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.The hierarchy is created by strategicallylocating routers between internetwork elements in the hierarchy.Figure 9-5 illustrates a simple hierarchical mesh.The internetwork illustrated in Figure 9-5illustrates a fully meshed backbone, with meshed regional internetworks and broadcast networks atthe outer periphery.The key advantages of the hierarchical mesh are that it scales well and localizes traffic.By placingrouters between fully meshed portions of the internetwork, you limit the number of DLCIs perphysical interface, segment your internetwork, and make the internetwork more manageable.However, consider the following two issues when implementing a hierarchical mesh:" Broadcast and packet replication In an environment that has a large number of multiple DLCIsper router interface, excessive broadcast and packet replication can impair overall performance.With a high level of meshing throughout a hierarchical mesh, excessive broadcast and packetreplication is a significant concern.In the backbone, where traffic throughput requirements aretypically high, preventing bandwidth loss due to broadcast traffic and packet replication isparticularly important." Increased costs associated with additional router interfaces Compared with a fully meshedtopology, additional routers are needed to separate the meshed backbone from the meshedperipheral internetworks.However, by using these routers, you can create much largerinternetworks that scale almost indefinitely in comparison to a fully meshed internetwork.9-8 Internetwork Design GuideHybrid Meshed Frame Relay InternetworksFigure 9-5 Fully meshed hierarchical Frame Relay environment.A1 A2RouterMeshedRegion ARouterB1RouterRouterMeshedMeshedFrame RelayRegion BRouter RouterbackboneRouterRouterB2RouterMeshedC1 C2Region CRouter RouterHybrid Meshed Frame Relay InternetworksThe economic and strategic importance of backbone environments often force internetworkdesigners to implement a hybrid meshed approach to WAN internetworks.Hybrid meshedinternetworks feature redundant, meshed leased lines in the WAN backbone and partially (or fully)meshed Frame Relay PSDNs in the periphery.Routers separate the two elements.Figure 9-6illustrates such a hybrid arrangement.Designing Packet Service Internetworks 9-9Frame Relay Internetwork DesignFigure 9-6 Hybrid hierarchical Frame Relay internetwork.Access networks A1 and A2also feature partiallymeshed topologyA1RouterFully meshed backbone Partially meshed regionalinterconnected with internetwork interconnectedpoint-to-point leased-lines via Frame Relay PSDNRouter RouterRouterFrame RelayRouter Router Routerregional networkRouterRouterPoint-to-pointbackbone A2RouterRouter RouterHybrid hierarchical meshes have the advantages of providing higher performance on the backbone,localizing traffic, and simplifying scaling of the internetwork.In addition, hybrid meshedinternetworks for Frame Relay are attractive because they can provide better traffic control in thebackbone and they allow the backbone to be made of dedicated links, resulting in greater stability.The disadvantages of hybrid hierarchical meshes include high costs associated with the leased linesas well as broadcast and packet replication that can be significant in access internetworks.Regional Topologies for Frame Relay InternetworksYou can adopt one of three basic design approaches for a Frame Relay-based packet service regionalinternetwork:" Star Topologies" Fully Meshed Topologies" Partially Meshed TopologiesEach of these is discussed in the following sections.In general, emphasis is placed on partiallymeshed topologies integrated into a hierarchical environment.Star and fully meshed topologies arediscussed for structural context.Star TopologiesThe general form of the star topology is addressed in the section  Topology Design earlier in thischapter.Stars are attractive because they minimize the number of DLCIs required and result in alow-cost solution.However, a star topology presents some inherent bandwidth limitations.Consideran environment where a backbone router is attached to a Frame Relay cloud at 256 Kbps, while theremote sites are attached at 56 Kbps.Such a topology will throttle traffic coming off the backboneintended for the remote sites.9-10 Internetwork Design GuideRegional Topologies for Frame Relay InternetworksAs suggested in the general discussion, a strict star topology does not offer the fault tolerance neededfor many internetworking situations.If the link from the hub router to a specific leaf router is lost,all connectivity to the leaf router is lost.Fully Meshed TopologiesA fully meshed topology mandates that every routing node connected to a Frame Relay internetworkis logically linked via an assigned DLCI to every other node on the cloud.This topology is nottenable for larger Frame Relay internetworks for several reasons:" Large, fully meshed Frame Relay internetworks require many DLCIs.One is required for eachlogical link between nodes.As shown in Figure 9-7, a fully connected topology requires theassignment of [n(n-1)]/2 DLCIs, where n is the number of routers to be directly connected.Figure 9-7 Fully meshed Frame Relay.Router RouterDLCIsRouter Router" Broadcast replication will choke internetworks in large, meshed Frame Relay topologies.Routersinherently treat Frame Relay as a broadcast medium.Each time a router sends a multicast frame(such as a routing update, spanning tree update, or SAP update), the router must copy the frameto each DLCI for that Frame Relay interface.These problems combine to make fully meshed topologies unworkable and unscalable for all butrelatively small Frame Relay implementations [ Pobierz całość w formacie PDF ]