2 introduction to ospf 1up
TRANSCRIPT
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1ISP/IXP Workshops 1999, Cisco Systems, Inc.
Introduction to OSPFIntroduction to OSPFIntroduction to OSPF
ISP/IXP WorkshopsISP/IXP WorkshopsISP/IXP Workshops
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AgendaAgendaAgenda
OSPF Primer
OSPF in Service Provider Networks
OSPF BCP - Adding Networks
OSPF Command Summary
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3ISP/IXP Workshops 1999, Cisco Systems, Inc.
OSPF PrimerOSPF PrimerOSPF Primer
3ISP/IXP Workshops 1999, Cisco Systems, Inc. www.cisco.com
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OSPFOSPFOSPF
Open ShortestPath First
Link state or SPFtechnology
Developed by OSPFworking group ofIETF (RFC 1247)
Designed for TCP/IPInternet environment
Fast convergence
Variable-lengthsubnet masks
Discontiguoussubnets
No periodic updates
Route authentication
Delivered two years
after IGRP
OSPF standarddescribed in RFC2328
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Link StateLink State
Topology Information IsKept in a DatabaseSeparate from the
Routing Table
AA
BB
CC
22
1313
1313
QQ
ZZ
XX
Xs Link State
ZZ
XX
YYQQ
Zs Link State
Qs Link State
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Link State RoutingLink State Routing
Neighbour discovery
Constructing an LSP
Distribute LSP
Compute routes
On network failure
New LSPs flooded
All routers recompute routing tables
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FDDIDual Ring
Low Bandwidth UtilisationLow Bandwidth Utilisation
Only changes propagated
Multicast on multi-access broadcastnetworks
R1
LSAX
LSA
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FDDI
Dual Ring
FDDI
Dual Ring
Optimal Path UtilisationOptimal Path Utilisation
N1
N2 N3
N4
N5R1
R2
R3
R4
Cost = 1 Cost = 1
Cost = 10
Cost = 10
The optimal path is determined by thesum of the interface costs
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Fast ConvergenceFast Convergence
Detection Plus LSA/SPF
XR1 R3
R2
N2
Alternate Path
Primary Path
N1
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Fast ConvergenceFast Convergence
Finding a new route
LSA flooded throughout area
Acknowledgement based
Topology databasesynchronised
Each router derives routingtable to destination networks
LSA
XR1
N1
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Utilises IP Multicast forUtilises IP Multicast forSending/Receiving UpdatesSending/Receiving Updates
Broadcast networks
DR and BDR > AllSPFRouters (224.0.0.5)All other routers > AllDRRouters (224.0.0.6)
Hello packets sent to AllSPFRouters(Unicast on point-to-point and virtuallinks)
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OSPF AreasOSPF Areas
Group of contiguoushosts and networks
Per area topologicaldatabase
Invisible outside the area
Reduction in routing traffic
Backbone areacontiguous
All other areas must beconnected to the backbone
Virtual Links
Area 1
Area 4
Area 0Backbone Area
Area 2 Area 3
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Classification of RoutersClassification of Routers
Internal Router (IR)
Area Border Router(ABR)
Backbone Router (BR)
Autonomous SystemBorder Router (ASBR)
Area 1
IR/BRArea 0
Area 2 Area 3
IR
ABR/BR
To other AS
ASBR
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OSPF Route TypesOSPF Route Types
Intra-area Routeall routes inside an area
Inter-area Route
routes advertised from one areato another by an Area Border
Router
External Route
routes imported into OSPF fromother protocol or static routes
Area 0Area 2 Area 3
ABR
To other AS
ASBR
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Inter-Area RouteInter-Area RouteSummarisationSummarisation
Prefix or all subnets
Prefix or all networks
Area range command
1.A 1.B 1.C
FDDI
Dual Ring
R1 (ABR)
R2
Network1
Next HopR1
Network1.A1.B1.C
Next HopR1R1R1
Withsummarisation
Withoutsummarisation
Backbone
Area 0
Area 1
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External RoutesExternal Routes
Redistributed into OSPF
Flooded unaltered throughout the AS
OSPF supports two types of external
metrics
Type 1 external metrics
Type 2 external metrics (Default)
RIP
IGRPEIGRPBGPetc.
OSPF
Redistribute
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External RoutesExternal Routes
Type 1 external metric: metrics are addedto the summarised internal link cost
NetworkN1N1
Type 11110
Next HopR2R3
Cost = 10
to N1External Cost = 1
to N1External Cost = 2R2
R3
R1
Cost = 8
Selected Route
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External RoutesExternal Routes
Type 2 external metric: metrics are comparedwithout adding to the internal link cost
NetworkN1N1
Type 212
Next HopR2R3
Cost = 10
to N1External Cost = 1
to N1External Cost = 2R2
R3
R1
Cost = 8
Selected Route
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Topology/Link State DatabaseTopology/Link State Database
A router has a separate LS databasefor each area to which it belongs
All routers belonging to the samearea have identical database
SPF calculation is performed separately
for each area LSA flooding is bounded by area
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Protocol FunctionalityProtocol Functionality
Bringing up adjacencies LSA types
Area classification
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The Hello ProtocolThe Hello Protocol
Responsible for establishing and maintainingneighbour relationships
Elects designated router on multi-access
networks
FDDI
Dual Ring
Hello
HelloHello
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The Hello PacketThe Hello Packet
Router priority
Hello interval
Router deadinterval
Network mask
Options: T-bit, E-bit
List of neighbours
FDDIDual Ring
Hello
HelloHello
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Designated RouterDesignated Router
One per multi-access network
Generates network links advertisements
Assists in database synchronization
DesignatedRouter
DesignatedRouter
BackupDesignated Router
BackupDesignated
Router
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Designated Router by PriorityDesignated Router by Priority
Configured priority (per interface)
Else determined by highest router ID
Router ID is the loopback interface address, ifconfigured, otherwise the highest IP address
144.254.3.5
R2 Router ID = 131.108.3.3
131.108.3.2 131.108.3.3
R1 Router ID = 144.254.3.5
DR
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Neighbouring StatesNeighbouring States
2-way
Router sees itself in other Hello packets
DR selected from neighbours in state2-way or greater
DR BDR
2-way
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Neighbouring StatesNeighbouring States
Full
Routers are fully adjacentDatabases synchronised
Relationship to DR andBDR
DR BDR
Full
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When to Become AdjacentWhen to Become Adjacent
Underlying network is point to point
Underlying network type is virtual link
The router itself is the designated router
The router itself is the backup designatedrouter
The neighbouring router is the designated
router The neighbouring router is the backup
designated router
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LSAsLSAs Propagate AlongPropagate AlongAdjacenciesAdjacencies
LSAs acknowledged alongadjacencies
DR BDR
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Routing Protocol PacketsRouting Protocol Packets
Share a common protocol header
Routing protocol packets are sent with typeof service (TOS) of 0
Five types of OSPF routing protocol packets
Hello - packet type 1
Database description - packet type 2
Link-state request - packet type 3
Link-state update - packet type 4
Link-state acknowledgement - packet type 5
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Different Types ofDifferent Types of LSAsLSAs
Five distinct type of LSAs
Type 1 : Router LSAType 2 : Network LSA
Type 3 and 4: Summary LSA
Type 5 and 7: External LSA
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Router LSA (Type 1)Router LSA (Type 1)
Describes the state and cost of therouters links to the area
All of the routers links in an area must bedescribed in a single LSA
Flooded throughout the particular areaand no more
Router indicates whether it is an ASBR,ABR, or end point of virtual link
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Network LSA (Type 2)Network LSA (Type 2)
Generated for every transit broadcastand NBMA network
Describes all the routers attached to thenetwork
Only the designated router originatesthis LSA
Flooded throughout the area and nomore
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Summary LSA (Type 3 and 4)Summary LSA (Type 3 and 4)
Describes the destination outside thearea but still in the AS
Flooded throughout a single area
Originated by an ABR
Only intra-area routes are advertisedinto the backbone
Type 4 is the information about theASBR
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External LSA (Type 5)External LSA (Type 5)
Defines routes to destination externalto the AS
Default route is also sent as external Two types of external LSA:
E1: Consider the total cost up to the externaldestination
E2: Considers only the cost of the outgoinginterface to the external destination
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Not Summarised: Specific LinksNot Summarised: Specific Links
BackboneArea #0
External links
1.A
1.C
1.B
1.D
TokenRing
TokenRing
Token
Ring
TokenRing
3.D
3.A
3.C
3.B
1.A1.B
1.C1.D
3.A3.B
3.C3.D
2.A2.B2.C
2.A
2.C
2.B
TokenRing
TokenRing
Specific link LSA advertised out
Link state changes propagate out ASBR
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Summarised: Summary LinksSummarised: Summary Links
Backbone
Area #0
ASBR
External links
1.A
1.C
1.B
1.D
TokenRing
TokenRing
TokenRing
TokenRing
3.D
3.A
3.C
3.B
2.A
2.B
TokenRing
TokenRing
Only summary LSA advertised out
Link state changes do notpropagate
1 3
2
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BackboneArea #0
External links
1.A
1.C
1.B
1.D
TokenRing
TokenRing
Token
Ring
TokenRing
3.D
3.A
3.C
3.B
2.A
2.C
2.B
TokenRing
TokenRing
ASBR
Not Summarised: Specific LinksNot Summarised: Specific Links
2.A2.B2.C3.A
3.B3.C3.D
1.A1.B
1.C1.D3.A3.B3.C
3.D
1.A1.B1.C1.D2.A
2.B2.C
Specific link LSA advertised in Link state changes propagate in
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Summarised: Summary LinksSummarised: Summary Links
BackboneArea #0
3.D
3.A
2.B
Only summary LSA advertised in
Link state changes do notpropagate ASBR
External links
1.A
1.C
1.B
1.D
TokenRing
TokenRing
TokenRing
TokenRing
3.C
3.B
2.A
TokenRing
TokenRing
2,3
1,3
1,2
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TokenRing
TokenRing
TokenRing
TokenRing
Regular Area (Not a Stub)Regular Area (Not a Stub)
From area 1s viewpoint
Summary networks from other areas injected
External networks injected, for examplenetwork X.1
ASBR
External Networks
1.A
1.C
1.B
1.DTokenRing
TokenRing
3.C
3.B
2.A
2,3
1,3
1,2
X.1
X.1
X.1X.1
2.D
2.C
2.B
3.A
3.D
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TokenRing
TokenRing
TokenRing
TokenRing
Normal Stub AreaNormal Stub Area
From area 1s viewpoint Summary networks from other areas injected
Default network injected into the area - represents external links
Default path to closest area border router
Define all routers in the area as stub
area x stub command ASBR
External Networks
1.A
1.C
1.B
1.DTokenRing
TokenRing
3.C
3.B
2.A
2,3 & Default
1,3
1,2
X.1
X.1
X.1X.1
2.D
2.C
2.B
3.A
3.D
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TokenRing
TokenRing
TokenRing
TokenRing
Totally Stubby AreaTotally Stubby Area
From area 1s viewpoint Only a default network is injected into the area
Represents external networks and all inter-area routes
Default path to closest area border router
Define all routers in the area as totally stubbyarea x stub no-summary command ASBR
External Networks
1.A
1.C
1.B
1.DTokenRing
TokenRing
3.C
3.B
2.A
Default 2&3
1,3
1,2
X.1
X.1
X.1X.1
2.D
2.C
2.B
3.A
3.D
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TokenRing
TokenRing
TokenRing
TokenRing
Not-So-Stubby AreaNot-So-Stubby Area
Capable of importing external routes in a limitedfashion
Type-7 LSAs carry external information within anNSSA
NSSA Border routers translate selected type-7
LSAs into type-5 external network LSAs ASBR
External Networks
1.A
1.C
1.B
1.DTokenRing
TokenRing
3.C
3.B
2.A
Default 2&3
1,3
1,2
X.1
X.1, X.2
X.1, X.2X.1
2.D
2.C
2.B
3.A
3.DExternal
NetworksX.2
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AddressingAddressing
Area 1network 131.108.0.0
subnets 17-31range 255.255.240.0
Area 2network 131.108.0.0
subnets 33-47range 255.255.240.0
Area 3network 131.108.0.0
subnets 49-63range 255.255.240.0
Area 0
network 192.117.49.0range 255.255.255.0
Assign contiguous ranges of subnets per area to facilitate summarisation
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SummarySummary
Scalable OSPF Network Design
Area hierarchy
Stub areas
Contiguous addressing
Route summarisation
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OSPF Design In
Service ProviderNetworks
OSPF Design InOSPF Design In
Service ProviderService Provider
NetworksNetworks
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BackboneBackboneRouterRouter
OSPF Areas and RulesOSPF Areas and RulesOSPF Areas and Rules
Area 1
Area 4
Area 0
Area 2 Area 3
InternalInternal
RouterRouter
AreaArea
BorderBorder
RouterRouter
AutonomousAutonomous
System (AS)System (AS)
Border RouterBorder Router
Internet
Backbone area (0)must be present
All other areasmust have
connectionto backbone
Backbone mustbe contiguous
Do not partition
area (0)
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OSPF DesignOSPF DesignOSPF Design
Attack addressing first - OSPF andAddressing go together.
Objective is to keep the Link State DataBase lean.
Create address hierarchy to matchtopology
Separate Blocks for infrastructure,customer interfaces, customers, etc.
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OSPF DesignOSPF DesignOSPF Design
Examine physical topology
Is it meshed or hub-and-spoke?
Try to use as Stubby an area as possible
It reduces overhead and LSA counts
Push the creation of a backbone
Reduces mesh and promotes hierarchy
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OSPF DesignOSPF Design
One SPF per area, flooding done per area
Watch out for overloading ABRs
Different types of areas do different
floodingNormal areas
Stub areas
Totally stubby (stub no-summary)
Not so stubby areas (NSSA)
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OSPF DesignOSPF Design
Redundancy
Dual Links out of each area - usingmetrics (cost) for traffic enginering
Too much redundancyDual links to backbone in stub areasmust be the same - other wise sub-optimal routing will result
Too Much Redundancy in the backbonearea without good summarization willeffect convergence in the area 0
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OSPF for ISPsOSPF for ISPsOSPF for ISPs
OSPF features should consider.
OSPF logging neighbour changesOSPF reference cost
OSPF Router ID Command
OSPF Process Clear/Restart
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OSPF BCP
Adding Networks
OSPF BCPOSPF BCP
Adding NetworksAdding Networks
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OSPF Addi N kOSPF Addi N kOSPF Addi N k
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OSPF - Adding NetworksOSPF - Adding NetworksOSPF - Adding Networks
BCP - Individual OSPFNetwork statement foreach infrastructure link.
Have separate IP address
blocks for infrastructureand customer links.
Use IP UnnumberedInterfaces or BGP to carry/30s to customers
OSPF should only carryinfrastructure routes in anISPs network.
OC12c
OC12c
Customer Connections
OC48
ISP Backbone
OSPF - Adding NetworksOSPF - Adding NetworksOSPF - Adding Networks
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OSPF Adding Networks(Method One)
OSPF Adding NetworksOSPF Adding Networks(Method One)(Method One)
redistributed connect subnets
Works for all connected interfaces on the
router but sends networks as externaltype-2s - which are not summarized
router ospf 100
redistributed connected subnets
Not recommended
OSPF Addi N t kOSPF Addi N t kOSPF Addi N t k
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OSPF - Adding NetworksOSPF - Adding NetworksOSPF - Adding Networks
Specific network statements
Every interface needs a OSPF networkstatement. Interface that should not be
broadcasting OSPF Hello packets needspassive-interface.
router ospf 100
network 192.168.1.4 0.0.0.3 area 51
network 192.168.1.6 0.0.0.3 area 51
passive interface Serial 1/0
OSPF Addi N t kOSPF Addi N t kOSPF Addi N t k
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OSPF - Adding NetworksOSPF - Adding NetworksOSPF - Adding Networks
Network statements - wildcard maskEvery interface covered by wildcard maskused in OSPF network statement.Interfaces that should not be broadcasting
OSPF Hello packets need passive-interfaceor default passive-interface.
router ospf 100
network 192.168.1.0 0.0.0.255 area 51default passive-interface default
no passive interface POS 4/0
OSPF Adding Net orksOSPF Addi N t kOSPF Adding Networks
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OSPF - Adding NetworksOSPF - Adding NetworksOSPF - Adding Networks
Key Theme when selecting atechnique: Keep the Link StateDatabase Lean
Increases Stability
Reduces the amount of information inthe Link State Advertisements (LSAs)
Speeds Convergence Time
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OSPF - New and Useful
Features
OSPF - New and UsefulOSPF - New and Useful
FeaturesFeatures
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OSPF Logging NeighbourOSPF Logging Neighbour
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gg g ggg g gChangesChanges
The router will generate a log messagewhenever an OSPF neighbour changes state
Syntax:
[no] ospf log-adjacency-changes[no] ospf log-adjacency-changes
Example of a typical log message:
%OSPF-5-ADJCHG: Process 1,%OSPF-5-ADJCHG: Process 1, NbrNbr223.127.255.223 on Ethernet0 from LOADING to223.127.255.223 on Ethernet0 from LOADING to
FULL, Loading DoneFULL, Loading Done
Number of State ChangesNumber of State Changes
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Number of State ChangesNumber of State Changes
The number of state transitions isavailable via SNMP (ospfNbrEvents)and the CLI:
show ip ospf neighbor [type number]show ip ospf neighbor [type number][neighbor-id] [detail][neighbor-id] [detail]
Detail(Optional) Displays all neighboursgiven in detail (list all neighbours). When
specified, neighbour state transition countersare displayed per interface or neighbour ID
State Changes (Continued)State Changes (Continued)
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State Changes (Continued)State Changes (Continued)
To reset OSPF-related statistics, usethe clear ip ospf countersclear ip ospf counters EXECcommand. At this point neighborneighbor is the
only available option; it will resetneighbour state transition counters perinterface or neighbour id
clear ip ospf counters [neighbor [] [neighbor-id]]
OSPF Cost: ReferenceOSPF Cost: Reference
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BandwidthBandwidth
Bandwidth used in Metric calculation
Cost = 10^8/BW
Not useful for BW > 100 Mbps
Syntax:
ospf auto-cost reference-bandwidth
Default reference bandwidth still 100Mbps for backward compatibility
OSPF Router IDOSPF Router IDOSPF Router ID
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OSPF Router IDOSPF Router IDOSPF Router ID
If the loopback interface exists and has anIP address, that is used as the router ID inrouting protocols - stability!
If the loopback interface does not exist, orhas no IP address, the router ID is thehighest IP address configured - danger!
New sub command to manually set the
OSPF Router ID:router-id
OSPF Clear/RestartOSPF Clear/RestartOSPF Clear/Restart
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OSPF Clear/RestartOSPF Clear/RestartOSPF Clear/Restart
clear ip ospf [pid] redistribution
This command can now clear redistribution based on OSPF routingprocess ID. If no pid is given, it assumes all OSPF processes.
clear ip ospf [pid] counters
This command can now clear counters based on OSPF routingprocess ID. If no pid is given, it assumes all OSPF processes.
clear ip ospf [pid] process
This command will restart the specified OSPF process. If no pid isgiven, it assumes all OSPF processes. It attempts to keep the oldrouter-id, except in cases, where a new router-id was configured, or
an old user configured router-id was removed. Since this commandcan potentially cause a network churn, a user confirmation isrequired before performing any action.
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OSPF Command
Summary
OSPF CommandOSPF Command
SummarySummary
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Redistributing Routes intoRedistributing Routes intoOSPF
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OSPFOSPF
ROUTER OSPF
REDISTRIBUTE {protocol}
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Router Sub commandsRouter Sub commands
NETWORK AREA
AREA STUB {no-summary}
AREA AUTHENTICATION
AREA DEFAULT_COST
AREA VIRTUAL-LINK ...
AREA RANGE
Interface SubcommandsInterface Subcommands
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Interface SubcommandsInterface Subcommands
IP OSPF COST
IP OSPF PRIORITY
IP OSPF HELLO-INTERVAL
IP OSPF DEAD-INTERVAL
IP OSPF AUTHENTICATION-KEY
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