Bài giảng Mạng máy tính nâng cao - Chapter 11: Open Shortest Path First

Chapter 11Open Shortest Path First MẠNG MÁY TÍNH NÂNG CAOObjectives Describe 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.Introduction Introduction to OSPF Background 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 OSPF OSPF 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 OSPF OSPF Message EncapsulationData link frame headerContains - Source MAC address and Destination MAC addressIntroduction to OSPF OSPF Packet TypesHello ProtocolOSPF 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 routerHello ProtocolOSPF Hello PacketHello ProtocolOSPF Hello PacketContents of a Hello Packet router ID of transmitting routerOSPF Hello IntervalsUsually multicast (224.0.0.5)By default, OSPF Hello packets are sent every 10 seconds on multiaccess and point-to-point segments and every 30 seconds on non-broadcast multiaccess (NBMA) segments (Frame Relay, X.25, ATM). OSPF Dead IntervalsThis is the time that must transpire before the neighbor is considered downDefault time is 4 times the hello intervalHello ProtocolElecting a DR and BDRTo reduce the amount of OSPF traffic on multiaccess networks, OSPF elects a Designated Router (DR) and Backup Designated Router (BDR). Hello 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 failsLink-state updates (LSUs) Link-state updates (LSUs) are the packets used for OSPF routing updates. An LSU packet can contain 11 different types of Link-State Advertisements (LSAs), as shown in the figure. The difference between the terms Link-State Update (LSU) and Link-State Advertisement (LSA) can sometimes be confusing. At times, these terms are used interchangeably. An LSU contains one or more LSAs and either term can be used to refer to link-state information propagated by OSPF routers.Link-state updates (LSUs)OSPF 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 tableAdministrative DistanceDefault Administrative Distance for OSPF is 110OSPF 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 Configuration Topology used for this chapter Discontiguous IP addressing schemeSince OSPF is a classless routing protocol the subnet mask is configured inBasic OSPF ConfigurationTo 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 Configuration-OSPF Router ID Router 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 Configuration OSPF 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-mask area area-id Basic OSPF Configuration-OSPF Router IDCommands used to verify current router IDShow ip protocolsShow ip ospfShow ip ospf interfaceBasic OSPF Configuration -OSPF Router IDRouter ID & Loopback addressesHighest loopback address will be used as router ID if router-id command isn’t usedThe OSPF router-id commandIntroduced in IOS 12.0Command syntaxRouter(config)#router ospf process-idRouter(config-router)#router-id ip-addressModifying the Router IDRouter#clear ip ospf processModifying a router ID with a new loopback or physical interface IP address may require reloading the router.Basic OSPF Configuration-Verifying OSPFUse the show ip ospf neighbor command to verify & trouble shoot OSPF networks. Command 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 tables Verifying OSPF - Additional CommandsCommandDescriptionShow 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 intervalExamining 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 Metric OSPF 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 Metric COST of an OSPF routeIs the accumulated value from one router to the nextOSPF Metric Usually the actual speed of a link is different than the default bandwidthIt is important that the bandwidth value reflect the actual speed of the link so that the routing table has accurate best path information. The show interface command will display interface’s bandwidth: Most serial link default to 1.544MbpsOSPF Metric -Modifying the Cost of a linkBoth sides of a serial link should be configured with the same bandwidthCommands used to modify bandwidth value is Bandwidth command Example: Router(config-if)#bandwidth bandwidth-kbpsip ospf cost command – allows you to directly specify interface cost Example: R1(config)#interface serial 0/0/0 R1(config-if)#ip ospf cost 1562OSPF Metric -Modifying the Cost of a linkOSPF Metric -Modifying the Cost of a linkDifference between bandwidth command & the ip ospf cost commandIp ospf cost commandSets cost to a specific valueBandwidth commandLink cost is calculated OSPF and Multiaccess Networks OSPF defines five network types:Point-to-point Broadcast Multiaccess Nonbroadcast Multiaccess (NBMA) Point-to-multipoint Virtual linksOSPF in Multiaccess Networks 2 challenges presented by multiaccess networksMultiple adjacenciesExtensive LSA floodingOSPF in Multiaccess Networks Extensive 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 Networks OSPF in Multiaccess Networks DR/BDR Election ProcessDR/BDR elections DO NOT occur in point to point networksOSPF in Multiaccess Networks DR/BDR elections will take place on multiaccess networks as shown belowOSPF in Multiaccess Networks OSPF in Multiaccess Networks Timing 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 Networks Manipulating the election process If 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 Networks More OSPF Configuration More OSPF Configuration More OSPF Configuration Modifying 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 neighbors