many Layers to Use in Network Design?
Which network design is better: two layers or three layers? As with almost all things in network design, it all depends. Examine some of the following factors involved in deciding whether to build a two- or three-layer network:
Network geography—Networks that cover a smaller geographic space, such as a single campus or a small number of interconnected campuses, tend to work well as two-layer designs. Networks spanning large geographic areas, such as a country, continent, or even the entire globe, often work better as three layer designs.
Network topology depth—Networks with a compressed, or flattened, topology tend to work better as two-layer hierarchies. For instance, service provider networks cover large geographic areas, but reducing number of hops through the network is critical in providing the services they sell; therefore, they are often built on a two-layer design. Networks with substantial depth in their topologies, however, tend to work better as three-layer designs.
Network topology design—Highly meshed networks, with many requirements for interzone traffic flows, tend to work better as two-layer designs. Simplifying the hierarchy to two levels tends to focus the design elements into meshier zones. Networks that focus traffic flows on well-placed distributed resources, or centralized resources, such as a network with a large number of remote sites connecting to a number of centralized Data Centers, tend to work better as three-layer designs.
Policy implementation—If policies of a network tend to focus on traffic engineering, two-layer designs tend to work better. Networks that attempt to limit access to resources attached to the network and other types of policies tend to work better as three-layer designs.
Again, however, these are simple rules of thumb. No definitive way exists to decide whether a network should have two or three layers. Likewise, you cannot point to a single factor and say, “Because of this, the network we are working on should have three layers instead of two.”
I hope that this helps you to understand the purposes of Two Layer & Three layer Hierarchical Model.
Best regards,
Ahmad Manzoor