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      You can see that IaaS allows the customer to manage most of the network, whereas SaaS doesn't allow any management by the customer, and PaaS is somewhere in the middle of the two. Clearly, choices can be cost driven, so the most important thing is that the customer pays only for the services or infrastructure they use.

      Let's take a look at each service:

      Infrastructure as a Service (IaaS): Provides only the network Delivers computer infrastructure – a platform virtualization environment – where the customer has the most control and management capability.

      Platform as a Service (PaaS): Provides the operating system and the network Delivers a computing platform and solution stack, allowing customers to develop, run, and manage applications without the complexity of building and maintaining the infrastructure typically associated with developing and launching an application. An example is Windows Azure.

      Software as a Service (SaaS): Provides the required software, operating system, and network SaaS is common application software such as databases, web servers, and email software that's hosted by the SaaS vendor. The customer accesses this software over the Internet. Instead of having users install software on their computers or servers, the SaaS vendor owns the software and runs it on computers in its data center. Microsoft Office 365 and many Amazon Web Services (AWS) offerings are perfect examples of SaaS.

      So depending on your business requirements and budget, cloud service providers market a very broad offering of cloud computing products from highly specialized offerings to a large selection of services.

      What's nice here is that you're is offered a fixed price for each service that you use, which allows you to easily budget wisely for the future. It's true – at first, you'll have to spend a little cash on staff training, but with automation you can do more with less staff because administration will be easier and less complex. All of this works to free up the company resources to work on new business requirements and be more agile and innovative in the long run.

      Basic Virtual Network Infrastructure

      Having centralized resources is critical for today's workforce. For example, if you have your documents stored locally on your laptop and your laptop gets stolen, you're pretty much screwed unless you're doing constant local backups. That is so 2005!

      After I lost my laptop and all the files for the book I was writing at the time, I swore (yes, I did that too) to never have my files stored locally again. I started using only Google Drive, OneDrive, and Dropbox for all my files, and they became my best backup friends. If I lose my laptop now, I just need to log in from any computer from anywhere to my service provider's logical drives and presto, I have all my files again. This is clearly a simple example of using cloud computing, specifically SaaS, and it's wonderful!

      So cloud computing provides for the sharing of resources, lower cost operations passed to the cloud consumer, computing scaling, and the ability to dynamically add new servers without going through the procurement and deployment process.

      Exam Essentials

      Understand basic cloud technology. Understand cloud services such as SaaS and others and how virtualization works.

      Compare and contrast collapsed core and three-tier architectures

      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

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