OSPF Version 2 (RFC 2328)

smashlizardsNetworking and Communications

Oct 29, 2013 (3 years and 10 months ago)

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OSPFVersion2(RFC2328)
￿
Interiorgatewayprotocol(IGP).
￿
Routersmaintainlink-statedatabase.
DescribesAutonomousSystems(AS)topology.
Propagatedbyßooding:LinkStateAdvertisements(LSAs).
￿
Routerconstructstreerootedatitself.
￿
TrafÞcdistributedequallyovermultipleequal-costpathsto
destination.
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OSPFMetrics
￿
Internalmetrics:costassociatedwithoutputsideofrouter
interfaces.
￿
Externalmetrics:denoteexternallyderivedroutingdata.
Type1:treatedthesameasinternalmetrics.
Type2:alwaysgreaterthancostofanyinternalpath.
￿
BothexternalmetrictypescansimultaneouslyexistinanAS.
￿
Abilitytospecify
forwardingaddress
inLSAto:
Reduceextrahops.
Enablesomerouterstoactsasrouteservers.
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OSPFHierarchicalOrganization
￿
AutonomousSystemsplitintoareas.
￿
Eachareahasitsownlink-statedatabase.
￿
Motivationforhierarchy:scalability.
￿
Areaborderrouter:routerconnectedtomultipleareas.
Storesdifferentlink-statedatabaseforeachofitsareas.
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AutonomousSystemBackbone
￿
Specialarea0(0.0.0.0).
￿
Containsallareaborderrouters.
￿
Logicallycontiguous:
Employvirtuallinksifphysicalcontiguityisnotpossible.
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OSPFClassicationofRouters
￿
Internalrouters.
￿
Areaborderrouters.
￿
Backbonerouters.
￿
ASboundaryrouters.
Aroutermaybelongtomultiplecategories.
EveryrouterinanASknowsthepathtoevery
ASboundaryrouter
.
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OSPFAdjacencyDetermination
￿
Routerssendandreceive
OSPFHello
packetstoacquire
neighbors.
￿
Hellopacketssentto
AllSPFRouters
multicastaddress(224.0.0.5).
￿
Hellopacketsalsousedtoelect
DesignatedRouter
in
broadcastandnon-broadcastmultiaccess(NBMA)networks.
￿
Routersformadjacencieswithsomenewlyacquiredneighbors
andsynchronizeLSdatabase.
￿
InbroadcastandNBMAnetworks,adjacenciesfromDesignated
Routerandotherrouters,andBackupDesignatedRouterand
otherrouters.
￿
AllDRouters
address(224.0.0.6)usedbyotherrouterstosend
informationtoDesignatedandBackupDesignatedrouters.
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Intra-areaRouting
￿
RoutersßoodLSAsinthearea:periodicallyandontopology
change.
￿
Reliableßoodingthroughacknowledgments.
￿
AllroutersinanareahaveidenticalLSdatabase.
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Inter-areaRouting
Areaborderrouter:
￿
Summarizestopologyinformationofitsarea.
￿
SendsthesummarytoallotherareaborderroutersintheAS.
￿
Receivessummaryfromotherareaborderroutersand
calculatesinter-areapaths.
￿
Advertisesinter-areapathstoitsattachedarea.
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AS-ExternalRouting
￿
ASboundaryroutersßoodtheirexternalroutinginformationto
allrouters.
￿
Exception:notßoodedinto
stub
areas.
￿
Allrouters,exceptthoseinstubareas,knowthepathtoAS
boundaryrouters.
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Authentication
￿
AllOSPFpacketsareauthenticated.
￿
64-bitauthenticationtypeÞeldinOSPFpacketheader.
￿
AuthenticationtypeconÞgurableonper-interfacebasis:
0:nullauthentication.
1:simplepassword.
2:cryptographicauthentication.
others:reservedforfutureuse.
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SimplePasswordAuthentication
￿
AllOSPFpacketsinanetworkusethesamepassword.
￿
64-bitclearpassword.
￿
Guardsagainstrouterinadvertentlyjoiningaroutingdomain.
￿
Possibletodeterminepasswordifonehasphysicalaccessto
network.
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CryptographicAuthentication
￿
SharedkeyconÞguredinallroutersattachedtoacommon
network.
￿
Messagedigest
generatedusingthekey,andappendedtopacket.
￿
Implicitagreementamongroutersaboutalgorithm.
￿
Cryptographicsequencenumbertopreventreplayattacks:
Non-decreasing,32-bitvalue.
Initializedtozero.
RolloverprocedurenotspeciÞed.
MaybeusedtodenoteÒsecondssincerebootÓ.
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