Although the focus of this chapter is designing a logical topology for an enterprise network, a quick discussion of service providers is warranted at this point. The Enterprise Composite Network Model includes the service provider edge module, and, although you aren't expected to design this module as an enterprise network designer, you need to have some understanding of it and be able to select the appropriate provider (or providers) for your design customers. The selection of a service provider is something you should consider during the logical design phase, which is the focus of this part (Part II) of the book. Part III addresses the topic again because during that phase you should make some definite selections of WAN technologies, devices, and providers.
In the early days of data communications, there were the regional or national telephone companies and their customers, and nothing else. A customer's choice of provider was dictated by location. The level of service and pricing was dictated by the provider. Today, there is a broad range of service providers. Service levels and pricing are more negotiable.
Finding a provider that matches an enterprise's needs requires a good understanding of those needs and the culture of the enterprise and the potential provider. Many ISPs are small startups that offer dialup and cable modem service to end users. These ISPs may not have the expertise to support a large enterprise, although they may be appropriate for home users accessing the corporate network using VPN software. Some ISPs focus mostly on hosting servers and don't support end users. Some ISPs are actually network service providers (NSPs), which means that their main business is connecting other ISPs rather than enterprises or end users. Selecting providers for your network design requires you to understand which of these types of services you actually need.
ISPs and NSPs are sometimes classified as being Tier 1 through Tier 5. Although these categories don't have universal meaning, if a provider calls itself a Tier 1 provider and you are looking for an inexpensive provider to connect a small office or home, then you know to look elsewhere. Tier 1 ISPs are large international providers, whereas Tier 5 ISPs are small, specialized providers, sometimes located in a town or rural area. A Tier 5 provider could be as small as an Internet cafe.
One important difference between the tiers has to do with the relationship a provider has with other ISPs. Using an economic definition of peer (rather than the BGP definition), a peer relationship means that two ISPs do not charge each other to carry each other's traffic. They are both about the same size and it is to their mutual advantage to let their customers have access to each other, without worrying about billing. This differs from the other common ISP relationship, which is a customer-provider one, where a smaller ISP pays a larger ISP for the privilege of sending traffic through the larger ISP's network. This is often called buying transit.
A Tier 1 provider doesn't buy transit. A Tier 1 provider has a 24/7 network operations center and a national or international backbone with at least DS-3 connectivity, and more likely OC-3 to OC-48. The provider gets all its routes from bilateral peering arrangements. Its customers are primarily other providers, but it may support a large enterprise also. Examples of Tier 1 providers include UUNet, Cable & Wireless (C&W), Sprint, Qwest, Verio, Level 3, and AT&T. Tier 2 providers also have high-bandwidth backbones and 24/7 operations, but they are limited to a regional or national presence and they buy transit (often at a bulk discount) from a Tier 1 provider for traffic that goes outside the region. A Tier 2 provider gets all its regional routes through peering arrangements. Examples of Tier 2 providers include SBC, Earthlink, and AOL.
A Tier 3 provider is typically a regional provider for a small or medium-sized region. For example, Log On America is a Tier 3 provider in the Northeast region of the United States. A Tier 3 provider buys transit from multiple upstream providers and runs a default-free routing table. There's no general definition of Tier 4 or Tier 5, but Tier 4 could be a metropolitan provider that is multihomed to two regional providers, and Tier 5 might be a small, single-homed provider that connects end users via dialup, cable modem, or wireless service.
At this point in the design process, you should have analyzed requirements and topologies to the extent that you have a good idea of the tier you will need. During the logical design phase, you should start making a list of criteria for selecting providers and develop a plan and set of standards for evaluating candidates. Investigate the availability of service providers in the relevant regions and start making inquiries. Be specific in your requests for information from candidates. Prioritize the requested information and indicate how quickly you need a response. You may also want to ask for references and to start asking questions about the provider of other users in the region. See the "Selecting a WAN Service Provider" section in Chapter 11, "Selecting Technologies and Devices for Enterprise Networks," for more information on this topic.
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