2 introduction to ospf 1up

8/6/2019 2 Introduction to OSPF 1up

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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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2ISP/IXP Workshops 1999, Cisco Systems, Inc. www.cisco.com

AgendaAgendaAgenda

OSPF Primer

OSPF in Service Provider Networks

OSPF BCP - Adding Networks

OSPF Command Summary


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OSPF PrimerOSPF PrimerOSPF Primer



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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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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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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


subnets 33-47range 255.255.240.0


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


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



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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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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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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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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


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


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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