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The Art of Network Architecture: Applying Modularity

Chapter Description

This chapter focuses on why we use specific design patterns to implement modularity, discussing specifically why we should use hierarchical design to create a modular network design, why we should use overlay networks to create virtualization, and the results of virtualization as a mechanism to provide modularity.


Virtualization is a key component of almost all modern network design. From the smallest single campus network to the largest globe-spanning service provider or enterprise, virtualization plays a key role in adapting networks to business needs.

What Is Virtualization?

Virtualization is deceptively easy to define: the creation of virtual topologies (or information subdomains) on top of a physical topology. But is it really this simple? Let’s look at some various network situations and determine whether they are virtualization.

  • A VLAN used to segregate voice traffic from other user traffic across a number of physical Ethernet segments in a network
  • An MPLS-based L3VPN offered as a service by a service provider
  • An MPLS-based L2VPN providing interconnect services between two data centers across an enterprise network core
  • A service provider splitting customer and internal routes using an interior gateway protocol (such as IS-IS) paired with BGP
  • An IPsec tunnel connecting a remote retail location to a data center across the public Internet
  • A pair of physical Ethernet links bonded into a single higher bandwidth link between two switches

The first three are situations just about any network engineer would recognize as virtualization. They all involve full-blown technologies with their own control planes, tunneling mechanisms to carry traffic edge to edge, and clear-cut demarcation points. These are the types of services and configurations we normally think of when we think of virtualization.

What about the fourth situation—a service provider splitting routing information between two different routing protocols in the same network? There is no tunneling of traffic from one point in the network to another, but is tunneling really necessary in order to call a solution “virtualization”? Consider why a service provider would divide routing information into two different domains. Breaking up networks in this way creates multiple mutually exclusive sets of information within the networks. The idea is that internal and external routing information should not be mixed. A failure in one domain is split off from a failure in another domain (just like failures in one module of a hierarchical design are prevented from leaking into a second module in the same hierarchical design), and policy is created that prevents reachability to internal devices from external sources.

All these reasons and results sound like modularization in a hierarchical network. Thus, it only makes sense to treat the splitting of a single control plane to produce mutually exclusive sets of information as a form of virtualization. To the outside world, the entire network appears to be a single hop, edge-to-edge. The entire internal topology is hidden within the operation of BGP—hence there is a virtual topology, even if there is no tunneling.

The fifth situation, a single IPsec tunnel from a retail store location into a data center, seems like it might even be too simple to be considered a case of virtualization. On the other hand, all the elements of virtualization are present, aren’t they? We have the hiding of information from the control plane—the end site control plane doesn’t need to be aware of the topology of the public Internet to reach the data center, and the routers along the path through the public Internet don’t know about the internal topology of the data center to which they’re forwarding packets. We have what is apparently a point-to-point link across multiple physical hops, so we also have a virtual topology, even if that topology is limited to a single link.

The answer, then, is yes, this is virtualization. Anytime you encounter a tunnel, you are encountering virtualization—although tunneling isn’t a necessary part of virtualization.

With the sixth situation—bonding multiple physical links into a single Layer 2 link connecting two switches—again we have a virtual link that runs across multiple physical links, so this is virtualization as well.

Essentially, virtualization appears anytime we have the following:

  • A logical topology that appears to be different from the physical topology
  • More than one control plane (one for each topology), even if one of the two control planes is manually configured (such as static routes)
  • Information hiding between the virtual topologies

Virtualization as Vertical Hierarchy

One way of looking at virtualization is as vertical hierarchy as Figure 7-6 illustrates.

Figure 7-6

Figure 7-6 Virtualization as Vertical Hierarchy

In this network, Routers A through F are not only a part of the physical topology, they are also part of a virtual topology, shown offset and in a lighter shade of gray. The primary topology is divided into three modules:

  • Routers A, B, G, and H
  • Routers C, D, J, and K (the network core)
  • Routers E, F, and L

What’s important to note is that the virtual topology cuts across the hierarchical modules in the physical topology, overlaying across all of them, to form a separate information domain within the network. This virtual topology, then, can be seen as yet another module within the hierarchical system—but because it cuts across the modules in the physical topology, it can be seen as “rising out of” the physical topology—a vertical module rather than a topological module, built on top of the network, cutting through the network.

How does seeing virtualization in this way help us? Being able to understand virtualization in this way allows us to understand virtual topologies in terms of the same requirements, solutions, and problems as hierarchical modules. Virtualization is just another mechanism network designers can use to hide information.

Why We Virtualize

What business problem can we solve through virtualization? If you listen to the chatter in modern network design circles, the answer is “almost anything.” But like any overused tool (hammer, anyone?), virtualization has some uses for which it’s very apt and others for which it’s not really such a good idea. Let’s examine two specific use cases.

Communities of Interest

Within any large organization there will invariably be multiple communities of interest—groups of users who would like to have a small part of the network they can call their own. This type of application is normally geared around the ability to control access to specific applications or data so only a small subset of the entire organization can reach these resources.

For instance, it’s quite common for a human resources department to ask for a relatively secure “network within the network.” They need a way to transfer and store information without worrying about unauthorized users being able to reach it. An engineering, design, or animation department might have the same requirements for a “network within the network” for the same reasons.

These communities of interest can often best be served by creating a virtual topology that only people within this group can access. Building a virtual topology for a community of interest can, of course, cause problems with the capability to share common resources—see the section “Consequences of Network Virtualization” later in the chapter.

Network Desegmentation

Network designers often segment networks by creating modules for various reasons (as explained in the previous sections of this chapter). Sometimes, however, a network can be unintentionally segmented. For instance, if the only (or most cost effective) way to connect a remote site to a headquarters or regional site is to connect them both to the public Internet, the corporate network is now unintentionally segmented. Building virtual networks that pass over (over the top of) the network in the middle is the only way to desegment the network in this situation.

Common examples here include the following:

  • Connecting two data centers through a Layer 3 VPN service (provided by a service provider)
  • Connecting remote offices through the public Internet
  • Connecting specific subsets of the network between two partner networks connected through a single service provider

Separation of Failure Domains

As we’ve seen in the first part of this chapter, designers modularize networks to break large failure domains into smaller pieces. Because virtualization is just another form of hiding information, it can also be used to break large failure domains into smaller pieces.

A perfect example of this is building a virtual topology for a community of interest that has a long record of “trying new things.” For instance, the animation department in a large entertainment company might have a habit of deploying new applications that sometimes adversely impact other applications running on the same network. By first separating a department that often deploys innovative new technology into its own community of interest, or making it a “network within the network,” the network designer can reduce or eliminate the impact of new applications deployed by this one department.

Another version of this is the separation of customer and internal routes across two separate routing protocols (or rather two different control planes) by a service provider. This separation protects the service provider’s network from being impacted by modifications in any particular customer’s network.

Consequences of Network Virtualization

Just as modularizing a network has negative side effects, so does virtualization—and the first rule to return to is the one about hiding information and its effect on stretch in networks. Just as aggregation of control plane information to reduce state can increase the stretch in a network (or rather cause the routing of traffic through a network to be suboptimal), virtualization’s hiding of control plane information has the same potential effect. To understand this phenomenon, take a look at the network in Figure 7-7.

Figure 7-7

Figure 7-7 Example of Stretch Through Virtualized Topologies

In this case, host G is trying to reach a service on a server located in the rack represented by H. If both the host and the server were on the same virtual topology, the path between them would be one hop. Because they are on different topologies, however, traffic between the two devices must travel to the point where the two topologies meet, at Router F/F’, to be routed between the two topologies (to leak between the VLANs).

If there are not services that need to be reached by all the hosts on the network, or each virtual topology acts as a complete island of its own, this problem may not arise in this specific form. But other forms exist, particularly when traffic must pass through filtering and other security devices while traveling through the network, or in the case of link or device failures along the path.

A second consequence of virtualization is fate sharing. Fate sharing exists anytime there are two or more logical topologies that share the same physical infrastructure—so fate sharing and virtualization go hand in hand, no matter what the physical layer and logical overlays look like. For instance, fate sharing occurs when several VLANs run across the same physical Ethernet wire, just as much as it occurs when several L3VPN circuits run across the same provider edge router or when multiple frame relay circuits are routed across a single switch. There is also fate sharing purely at the physical level, such as two optical strands running through the same conduit. The concepts and solutions are the same in both cases.

To return to the example in Figure 7-7, when the link between Routers E and F fails, the link between Routers E’ and F’ also fails. This may seem like a logical conclusion on its face, but fate sharing problems aren’t always so obvious, or easy to see.

The final consequence of virtualization isn’t so much a technology or implementation problem as it is an attitude or set of habits on the part of network engineers, designers, and architects. RFC1925, rule 6, and the corollary rule 6a, state: “It is easier to move a problem around (for example, by moving the problem to a different part of the overall network architecture) than it is to solve it. ...It is always possible to add another level of indirection.”

In the case of network design and architecture, it’s often (apparently) easier to add another virtual topology than it is to resolve a difficult and immediately present problem. For instance, suppose you’re deploying a new application with quality of service requirements that will be difficult to manage alongside existing quality of service configurations. It might seem easier to deploy a new topology, and push the new application onto the new topology, than to deal with the complex quality of service problems. Network architects need to be careful with this kind of thinking, though—the complexity of multiple virtual topologies can easily end up being much more difficult to manage than the alternative.

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