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.Adynamicrouter, on the other hand, reconfigures the routing tableautomatically and recalculates the most efficient path interms of load, line delay, or bandwidth.In wired networks, some routers balance the traffic loadacross multiple access links, providing an N × T1 inverse mul-tiplexer function.This allows multiple T1 access lines oper-ating at 1.544 Mbps each to be used as a singlehigher-bandwidth facility.If one of the links fails, the otherlinks remain in place to handle the offered traffic.As soon asthe failed link is restored to service, traffic is spread across theentire group of lines as in the original configuration.Routing ProtocolsEach router on the network keeps a routing table and movesdata along the network from one router to the next usingsuch protocols as the Open Shortest Path First (OSPF) pro-tocol and the Routing Information Protocol (RIP).Although still supported by many vendors, RIP does notperform well in today s increasingly complex networks.Asthe network expands, routing updates grow larger under ROUTERS 307RIP and consume more bandwidth to route the information.When a link fails, the RIP update procedure slows route dis-covery, increases network traffic and bandwidth usage, andmay cause temporary looping of data traffic.Also, RIP can-not base route selection on such factors as delay and band-width, and its line-selection facility is capable of choosingonly one path to each destination.The newer routing standard, OSPF, overcomes the limita-tions of RIP and even provides capabilities not found in RIP.The update procedure of OSPF requires that each router onthe network transmit a packet with a description of its locallinks to all other routers.On receiving each packet, the otherrouters acknowledge it, and in the process, distributed rout-ing tables are built from the collected descriptions.Sincethese description packets are relatively small, they producea minimum of overhead.When a link fails, updated infor-mation floods the network, allowing all the routers to simul-taneously calculate new tables.Types of RoutersMultiprotocol nodal, or hub, routers are used for buildinghighly meshed internetworks.In addition to allowing sev-eral protocols to share the same logical network, thesedevices pick the shortest path to the end node, balance theload across multiple physical links, reroute traffic aroundpoints of failure or congestion, and implement flow controlin conjunction with the end nodes.They also provide themeans to tie remote branch offices into the corporate back-bone, which might use such WAN services as TransmissionControl Protocol/Internet Protocol (TCP/IP), T1, ISDN,and ATM.Access routers are typically used at branch offices.Theseare usually fixed-configuration devices available inEthernet and Token Ring versions that support a limitednumber of protocols and physical interfaces.They provideconnectivity to high-end multiprotocol routers, allowing 308 ROUTERSlarge and small nodes to be managed as a single logicalenterprise network.Although low-cost, plug-and-playbridges can meet the need for branch office connectivity,low-end routers can offer more intelligence and configura-tion flexibility at comparable cost.The newest access routers are multiservice devices that aredesigned to handle a mix of data, voice, and video traffic.Theysupport a variety of WAN connections through built-in inter-faces that include dual ISDN Basic Rate Interface (BRI) inter-faces, dual analog ports, T1/Frame Relay ports, and an ISDNinterface for videoconferencing.Such routers can run soft-ware that provides Internet Protocol Secure (IPSec) virtualprivate network (VPN), firewall, and encryption services.Midrange routers provide network connectivity betweencorporate locations in support of workgroups or the corporateintranet, for example.These routers can be stand-alonedevices or packaged as modules that occupy slots in an intel-ligent wiring hub or LAN switch.In fact, this type of router isoften used to provide connectivity between multiple wiringhubs or LAN switches over high-speed LAN backbones suchas ATM, Fiber Distributed Data Interface (FDDI), and FastEthernet.There is a consumer class of routers for 2 [ Pobierz caÅ‚ość w formacie PDF ]
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.Adynamicrouter, on the other hand, reconfigures the routing tableautomatically and recalculates the most efficient path interms of load, line delay, or bandwidth.In wired networks, some routers balance the traffic loadacross multiple access links, providing an N × T1 inverse mul-tiplexer function.This allows multiple T1 access lines oper-ating at 1.544 Mbps each to be used as a singlehigher-bandwidth facility.If one of the links fails, the otherlinks remain in place to handle the offered traffic.As soon asthe failed link is restored to service, traffic is spread across theentire group of lines as in the original configuration.Routing ProtocolsEach router on the network keeps a routing table and movesdata along the network from one router to the next usingsuch protocols as the Open Shortest Path First (OSPF) pro-tocol and the Routing Information Protocol (RIP).Although still supported by many vendors, RIP does notperform well in today s increasingly complex networks.Asthe network expands, routing updates grow larger under ROUTERS 307RIP and consume more bandwidth to route the information.When a link fails, the RIP update procedure slows route dis-covery, increases network traffic and bandwidth usage, andmay cause temporary looping of data traffic.Also, RIP can-not base route selection on such factors as delay and band-width, and its line-selection facility is capable of choosingonly one path to each destination.The newer routing standard, OSPF, overcomes the limita-tions of RIP and even provides capabilities not found in RIP.The update procedure of OSPF requires that each router onthe network transmit a packet with a description of its locallinks to all other routers.On receiving each packet, the otherrouters acknowledge it, and in the process, distributed rout-ing tables are built from the collected descriptions.Sincethese description packets are relatively small, they producea minimum of overhead.When a link fails, updated infor-mation floods the network, allowing all the routers to simul-taneously calculate new tables.Types of RoutersMultiprotocol nodal, or hub, routers are used for buildinghighly meshed internetworks.In addition to allowing sev-eral protocols to share the same logical network, thesedevices pick the shortest path to the end node, balance theload across multiple physical links, reroute traffic aroundpoints of failure or congestion, and implement flow controlin conjunction with the end nodes.They also provide themeans to tie remote branch offices into the corporate back-bone, which might use such WAN services as TransmissionControl Protocol/Internet Protocol (TCP/IP), T1, ISDN,and ATM.Access routers are typically used at branch offices.Theseare usually fixed-configuration devices available inEthernet and Token Ring versions that support a limitednumber of protocols and physical interfaces.They provideconnectivity to high-end multiprotocol routers, allowing 308 ROUTERSlarge and small nodes to be managed as a single logicalenterprise network.Although low-cost, plug-and-playbridges can meet the need for branch office connectivity,low-end routers can offer more intelligence and configura-tion flexibility at comparable cost.The newest access routers are multiservice devices that aredesigned to handle a mix of data, voice, and video traffic.Theysupport a variety of WAN connections through built-in inter-faces that include dual ISDN Basic Rate Interface (BRI) inter-faces, dual analog ports, T1/Frame Relay ports, and an ISDNinterface for videoconferencing.Such routers can run soft-ware that provides Internet Protocol Secure (IPSec) virtualprivate network (VPN), firewall, and encryption services.Midrange routers provide network connectivity betweencorporate locations in support of workgroups or the corporateintranet, for example.These routers can be stand-alonedevices or packaged as modules that occupy slots in an intel-ligent wiring hub or LAN switch.In fact, this type of router isoften used to provide connectivity between multiple wiringhubs or LAN switches over high-speed LAN backbones suchas ATM, Fiber Distributed Data Interface (FDDI), and FastEthernet.There is a consumer class of routers for 2 [ Pobierz caÅ‚ość w formacie PDF ]