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.A meshed topology has three distinct advantages over a redundant star topology:" A meshed topology is usually slightly less expensive (at least by the cost of one WAN link)." A meshed topology provides more direct (and potentially faster) communication between remotesites, which translates to greater application availability.This can be useful if direct trafficvolumes between remote sites are relatively high." A meshed topology promotes distributed operation, preventing bottlenecks on the corporaterouter and further increasing application availability.A redundant star is a reasonable solution under the following conditions:" Relatively little traffic must travel between remote offices." Traffic moving between corporate and remote offices is delay sensitive and mission critical.Thedelay and potential reliability problems associated with making an extra hop when a link betweena remote office and the corporate office fails might not be tolerable.Redundant Power SystemsPower faults are common in large-scale networks.Because they can strike across a very local or avery wide scale, power faults are difficult to preempt.Simple power problems include dislodgedpower cords, tripped circuit breakers, and local power supply failures.More extensive powerproblems include large-scale outages caused by natural phenomena (such as lightning strikes) orbrown-outs.Each organization must assess its needs and the probability of each type of power outagebefore determining which preventative actions to take.You can take many precautions to try to ensure that problems, such as dislodged power cords, do notoccur frequently.These fall outside the scope of this publication and will not be discussed here.Thischapter focuses on issues addressable by internetworking devices.From the standpoint of internetworking devices, dual power systems can prevent otherwisedebilitating failures.Imagine a situation where the so-called backbone-in-a-box configuration isbeing used.This configuration calls for the connection of many networks to a router being used as aconnectivity hub.Benefits include a high-speed backbone (essentially the router s backplane) andcost efficiency (less media).Unfortunately, if the router s power system becomes faulty, eachnetwork connected to that router loses its capability to communicate with all other networksconnected to that router.Some backbone-in-a-box routers can address this requirement by providing redundant powersystems.In addition, many sites connect one power system to the local power grid and the other toan uninterruptable power supply.If router power fails, the router can continue to provideconnectivity to each connected network.General power outages are usually more common than failures in a router s power system.Considerthe effect of a site-wide power failure on redundant star and meshed topologies.If the power fails inthe corporate office, the organization might be seriously inconvenienced.Key network applicationsare likely to be placed at a centralized, corporate location.The organization could easily lose revenuefor every minute its network is down.The meshed network configuration is superior in this casebecause links between the remote offices would still be able to communicate with each other.2-30 Cisco CCIE Fundamentals: Network Design Choosing Internetworking Reliability OptionsIf power fails at a remote site, all connections to that remote site will be terminated unless otherwiseprotected.Neither the redundant star nor the meshed topology is superior.In both cases, all otherremote offices will still be able to communicate with the corporate office.Generally, power failuresin a remote office are more serious when network services are widely distributed.To protect against local and site-wide power outages, some companies have negotiated anarrangement with local power companies to use multiple power grids within their organization.Failure within one power grid will not affect the network if all critical components have access tomultiple power grids.Unfortunately, this arrangement is very expensive and should only beconsidered by companies with substantial resources, extremely mission-critical operations, and arelatively high likelihood of power failures.The effect of highly localized power failures can be minimized with prudent network planning.Wherever possible, redundant components should use power supplied by different circuits.Further,these redundant components should not be physically colocated.For example, if redundant routersare employed for all stations on a given floor, these routers can be physically stationed in wiringclosets on different floors.This prevents local wiring closet power problems from affecting thecapability of all stations on a given floor to communicate.Figure 2-21 shows such a configuration.Figure 2-21 Redundant components on different floors.To other routers To end stations on floor X+2Floor X+1RouterEnd stationsRouterShared media LANsFloor XRouterEnd stationsRouterShared media LANsTo other routers Router on floor X-1For some organizations, the need for fault tolerance is so great that potential power failures areprotected against with a duplicate corporate data center.Organizations with these requirements oftenlocate a redundant data center in another city, or in a part of the same city that is some distance fromthe primary data center.All backend services are duplicated, and transactions coming in from remoteoffices are sent to both data centers.This configuration requires duplicate WAN links from all remoteoffices, duplicate network hardware, duplicate servers and server resources, and leasing anotherbuilding.Because this approach is so costly, it is typically the last step taken by companies desiringthe ultimate in fault tolerance.Partial duplication of the data center is also a possibility [ Pobierz całość w formacie PDF ]
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.A meshed topology has three distinct advantages over a redundant star topology:" A meshed topology is usually slightly less expensive (at least by the cost of one WAN link)." A meshed topology provides more direct (and potentially faster) communication between remotesites, which translates to greater application availability.This can be useful if direct trafficvolumes between remote sites are relatively high." A meshed topology promotes distributed operation, preventing bottlenecks on the corporaterouter and further increasing application availability.A redundant star is a reasonable solution under the following conditions:" Relatively little traffic must travel between remote offices." Traffic moving between corporate and remote offices is delay sensitive and mission critical.Thedelay and potential reliability problems associated with making an extra hop when a link betweena remote office and the corporate office fails might not be tolerable.Redundant Power SystemsPower faults are common in large-scale networks.Because they can strike across a very local or avery wide scale, power faults are difficult to preempt.Simple power problems include dislodgedpower cords, tripped circuit breakers, and local power supply failures.More extensive powerproblems include large-scale outages caused by natural phenomena (such as lightning strikes) orbrown-outs.Each organization must assess its needs and the probability of each type of power outagebefore determining which preventative actions to take.You can take many precautions to try to ensure that problems, such as dislodged power cords, do notoccur frequently.These fall outside the scope of this publication and will not be discussed here.Thischapter focuses on issues addressable by internetworking devices.From the standpoint of internetworking devices, dual power systems can prevent otherwisedebilitating failures.Imagine a situation where the so-called backbone-in-a-box configuration isbeing used.This configuration calls for the connection of many networks to a router being used as aconnectivity hub.Benefits include a high-speed backbone (essentially the router s backplane) andcost efficiency (less media).Unfortunately, if the router s power system becomes faulty, eachnetwork connected to that router loses its capability to communicate with all other networksconnected to that router.Some backbone-in-a-box routers can address this requirement by providing redundant powersystems.In addition, many sites connect one power system to the local power grid and the other toan uninterruptable power supply.If router power fails, the router can continue to provideconnectivity to each connected network.General power outages are usually more common than failures in a router s power system.Considerthe effect of a site-wide power failure on redundant star and meshed topologies.If the power fails inthe corporate office, the organization might be seriously inconvenienced.Key network applicationsare likely to be placed at a centralized, corporate location.The organization could easily lose revenuefor every minute its network is down.The meshed network configuration is superior in this casebecause links between the remote offices would still be able to communicate with each other.2-30 Cisco CCIE Fundamentals: Network Design Choosing Internetworking Reliability OptionsIf power fails at a remote site, all connections to that remote site will be terminated unless otherwiseprotected.Neither the redundant star nor the meshed topology is superior.In both cases, all otherremote offices will still be able to communicate with the corporate office.Generally, power failuresin a remote office are more serious when network services are widely distributed.To protect against local and site-wide power outages, some companies have negotiated anarrangement with local power companies to use multiple power grids within their organization.Failure within one power grid will not affect the network if all critical components have access tomultiple power grids.Unfortunately, this arrangement is very expensive and should only beconsidered by companies with substantial resources, extremely mission-critical operations, and arelatively high likelihood of power failures.The effect of highly localized power failures can be minimized with prudent network planning.Wherever possible, redundant components should use power supplied by different circuits.Further,these redundant components should not be physically colocated.For example, if redundant routersare employed for all stations on a given floor, these routers can be physically stationed in wiringclosets on different floors.This prevents local wiring closet power problems from affecting thecapability of all stations on a given floor to communicate.Figure 2-21 shows such a configuration.Figure 2-21 Redundant components on different floors.To other routers To end stations on floor X+2Floor X+1RouterEnd stationsRouterShared media LANsFloor XRouterEnd stationsRouterShared media LANsTo other routers Router on floor X-1For some organizations, the need for fault tolerance is so great that potential power failures areprotected against with a duplicate corporate data center.Organizations with these requirements oftenlocate a redundant data center in another city, or in a part of the same city that is some distance fromthe primary data center.All backend services are duplicated, and transactions coming in from remoteoffices are sent to both data centers.This configuration requires duplicate WAN links from all remoteoffices, duplicate network hardware, duplicate servers and server resources, and leasing anotherbuilding.Because this approach is so costly, it is typically the last step taken by companies desiringthe ultimate in fault tolerance.Partial duplication of the data center is also a possibility [ Pobierz całość w formacie PDF ]