Cisco CCIE Fundamentals Network Design

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.The scope of the dialer cloud includes only the intended interconnected devices anddoes not include the entire switched media (the entire ISDN spans the globe and is beyond the scopeof the dialer cloud).The exposure to the ISDN must be considered when designing security.The fundamental characteristics of dialer clouds are as follows:" Dialer clouds are collective bundles of potential and active point-to-point connections." On active connections, dialer clouds form an NBMA (non-broadcast multiaccess) media similarto Frame Relay." For outbound dialing on switched circuits (such as ISDN) network protocol address to directorynumber mapping must be configured." Inactive DDR connections are spoofed to appear as active to routing tables." Unwanted broadcast or other traffic causing unneeded connections can be prohibitivelyexpensive.Potential costs on Tariffed media (such as ISDN) should be closely analyzed andmonitored to prevent such loss.The characteristics of dialer clouds affect every stage of DDR internetworking design.A solidunderstanding of network protocol addressing, routing, and filtering strategies can result in veryrobust and cost-effective internetworks.Traffic and Topology of DDRTo determine the optimum topology, the DDR designer should perform a traffic analysis ofinternetworking applications that must be supported.This includes answering the followingquestions:" How often does data traffic need to move between the DDR sites?" What side of the DDR connection can establish the connection? How many remote sites?" Is this a point-to-point solution or a multipoint solution?10-2 Cisco CCIE Fundamentals: Network DesignTopologiesTopologiesThe most important factor in selecting the topology is the number of sites that will be supported.Ifonly two sites will be involved, the point-to-point topology is used.If more than two sites are to besupported, the hub-and-spoke topology is typically used.For small numbers of sites with very lowtraffic volumes, the fully meshed topology may be the most appropriate solution.Topologies for DDR covered in this section include:" Point-to-point" Fully meshed" Hub-and-spokePoint-to-Point TopologyIn a simple point-to-point topology (see Figure 10-2), two sites are connected to each other.Eachsite has a dialer interface and maps the other site s address to a telephone number.If additionalbandwidth is required, multiple links can be aggregated using Multilink PPP.Figure 10-2 Point-to-point topology.Router A Router BFully Meshed TopologyThe fully meshed configuration (see Figure 10-3) is recommended only for very small DDRnetworks.Fully meshed topologies can streamline the dialing process for any-to-any connectivity aseach site can call any other site directly, rather than having to call through a central site, which thenplaces another call to the target site.However, the configuration for each site is more complexbecause each site must have mapping information for every other site.If load sharing is desired, interfaces can be configured for MultiLink PPP capability.In addition tothe complexity of the configuration, either sufficient interfaces must be available on each device todeal with the possibility of all of the other devices calling in, or the possibility of contention forinterfaces needs to be understood and dealt with.Designing DDR Internetworks 10-3Traffic and Topology of DDRFigure 10-3 Fully meshed topologyRouter ARouter D Router BRouter CHub-and-Spoke DDR SolutionsIn a hub-and-spoke topology (see Figure 10-4), a central site is connected to several remote sites.The remote sites communicate with the central site directly; they do not call any of the other remotesites.This topology works very well for scaling large solutions.Figure 10-4 Hub-and-spoke topology.Router ARouter DRouter BRouter CHub-and-spoke topologies are easier to configure than fully meshed topologies when multipointtopologies are required because remote site dialer interfaces are mapped only to the central site.Thisallows most of the design complexity (such as addressing, routing, and authentication) to bemanaged on the DDR Hub.Configuration support of the remote sites can be greatly simplified(similar to one end of a point-to-point topology).10-4 Cisco CCIE Fundamentals: Network DesignTraffic AnalysisIf any-to-any connectivity initiation is required between remote sites, routing behavior may need tobe modified depending on dialer interface behavior (that is, it may be necessary to disablesplit-horizon on distance vector routing protocols).Multiple hubs can be used to provide further scaling of hub-and-spoke technologies.When usingMultiLink PPP, as is very common in ISDN solutions, designers can implement Cisco IOSMultiChassis MultiLink PPP to scale the dial-in rotary group between multiple Network AccessServers.MultiChassis MultiLink PPP (MMP) is discussed further in Chapter 11, Designing ISDNInternetworks, in the section Designing ISDN Internetworks.Traffic AnalysisFor traffic analysis, develop a chart of which protocols need to be able to support DDR-based dialingfrom which devices.This will form the basis of the rest of the DDR design.For example, Company KDT has selected a hub-and-spoke topology (to provide for scaling) and hasdeveloped the needs shown in Table 10-1 for its DDR cloud requirements.Table 10-1 DDR Protocol Connectivity Requirements for KDTRemote Site Dial-In Protocols Dial-Out Protocols Notesc700A IP, IPX Nonec700B IP Nonec1600A IP, AppleTalk IPc2500A IP, IPX, AppleTalk IP, IPX, AppleTalkc2500B IP, IPX IPNAS3600A IP, IPX, AppleTalk IP, IPX, AppleTalkThe purpose of Table is to identify which sites and protocols require the capability to initiate theDDR connections [ Pobierz całość w formacie PDF ]

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