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ters users and central servers are treated as a virtual region, rather than being put into the core The core should only be used for communications and carefully selected network management devices, never for application servers Not every enterprise has the same requirements Figure 26 shows my logical design for a worldwide transportation company that had extensive interregional communications plus an Internet connectivity requirement for each region This
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Corporate Collapsed Backbone
Regional Core
Regional Core
Regional Core
HQ Distribution
Distribution
Distribution
Distribution
HQ Users
Corporate Servers
Firewall
Access
Access
Access
Interserver Links
Figure 25 Multilevel enterprise core centralized organization
The Service Provider Landscape
Worldwide BGP Routed Network
Regional AS Core
Regional AS Border
Regional AS Core
Regional AS Border
Regional AS Core
Regional AS Border
Distribution
Distribution
Distribution
Access
Access
Access
Figure 26 Multilevel enterprise core distributed organization
model works acceptably for centrally controlled enterprises, but does not scale well for interenterprise networks such as credit card authorization Large banks, for example, need to optimize their own cores for internal use, but need to connect to the credit authorization network The logical characteristics of such networks fit best into the distribution tier, which becomes the place of interconnection Interconnecting at the distribution tier allows the core to return to its original simple and fast role of interconnecting sites inside one organization The requirement for a distribution layer function between access and core, however, does not disappear Increasingly, network architects define two distinct sets of function at the distribution tier: the traditional one between core and access, and a border function concerned with interorganizational connectivity (Figure 27) Border functions can deal both with controlled cooperative relationships (for example, a bank to the Visa or MasterCard service networks, or to the Federal Reserve), and with access to the Internet via firewalls
Core
Core
Distribution
Firewall
Firewall
Distribution
Access
Access
Figure 27 Distribution tier evolution
2
This model, however, has its limitations in dealing with provider environments Figure 28 shows some of the ambiguity with which many providers approach the model The providers call their own POP entry point access There are a variety of names for interprovider connection devices, but border router is gaining popularity Matters become especially confusing when referring to the thing at the customer site that connects to the provider This thing is sometimes called a subscriber access device, but certainly that makes the term access rather ambiguous To complicate matters even further, the subscriber access device, with respect to the enterprise network, is probably a device in the enterprise network s distribution tier Entangling the terminology to yet another level, there is usually a device at the customer location that establishes the demarcation of responsibility between subscriber and provider It may be either a simple interface converter and diagnostic box or a fullfunctioned router or switch For this, the general terms customer premises equipment (CPE) and customer location equipment (CLE) have emerged, but still may contain some ambiguity The basic assumption is that the customer owns the CPE and the provider owns the CLE, but operational responsibility may vary For example, I own my digital subscriber line (DSL) access router, but I don t have the configuration password to it; my ISP does The customer, of course, may have a complex enterprise network What we think of as CLE or CPE, however, is an increasingly intelligent interface between customer and provider It also is a point of economic and legal demarcation at which responsibilities change and service level guarantees are monitored The interface may contain firewall functionality, which can be either at
Core
Core Border Router Border Router
Distribution
Distribution POP Routers Access
Access
POP Routers
Subscriber Access
Subscriber Access
Subscriber Access
Subscriber Access
Figure 28 Data carrier interconnection evolution
The Service Provider Landscape
CPE, NOT CPE A telephony tradition has resulted in a good deal of confusion due to acronym collision Traditionally, CPE meant customer premises equipment In the traditional telco environment, CPE was, of course, owned and operated by the carrier As more and more deregulation affected the industry, customer premises equipment variously could be owned and operated by the customer, leased to the customer by the provider, owned by the customer but operated by the provider, or owned and operated by the subscriber Redefining the former CPE into CPE and CLE at least identified operational responsibilities
the customer site or at the POP As seen in Figure 29, the customer edge function may contain equipment to multiplex outgoing Internet traffic, virtual private networks (VPNs), and voice over IP (VoIP) onto a broadband access facility The provider may manage any of the edge devices; at least one device normally will be managed this way If the provider allows the subscriber to manage its own device, the provider will have ironclad configuration settings, which are not negotiable