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When arranging the infrastructure devices in the network there are a number of different models that can aid in defining these relationships. In this section we'll look at two such models.

The Cisco Three-Layer Hierarchical Model

Most of us were exposed to hierarchy early in life. Anyone with older siblings learned what it was like to be at the bottom of the hierarchy. Regardless of where you first discovered the concept of hierarchy, most of us experience it in many aspects of our lives. It's hierarchy that helps us understand where things belong, how things fit together, and what functions go where. It brings order to otherwise complex models. If you want a pay raise, for instance, hierarchy dictates that you ask your boss, not your subordinate, because that's the person whose role it is to grant or deny your request. So basically, understanding hierarchy helps us discern where we should go to get what we need.

Hierarchy has many of the same benefits in network design that it does in other areas of life. When used properly, it makes networks more predictable and helps us define which areas should perform certain functions. Likewise, you can use tools such as access lists at certain levels in hierarchical networks and avoid them at others.

Let's face it: Large networks can be extremely complicated, with multiple protocols, detailed configurations, and diverse technologies. Hierarchy helps us summarize a complex collection of details into an understandable model, bringing order from the chaos. Then, as specific configurations are needed, the model dictates the appropriate manner in which to apply them.

The Cisco hierarchical model can help you design, implement, and maintain a scalable, reliable, cost-effective hierarchical internetwork. Cisco defines three layers of hierarchy, as shown in Figure 1.10, each with specific functions.

Figure 1.10 The Cisco hierarchical model

Each layer has specific responsibilities. Keep in mind that the three layers are logical and are not necessarily physical devices. Consider the OSI model, another logical hierarchy. Its seven layers describe functions but not necessarily protocols, right? Sometimes a protocol maps to more than one layer of the OSI model, and sometimes multiple protocols communicate within a single layer. In the same way, when we build physical implementations of hierarchical networks, we may have many devices in a single layer, or there may be a single device performing functions at two layers. Just remember that the definition of the layers is logical, not physical!

So let's take a closer look at each of the layers now.

The Core Layer

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. But remember that user data is processed at the distribution layer, which forwards the requests to the core if needed.

If there's a failure in the core, every single user can be affected! This is why fault tolerance at this layer is so important. 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:

■ We don't want 24/7 connectivity.

■ Never do anything to slow down traffic. This includes making sure you don't use access lists, perform routing between virtual local area networks, or implement packet filtering.

■ Don't support workgroup access here.

■ Avoid expanding the core (e.g., adding routers when the internetwork grows). If performance becomes an issue in the core, give preference to upgrades over expansion.

Here's a list of things that we want to achieve as we design the core:

■ Design the core for high reliability. Consider data-link technologies that facilitate both speed and redundancy, like Gigabit Ethernet with redundant links or even 10 Gigabit Ethernet.

■ 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 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 if necessary. The core layer then quickly transports the request to the correct service.

The distribution layer is where we want to implement policies for the network because we are allowed a lot of flexibility in defining network operation here. There are several things that should generally be handled at the distribution layer:

■ Routing

■ Implementing tools (such as access lists), packet filtering, and queuing

■ Implementing security and network policies, including address translation and firewalls

■ Redistributing between routing protocols, including static routing

■ Routing between VLANs and other workgroup support functions

■ Defining broadcast and multicast domains

Key things to avoid at the distribution layer are those that are limited to 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 because the distribution layer handles any traffic for remote services.

The following are some of the functions to be included at the access layer:

■ Continued (from distribution layer) use of access control and policies

■ Creation of separate collision domains (microsegmentation/switches)

■ Workgroup connectivity into the distribution layer

■ Device connectivity

■ Resiliency and security services

■ Advanced technology capabilities (voice/video, etc.)

Technologies like Gigabit or Fast Ethernet switching are frequently seen in the access layer.

I can't stress this enough – just because there are three separate levels does not imply three separate devices! There could be fewer or there could be more. After all, this is a layered approach.

Collapsed Core

In the collapsed core approach the distribution layer and the core layer are combined into a single layer, thus the name collapsed core. When using this design it is critical that the devices operating as both distribution and core devices must exhibit the following characteristics:

■ High speed paths connecting to the network

■ Must be a Layer-2 aggregation point

■ Must enforce routing and network access policies

■ Must be capable of Intelligent network services such as QoS, and network virtualization.

The benefits are reduced cost in equipment, while the drawbacks can be slower performance and reduced network availability as compared to the three tier model.

Exam Essentials

Identify the layers in the Cisco three-layer model, and describe the ideal function of each layer. The three layers in the Cisco hierarchical model are the core (responsible for transporting large amounts of traffic both reliably and quickly), distribution (provides routing, filtering, and WAN access), and access (workgroup connectivity into the distribution layer).