Quản trị mạng - Ospf

OSPFRouting Protocols and Concepts – Chapter 11ObjectivesDescribe the background and basic features of OSPF.Identify and apply the basic OSPF configuration commands.Describe, modify and calculate the metric used by OSPF.Describe the Designated Router/Backup Designated Router (DR/BDR) election process in multiaccess networks.Describe the uses of additional configuration commands in OSPF.IntroductionIntroduction to OSPFBackground of OSPFBegan in 19871989 OSPFv1 released in RFC 1131This version was experimental & never deployed1991 OSPFv2 released in RFC 12471998 OSPFv2 updated in RFC 23281999 OSPFv3 published in RFC 2740 Introduction to OSPFOSPF Message EncapsulationOSPF packet typeThere exist 5 typesOSPF packet header Contains - Router ID and area ID and Type code for OSPF packet typeIP packet headerContains - Source IP address, Destination IP address, & Protocol field set to 89 Introduction to OSPFOSPF Message EncapsulationData link frame headerContains - Source MAC address and Destination MAC addressIntroduction to OSPFOSPF Packet TypesIntroduction to OSPFHello ProtocolOSPF Hello PacketPurpose of Hello PacketDiscover OSPF neighbors & establish adjacenciesAdvertise guidelines on which routers must agree to become neighborsUsed by multi-access networks to elect a designated router and a backup designated routerIntroduction to OSPFHello Packets continuedContents of a Hello Packet router ID of transmitting routerOSPF Hello IntervalsUsually multicast (224.0.0.5)Sent every 30 seconds for NBMA segmentsOSPF Dead IntervalsThis is the time that must transpire before the neighbor is considered downDefault time is 4 times the hello intervalIntroduction to OSPFHello protocol packets contain information that is used in electingDesignated Router (DR)DR is responsible for updating all other OSPF routersBackup Designated Router (BDR)This router takes over DR’s responsibilities if DR failsIntroduction to OSPFOSPF Link-state UpdatesPurpose of a Link State Update (LSU)Used to deliver link state advertisementsPurpose of a Link State Advertisement (LSA)Contains information about neighbors & path costsIntroduction to OSPFOSPF AlgorithmOSPF routers build & maintain link-state database containing LSA received from other routersInformation found in database is utilized upon execution of Dijkstra SPF algorithmSPF algorithm used to create SPF treeSPF tree used to populate routing tableIntroduction to OSPFAdministrative DistanceDefault Administrative Distance for OSPF is 110Introduction to OSPFOSPF AuthenticationPurpose is to encrypt & authenticate routing informationThis is an interface specific configurationRouters will only accept routing information from other routers that have been configured with the same password or authentication informationBasic OSPF ConfigurationLab TopologyTopology used for this chapter Discontiguous IP addressing schemeSince OSPF is a classless routing protocol the subnet mask is configured inBasic OSPF ConfigurationThe router ospf commandTo enable OSPF on a router use the following command R1(config)#router ospf process-id Process id A locally significant number between 1 and 65535This means it does not have to match other OSPF routersBasic OSPF ConfigurationOSPF network commandRequires entering: network addresswildcard mask - the inverse of the subnet maskarea-id - area-id refers to the OSPF area – OSPF area is a group of routers that share link state informationExample: Router(config-router)#network network-address wildcard-ask area area-id Basic OSPF ConfigurationRouter IDThis is an IP address used to identify a router3 criteria for deriving the router IDUse IP address configured with OSPF router-id commandTakes precedence over loopback and physical interface addressesIf router-id command not used then router chooses highest IP address of any loopback interfacesIf no loopback interfaces are configured then the highest IP address on any active interface is usedBasic OSPF ConfigurationOSPF Router IDCommands used to verify current router IDShow ip protocolsShow ip ospfShow ip ospf interfaceBasic OSPF ConfigurationOSPF Router IDRouter ID & Loopback addressesHighest loopback address will be used as router ID if router-id command isn’t usedAdvantage of using loopback addressThe loopback interface cannot fail  OSPF stabilityThe OSPF router-id commandIntroduced in IOS 12.0Command syntaxRouter(config)#router ospfprocess-idRouter(config-router)#router-idip-addressModifying the Router IDUse the command Router#clear ip ospf processBasic OSPF ConfigurationVerifying OSPFUse the show ip ospf command to verify & trouble shoot OSPF networksCommand will display the following:Neighbor adjacencyNo adjacency indicated byNeighboring router’s Router ID is not displayedA state of full is not displayedConsequence of no adjacencyNo link state information exchangedInaccurate SPF trees & routing tablesBasic OSPF ConfigurationCommandDescriptionShow ip protocolsDisplays OSPF process ID, router ID, networks router is advertising & administrative distanceShow ip ospfDisplays OSPF process ID, router ID, OSPF area information & the last time SPF algorithm calculatedShow ip ospf interfaceDisplays hello interval and dead intervalVerifying OSPF - Additional CommandsBasic OSPF ConfigurationExamining the routing tableUse the show ip route command to display the routing tableAn “O’ at the beginning of a route indicates that the router source is OSPFNote OSPF does not automatically summarize at major network boundariesOSPF MetricOSPF uses cost as the metric for determining the best routeThe best route will have the lowest costCost is based on bandwidth of an interfaceCost is calculated using the formula108 / bandwidthReference bandwidthDefaults to 100MbpsCan be modified usingAuto-cost reference-bandwidth commandOSPF MetricCOST of an OSPF routeIs the accumulated value from one router to the nextOSPF MetricUsually the actual speed of a link is different than the default bandwidthThis makes it imperative that the bandwidth value reflects link’s actual speedReason: so routing table has best path informationThe show interface command will display interface’s bandwidthMost serial link default to 1.544MbpsBasic OSPF ConfigurationModifying the Cost of a linkBoth sides of a serial link should be configured with the same bandwidthCommands used to modify bandwidth valueBandwidth commandExample: Router(config-if)#bandwidthbandwidth-kbpsip ospf cost command – allows you to directly specify interface costExample: R1(config)#interface serial 0/0/0R1(config-if)#ip ospf cost 1562 Basic OSPF ConfigurationModifying the Cost of the linkDifference between bandwidth command & the ip ospf cost commandIp ospf cost commandSets cost to a specific valueBandwidth commandLink cost is calculatedOSPF and Multiaccess NetworksChallenges in Multiaccess NetworksOSPF defines five network types:Point-to-point Broadcast Multiaccess Nonbroadcast Multiaccess (NBMA) Point-to-multipoint Virtual linksOSPF in Multiaccess Networks2 challenges presented by multiaccess networksMultiple adjacenciesExtensive LSA floodingOSPF in Multiaccess NetworksExtensive flooding of LSAsFor every LSA sent out there must be an acknowledgement of receipt sent back to transmitting routerConsequence: lots of bandwidth consumed and chaotic trafficOSPF in Multiaccess NetworksSolution to LSA flooding issue is the use of Designated router (DR)Backup designated router (BDR)DR & BDR selectionRouters are elected to send & receive LSASending & Receiving LSADR others send LSAs via multicast 224.0.0.6 to DR & BDRDR forward LSA via multicast address 224.0.0.5 to all other routersOSPF in Multiaccess NetworksDR/BDR Election ProcessDR/BDR elections DO NOT occur in point to point networksOSPF in Multiaccess NetworksDR/BDR elections will take place on multiaccess networks as shown belowOSPF in Multiaccess NetworksCriteria for getting elected DR/BDRDR: Router with the highest OSPF interface priorityBDR: Router with the second highest OSPF interface priorityIf OSPF interface priorities are equal, the highest router ID is used to break the tieOSPF in Multiaccess NetworksTiming of DR/BDR ElectionOccurs as soon as 1st router has its interface enabled on multiaccess networkWhen a DR is elected it remains as the DR until one of the following occursThe DR failsThe OSPF process on the DR failsThe multiaccess interface on the DR failsOSPF in Multiaccess NetworksManipulating the election processIf you want to influence the election of DR & BDR then do one of the following:Boot up the DR first, followed by the BDR, and then boot all other routers ORShut down the interface on all routers, followed by a no shutdown on the DR, then the BDR, and then all other routersOSPF in Multiaccess NetworksOSPF Interface PriorityManipulating the DR/BDR election process continuedUse the ip ospf priority interface command.Example:Router(config-if)#ip ospf priority {0 - 255}Priority number range 0 to 2550 means the router cannot become the DR or BDR1 is the default priority valueMore OSPF ConfigurationRedistributing an OSPF Default RouteTopology includes a link to ISPRouter connected to ISPCalled an autonomous system border routerUsed to propagate a default routeExample of static default route: R1(config)#ip route 0.0.0.0 0.0.0.0 loopback 1Requires the use of the default-information originate commandExample of default-information originate command: R1(config-router)#default-information originateMore OSPF ConfigurationFine-Tuning OSPFSince link speeds are getting faster it may be necessary to change reference bandwidth valuesDo this using the auto-cost reference-bandwidth commandExample:R1(config-router)#auto-cost reference-bandwidth 10000More OSPF ConfigurationFine-Tuning OSPFModifying OSPF timersReason to modify timersFaster detection of network failuresManually modifying Hello & Dead intervalsRouter(config-if)#ip ospf hello-interval  secondsRouter(config-if)#ip ospf dead-interval secondsPoint to be madeHello & Dead intervals must be the same between neighborsSummaryRFC 2328 describes OSPF link state concepts and operationsOSPF CharacteristicsA commonly deployed link state routing protocolEmploys DRs & BDRs on multi-access networksDRs & BDRs are electedDR & BDRs are used to transmit and receive LSAsUses 5 packet types:1: HELLO2: DATABASE DESCRIPTION3: LINK STATE REQUEST4: LINK STATE UPDATE 5: LINK STATE ACKNOWLEDGEMENTSummaryOSPF CharacteristicsMetric = costLowest cost = best pathConfigurationEnable OSPF on a router using the following commandR1(config)#router ospf process-idUse the network command to define which interfaces will participate in a given OSPF processRouter(config-router)#network network-address wildcard-mask area area-idSummaryVerifying OSPF configurationUse the following commands:show ip protocolshow ip routeshow ip ospf interfaceshow ip ospf neighbor