[ Pobierz całość w formacie PDF ]
.cisco.com/cpress/cc/td/cpress/fund/iprf/ip2913.htm (12 of 21) [02/02/2001 11.40.09] Building Internetworksare interconnected.Having three or more concentrator routers introduced requires the network designerto select a subtopology for the concentrator tier.These routers can be either fully or partially meshed, orstrung together peer to peer.Regardless of the subtopology selected, hierarchical, multitiered topologies function best when somebasic implementation principles are followed.First, the concentration layer of routers should bededicated to the task.That is, they are not used to directly connect user communities.Second, theuser-premises routers should only internetwork with concentrator nodes and not with each other in apeer-to-peer fashion.Third, the interconnection of user-premises routers to concentrator routers shouldnot be done randomly.Some logic should be applied in determining their placement.Depending on thegeographic distribution of the users and the transmission facilities used, it may be prudent to place theconcentrator nodes to minimize the distances from the user premises.The reason for this is rooted ineconomics: Most WAN transmission facilities are priced according to their mileage.Longer transmissionfacilities are more costly than shorter transmission facilities.Therefore, arranging your topology tominimize the distances of transmission facilities will effectively reduce your network's monthly recurringfacilities charges.Given that one or more routers will be dedicated to route aggregation, this topology can be an expensiveundertaking.This tends to limit the use of these topologies to larger companies.Three-Tiered TopologyWANs that need to interconnect a very large number of sites, or are built using smaller routers that canonly support a few serial connections, may find the two-tiered architecture insufficiently scalable.Therefore, adding a third tier (or even a fourth or fifth tier) may provide the additional scalability theyrequire.Figure 13-7 illustrates this topology.Figure 13-7: The three-tiered WAN topology.A three-tiered WAN constructed with dedicated facilities offers even greater fault tolerance andhttp://wwwin.cisco.com/cpress/cc/td/cpress/fund/iprf/ip2913.htm (13 of 21) [02/02/2001 11.40.09] Building Internetworksscalability than the two-tiered topology.Three-tiered networks are expensive to build, operate, andmaintain.They should be used only for interconnecting very large numbers of locations.Given this, it isfoolish to develop a WAN of this magnitude and not fully mesh the uppermost, or backbone, tier ofrouters.Hybrid TopologiesHybridization of multiple topologies is useful in larger, more complex networks.It allows the WAN tobe tailored to actual traffic patterns instead of trying to force-fit those patterns into a rigid topologicalmodel.In other words, the basic topologies presented in this section are examples of constructs intendedto stimulate your creative thought.There are no limits on the topological variety that can be introduced toa WAN.The effectiveness of each topology, and subsequent combination of WAN technologies, dependsdirectly on your particular situation and performance requirements.Multitiered networks, in particular, lend themselves to hybridization.As previously discussed,multitiered WANs can be hybridized by fully or partially meshing the backbone tier of routers.Althoughthere is no right or wrong way to build a hybrid topology, one example of this WAN is illustrated inFigure 13-8.Figure 13-8: A hybrid topology.An effective hybrid topology may be developed in a multitiered WAN by using a fully meshed topologyfor the backbone nodes only.This affords fault tolerance to the network's backbone and can providesome of the hop-minimization of a full mesh network without experiencing all its costs or incurring itslimitations on scalability.http://wwwin.cisco.com/cpress/cc/td/cpress/fund/iprf/ip2913.htm (14 of 21) [02/02/2001 11.40.09] Building InternetworksFully meshing the backbone of a multitiered WAN is just one form of hybridized topology.Otherhybrids can also be highly effective.The key is to look for topologies, and subtopologies, that can beused in combination to satisfy your particular networking requirements.Before You Build the WANAfter you have collected your users' requirements and evaluated potential topologies, you might betempted to jump ahead to procurement and implementation.Before you do that, you should address just afew more issues.You must stillDetermine what the aggregate traffic loads will be across your WAN's backbone (if there is abackbone).Identify where to place inbound and outbound gateways (such as dial-in modem pools or gatewayrouters that connect the network to external networks).Select the transmission technologies that will be used for each link.Identify IP addressing and subnet masking schemes.Select which routing protocol(s) will be used.Determining Backbone LoadsA WAN backbone is a router, or routers, and a series of transmission facilities dedicated toWAN-to-WAN traffic transportation.Not every WAN topology features an organized backbone.In fact,of the topologies presented in this chapter, only a few of the larger, more complex, topologies afford thisluxury.Despite this, even the interior gateway routers in small and simple topologies such aspeer-to-peer and ring topologies have to function as WAN backbone devices.This capability is neededwhenever nonadjacent interior gateway routers need to communicate.For the purposes of this discussion,a WAN's backbone is the collection of routers and transmission facilities used to interconnectnonadjacent interior gateway routers, regardless of whether they also function as interior gatewayrouters.Calculating the LoadGiven this definition, all traffic routed outside a LAN by an interior gateway router can be consideredbackbone traffic.This simplification allows you to understand what kind of load you can expect yourWAN links to experience.Actually calculating the load on a backbone is difficult, if not impossible, todo proactively.This is especially true in startup scenarios in which there is no historical data on which tobase your design.In such cases, you have little choice but to interview users [ Pobierz całość w formacie PDF ]
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.cisco.com/cpress/cc/td/cpress/fund/iprf/ip2913.htm (12 of 21) [02/02/2001 11.40.09] Building Internetworksare interconnected.Having three or more concentrator routers introduced requires the network designerto select a subtopology for the concentrator tier.These routers can be either fully or partially meshed, orstrung together peer to peer.Regardless of the subtopology selected, hierarchical, multitiered topologies function best when somebasic implementation principles are followed.First, the concentration layer of routers should bededicated to the task.That is, they are not used to directly connect user communities.Second, theuser-premises routers should only internetwork with concentrator nodes and not with each other in apeer-to-peer fashion.Third, the interconnection of user-premises routers to concentrator routers shouldnot be done randomly.Some logic should be applied in determining their placement.Depending on thegeographic distribution of the users and the transmission facilities used, it may be prudent to place theconcentrator nodes to minimize the distances from the user premises.The reason for this is rooted ineconomics: Most WAN transmission facilities are priced according to their mileage.Longer transmissionfacilities are more costly than shorter transmission facilities.Therefore, arranging your topology tominimize the distances of transmission facilities will effectively reduce your network's monthly recurringfacilities charges.Given that one or more routers will be dedicated to route aggregation, this topology can be an expensiveundertaking.This tends to limit the use of these topologies to larger companies.Three-Tiered TopologyWANs that need to interconnect a very large number of sites, or are built using smaller routers that canonly support a few serial connections, may find the two-tiered architecture insufficiently scalable.Therefore, adding a third tier (or even a fourth or fifth tier) may provide the additional scalability theyrequire.Figure 13-7 illustrates this topology.Figure 13-7: The three-tiered WAN topology.A three-tiered WAN constructed with dedicated facilities offers even greater fault tolerance andhttp://wwwin.cisco.com/cpress/cc/td/cpress/fund/iprf/ip2913.htm (13 of 21) [02/02/2001 11.40.09] Building Internetworksscalability than the two-tiered topology.Three-tiered networks are expensive to build, operate, andmaintain.They should be used only for interconnecting very large numbers of locations.Given this, it isfoolish to develop a WAN of this magnitude and not fully mesh the uppermost, or backbone, tier ofrouters.Hybrid TopologiesHybridization of multiple topologies is useful in larger, more complex networks.It allows the WAN tobe tailored to actual traffic patterns instead of trying to force-fit those patterns into a rigid topologicalmodel.In other words, the basic topologies presented in this section are examples of constructs intendedto stimulate your creative thought.There are no limits on the topological variety that can be introduced toa WAN.The effectiveness of each topology, and subsequent combination of WAN technologies, dependsdirectly on your particular situation and performance requirements.Multitiered networks, in particular, lend themselves to hybridization.As previously discussed,multitiered WANs can be hybridized by fully or partially meshing the backbone tier of routers.Althoughthere is no right or wrong way to build a hybrid topology, one example of this WAN is illustrated inFigure 13-8.Figure 13-8: A hybrid topology.An effective hybrid topology may be developed in a multitiered WAN by using a fully meshed topologyfor the backbone nodes only.This affords fault tolerance to the network's backbone and can providesome of the hop-minimization of a full mesh network without experiencing all its costs or incurring itslimitations on scalability.http://wwwin.cisco.com/cpress/cc/td/cpress/fund/iprf/ip2913.htm (14 of 21) [02/02/2001 11.40.09] Building InternetworksFully meshing the backbone of a multitiered WAN is just one form of hybridized topology.Otherhybrids can also be highly effective.The key is to look for topologies, and subtopologies, that can beused in combination to satisfy your particular networking requirements.Before You Build the WANAfter you have collected your users' requirements and evaluated potential topologies, you might betempted to jump ahead to procurement and implementation.Before you do that, you should address just afew more issues.You must stillDetermine what the aggregate traffic loads will be across your WAN's backbone (if there is abackbone).Identify where to place inbound and outbound gateways (such as dial-in modem pools or gatewayrouters that connect the network to external networks).Select the transmission technologies that will be used for each link.Identify IP addressing and subnet masking schemes.Select which routing protocol(s) will be used.Determining Backbone LoadsA WAN backbone is a router, or routers, and a series of transmission facilities dedicated toWAN-to-WAN traffic transportation.Not every WAN topology features an organized backbone.In fact,of the topologies presented in this chapter, only a few of the larger, more complex, topologies afford thisluxury.Despite this, even the interior gateway routers in small and simple topologies such aspeer-to-peer and ring topologies have to function as WAN backbone devices.This capability is neededwhenever nonadjacent interior gateway routers need to communicate.For the purposes of this discussion,a WAN's backbone is the collection of routers and transmission facilities used to interconnectnonadjacent interior gateway routers, regardless of whether they also function as interior gatewayrouters.Calculating the LoadGiven this definition, all traffic routed outside a LAN by an interior gateway router can be consideredbackbone traffic.This simplification allows you to understand what kind of load you can expect yourWAN links to experience.Actually calculating the load on a backbone is difficult, if not impossible, todo proactively.This is especially true in startup scenarios in which there is no historical data on which tobase your design.In such cases, you have little choice but to interview users [ Pobierz całość w formacie PDF ]