Chapter 1: Introduction to WAN Architectures

The WAN is a place in the network that aggregates various types, speeds, and links running a disparate set of protocols together crossing metropolitan, state, and even country boundaries. The largest example of a WAN is the Internet itself, which can be regarded as the public WAN. The primary purpose of a WAN is to connect users and applications connected to various LANs.

As evident from its definition, the WAN is the central point for all data aggregation coming from various places within an enterprise network. Because of this, it is important to understand not only how a WAN is constructed, but also the underlying business drivers that have been and continue to bring changes to this place in the network.

In this book, you study the variety of WANs as they exist today, business models, and the associated emerging trends and how they are giving birth to “next-generation” WAN. Once you have the first four chapters (or Part I) behind you, it should become evident that the core requirement to building such networks hinges on the usage of modern routing/switching infrastructure that is highly available, scalable, flexible, and above all, service rich.

This chapter describes the various types of WAN architectures and their various associated aspects.

Introduction to WAN Solutions

Thanks to an increasingly dispersed global work force, businesses rely on their WANs more than ever. So much so that business performance is now directly tied to how well quality, reliability, and security are implemented when it comes to communications between main and regional headquarters, branch offices, suppliers, partners, and customers. Because of the development of new IP services and applications such as VoIP, video, and mobile data connectivity, and because of remotely connected road warriors and the unification of both wired and wireless networks, the headend router must perform a wide variety of functions.

Depending on the connectivity, transport protocols, and whether the medium is private or public, several different varieties of WAN might be in play. The four main WAN types are as follows:

• Branch/private WAN aggregation

• Internet edge

• Data center interconnect

• Large branch WAN

The WAN aggregation role can also be subdivided into the following three categories, based on what is typically found in the enterprise networks:

• Basic WAN aggregation (explained in the following section)

• Secure WAN aggregation (add-on with solutions based on IPsec or Secure Sockets Layer virtual private networking [SSL VPN])

• Optimized WAN aggregation (add-on with solutions based on WAN optimization with Web Cache Communication Protocol Version 2/Policy Based Routing [WCCPv2/PBR] and Wide Area Application Services [WAAS])

Figure 1-1 shows various WAN options and puts them into perspective as to how they come together.

Branch/Private WAN Aggregation Role

Branch WAN aggregation is a way to connect and aggregate all the enterprise branches into the WAN core router, or headend. On the cloud-facing side, router interfaces use various physical transport options (as outlined in Table 1-1), whereas on the campus core side, the connection is Gigabit Ethernet (GE) or 10 Gigabit Ethernet (10 GigE) that is acting as the uplink for the campus core switches to the WAN. Leased lines are one of the most common ways (now more so Ethernet) of interfacing with the WAN cloud. IPsec tunnel termination and firewall functions are usually not collapsed in the WAN aggregation/edge router. This is usually implemented as classical hub-and-spoke design with traditional Layer 2 connectivity.

Figure 1-2 shows the basic WAN aggregation topology.

Table 1-1 shows the various options in use for WAN connectivity.

Table 1-1  WAN Connectivity Options

Basic Feature Requirements

Table 1-2 outlines the basic requirements that a router must meet to be positioned as the WAN aggregation platform. Scale and performance for these services are driven based on how large the branch site concentration is for the given deployment. A platform with a separate control, data, and input/output plane is most preferred, for obvious reasons.

Table 1-2  Feature Matrix for WAN Aggregation Role

Basic Service Level Agreement Requirements

Table 1-3 outlines the usual service level agreement (SLA) requirements that need to be met for the converged WAN for voice, video, and data traffic types.

Table 1-3  Typical SLA Target

Traditional WANs (such as those based on Frame Relay) are assumed to be inherently secure, which is not the case (because providers do use shared physical infrastructure to carry this traffic). An MPLS VPN is another example where traffic is isolated (via Virtual Routing/Forwarding [VRF] instances and labels) but still share the same physical infrastructure while traversing the service provider cloud.

It is not uncommon to see some form of encryption used to achieve confidentiality, the drivers behind which could be company policy (such as any traffic leaving the premises must be encrypted) or regulatory compliance (such as with HIPAA or SOX).

Table 1-4 outlines the commonly used technologies to secure WAN traffic. Chapter 14, “Security Services Use Cases,” provides further detail.

Table 1-4  High-Level Details of Secure WAN Technologies

Internet Edge Role

The Internet edge is the boundary where an enterprise private network connects to the public Internet. In the simplest sense, the Internet edge device acts as the gateway for the inside network. Contrary to popular understanding, the Internet edge is not only just about accessing the Internet for web traffic for campus users.

The Internet edge serves various functions, including those outlined in Table 1-5.

Table 1-5  Internet Edge Router Functionality

Figure 1-3 shows the Internet edge topology.

Basic Feature Requirements

The primary function of a device at the Internet edge is to act as the demarcation between the private (campus or data center) and public network (that is, the Internet). Features required in a single device depend on how the Internet edge is designed, although typically the basic features are those outlined in Table 1-6.

Table 1-6  Internet Edge Network Device Feature Requirements

Data Center Interconnect

Data center interconnect (DCI) is yet another WAN function where someone is trying to connect two data centers together via Layer 2 or 3 links. Layer 2 extensions are much more common because of their capability to take all Ethernet frames (or even dot1Q or QinQ [IEEE 802.1Q-in-Q VLAN]) as is across the data centers. This is usually done with some kind of pseudowire (for example, Ethernet over MPLS [EoMPLS] for two data centers, and Virtual Private LAN Service [VPLS] for multisite data center connectivity). Major drivers behind DCI are as follows:

• Data center consolidation and virtualization (VMWare VMotion)

• Disaster recovery or data center HA

• Geo-clustering, where clusters are connected across geographies

• Layer 2 extensions for any reason

Figure 1-4 shows the DCI topology

Basic Feature Requirements

The primary function of the edge device at the DCI is to extend VLANs across the data centers for the previously listed applications such as VMWare’s VMotion or geo-clusters to function. Convergence and failover times for this type of connectivity are of extreme importance because the underlying assumptions from the application perspective usually require them to be on the same LAN.

The solution requirements in Table 1-7 call for an infrastructure that has the features outlined in Table 1-8.

Table 1-7  DCI Feature Requirements

The solution requirements in Table 1-7 call for an infrastructure that has the features outlined in Table 1-8.

Table 1-8  Router/Switch Feature Requirements Needed to Meet the DCI Solution Requirements

Large Branch WAN

As universally understood, not all branches are equal. This is not only true for the size of branch (as in number of users or perhaps application servers residing at the branch) but also for how critical the branch is to the overall business function. Consider bank branches, for example. Not all branches provide the entire portfolio of services. In the real world, some provide only basic banking services, whereas others provide full-blown services, including home mortgage, small business loans, and investment services to commercial customers.

Large branches (those that provide more services or services that are critical to the business, or in most cases both) tend to have slightly different requirements for a WAN infrastructure that connects them to the corporate backbone. Table 1-9 outlines the large branch WAN requirements.

Table 1-9  Large Branch Office Deployment Requirements

Table 1-10 maps the requirements onto the infrastructure needed to support such requirements.

Table 1-10  Large Branch Office Requirements/Traits

Summary

This opening chapter covered the basic building blocks of WAN architectures:

• Branch aggregation

• Internet edge

• Data center interconnect

• Large branch office

Although the basic requirements are common across the various roles, they differ significantly enough that you need to understand how they are architected, deployed, and troubleshot. If there were one word to describe the hardware required to meet these needs, that word would be flexibility. Infrastructure needs to be very flexible in terms of feeds and speeds, scale and performance, service richness, and interface diversity (to name a few).

The next chapter covers the various business drivers, and the underlying technical requirements that they are generating. It concludes with an analysis of how these are driving requirements for next-generation WAN infrastructure.

Review Questions

1. What are the four usual WAN architectures?

2. What does optimized WAN mean?

3. What are the few fundamental requirements for WAN aggregation?

4. Why is service richness so important to enterprises?

5. Why would an enterprise connect directly to a service provider or be multihomed?

6. What are the core business drivers for DCI?

Answers

1. Usual WAN architectures typical in today’s networks include the following:

   Branch and private WAN aggregation

   Internet edge

   Data center interconnect

   Large branch WAN

2. Optimization here refers to the capability of the network infrastructure to provide the voice, video, and data traffic optimization before it goes over the WAN links. This helps reduce the need for more bandwidth every time a new application is added to the network. Cisco WAAS and IOS provide such services.

3. WAN aggregation requires, at a minimum, infrastructure to support the following:

   Flexible routing/switching architecture that can evolve with the changing business requirements

   Capability to combine various types and speeds of interfaces into one common infrastructure

   Modular and highly available carrier-class design with the separation of control, data, and I/O planes

   Capability to add basic services without requiring new hardware

4. Service richness refers to an in structure (hardware and software) that can introduce a basic service and some of the advanced services into baseline hardware with simple software upgrades. Enterprises and their businesses thrive on applications, and that in turn requires network-based services for them to work on common physical infrastructure and with the ability for virtualization for today’s typical multitenant requirements.

5. Enterprises connect to multiple Internet providers (or what is generally known as multihoming) for a few key reasons, including the following:

   Fault tolerance and resiliency to failure in one provider’s network

   Granular routing control

   Path selection based on features such as performance-based routing or PfR

6. Core business drivers behind DCI include the following:

   Layer 2 extensions

   Data center consolidation

   Data center disaster recovery site

   Virtualization and clustering applications such as VMWare’s VMotion

Further Reading

Introduction to WAN, document: http://tinyurl.com/6g8cym

Cisco Validated Designs, document: http://tinyurl.com/lnnyjt

Unified WAN Services, document: http://www.cisco.com/en/US/netsol/index.html

DC Interconnect, document: http://tinyurl.com/rclv2f

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