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.A main difference between X.25 technology and Frame Relayis latency and overhead.X.25 technology assumes that the physical line is unreliable andtherefore X.25 uses error checking, whereas Frame Relay does not, thereby achieving greaterspeeds with minimal latency.More information on Frame Relay can be found in the section  Provisioning a Frame RelayNetwork later in this chapter.CH01.book Page 165 Friday, January 7, 2000 5:35 PMSelecting a WAN Technology 165Figure 5-4 Using A Frame Relay Network to Connect Remote OfficesRemoteMedia GroupOfficeSegmentMultimediaFile Servers256kInternet512k MultimediaWorkstationsFrame RelayT1RemoteOffice256kFile ServersT1File ServersWorkstationsDMZWorkstationsWEB WEB WEBWorkstationsServers Servers ServersCorporate SegmentOther WAN Technologies and Design FactorsThere are three basic WAN technology categories:" Leased line" Circuit switched" Packet switchedCH01.book Page 166 Friday, January 7, 2000 5:35 PM166 Chapter 5: WAN DesignLeased lines, also known as point-to-point links, are permanent connections establishedbetween two customer end points through a carrier s WAN.Usually these types of links are forprivate use by the customer for the duration of their lease or contract.Circuit switching is a technology used by ISDN.This is where the carrier establishes a physicalcircuit for the length of the session.This type of technology is similar to establishing a phonecall.Packet switching, used by ATM, Frame Relay, Switched Multimegabit Data Service (SMDS),and X.25, can share a single physical connection through the carrier network.In these types ofnetworks, broadcast traffic can greatly affect the network s performance.However, atechnology such as X.25 and SMDS that does not support broadcasting is called a nonbroadcastmulti-access (NBMA) network.Networks such as Frame Relay and ATM that do supportbroadcast are called multi-access networks.Beyond meshing topologies, there are two other WAN configuration or connection types thatneed to be discussed.The first is a point-to-point connection.This is a description of a type oflogical connection.There is a distinction between the point-to-point WAN technology and thepoint-to-point connection.A Frame Relay network can have one core router that only allowscommunication between the core and the remote offices.This means that the remote officescould not talk directly to each other, so logically the Frame Relay network is operating asmultiple point-to-point connections.This type of connection is very common in T1 WANconfigurations where one end system is connected to another.This connection is also seen inFrame Relay and ATM.The other type of connection, which is often used in packet-switched networks, is the point-to-multipoint type of connection.This is similar to the star topology configuration, where one coreor root device is connected to multiple end devices or leaves.These types of connections, aswell as the meshing topologies, pose problems in routing because of loops.Using the split-horizon update prevents a common problem with routing loops.Split horizon isa method of preventing a router from sending routing information out to the same interface fromwhich it received the update.However, in the case of point-to-multipoint topologies, alsoknown as hub-and-spoke topologies, where one central point handles all the traffic, how is thisproblem resolved? This problem is addressed in Frame Relay by using subinterfaces.Thatmeans that one physical interface has multiple virtual interfaces.Routing information from onevirtual interface is sent out another virtual interface, thereby avoiding the split horizon problem.After reviewing the WAN technology you want to implement, the next step is to select the righthardware.CH01.book Page 167 Friday, January 7, 2000 5:35 PMSelecting the WAN Hardware 167Selecting the WAN HardwareThe CCDA objectives covered in this section are the following:19 Recognize scalability constraints and performance budgets for majorCisco products.20 Recommend Cisco products and WAN technologies that will meet thecustomer s requirements for performance, capacity, and scalability in anenterprise network.Throughout this book there are checklists with questions and criteria to aid a designer inselecting a technology or method.Table 5-2 summarizes the criteria you need to understand tomeet cost, functionality, redundancy, and scalability requirements for selecting WAN hardware.Table 5-2 Checklist for Selecting WAN HardwareCriteria QuestionsCost How much has the customer budgeted for access?Is there a possibility of consolidating services over the same WANTechnology?Functionality What will be the function of this router in the network?Is the router a backbone router?Is the router an Internet access router?What type of technologies will it have to support?Are there serial connections?Is ISDN a consideration?What type of modules will the router need?Is there a WAN interface card?Are there Versatile Interface Processor (VIP) cards?What other modules and slots will the router need?How much processing power will the router need (processing powerdepends on the complexity of the routing)?How fast does the processor need to be? (The processor speed candepend on complex processes such as route calculation complexity.)Are there other features such as encryption to consider?What types of ports are needed on the router, and how many ports arenecessary?continuesCH01 [ Pobierz całość w formacie PDF ]