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1 ISP/IXP Workshops © 1999, Cisco Systems, Inc. Introduction to OSPF Introduction to OSPF Introduction to OSPF ISP/IXP Workshops ISP/IXP Workshops ISP/IXP Workshops
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Page 1: Introduction to OSPF - Internet Societyws.edu.isoc.org/workshops/2004/CEDIA2/material/b2-1up.pdf · Introduction to OSPF ISP/IXP Workshops. ... • Five types of OSPF routing protocol

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

OSPFOSPFOSPF

• Dynamic Routing Protocol

• Link State technology

• Runs over IP, protocol 89

• Designed by IETF for TCP/IP

• Supports VLSM

• Multi-vendor

• Fast rerouting

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3ISP/IXP Workshops © 1999, Cisco Systems, Inc. www.cisco.com

OSPF (continued)OSPF (continued)OSPF (continued)

• Minimises routing protocol traffic

• Low bandwidth requirements

• Supports different types of areas

• Route summarisation and authentication

• Cisco’s implementation is fully compliantwith the specification OSPF v2.

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4ISP/IXP Workshops © 1999, Cisco Systems, Inc. www.cisco.com

Link StateLink State

Topology Information IsKept in a DatabaseSeparate from the

Routing Table

AABBCC

2213131313

QQZZXX

X’s Link State

ZZ

XX

YYQQ

Z’s Link State

Q’s Link State

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5ISP/IXP Workshops © 1999, Cisco Systems, Inc. www.cisco.com

Link State RoutingLink State Routing

• Neighbour discovery

• Constructing an LSP

• Distribute LSP

• Compute routes

• On network failureNew LSPs flooded

All routers recompute routing tables

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6ISP/IXP Workshops © 1999, Cisco Systems, Inc. www.cisco.com

FDDIDual Ring

Low Bandwidth UtilisationLow Bandwidth Utilisation

• Only changes propagated

• Multicast on multi-access broadcastnetworks

R1

LSA

XLSA

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7ISP/IXP Workshops © 1999, Cisco Systems, Inc. www.cisco.com

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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8ISP/IXP Workshops © 1999, Cisco Systems, Inc. www.cisco.com

Fast ConvergenceFast Convergence

• Detection Plus LSA/SPF

XR1 R3

R2

N2

Alternate Path

Primary Path

N1

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9ISP/IXP Workshops © 1999, Cisco Systems, Inc. www.cisco.com

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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10ISP/IXP Workshops © 1999, Cisco Systems, Inc. www.cisco.com

Utilises IP Multicast forUtilises IP Multicast forSending/Receiving UpdatesSending/Receiving Updates

• Broadcast networksDR 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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11ISP/IXP Workshops © 1999, Cisco Systems, Inc. www.cisco.com

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 1Area 4

Area 0Backbone Area

Area 2 Area 3

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12ISP/IXP Workshops © 1999, Cisco Systems, Inc. www.cisco.com

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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13ISP/IXP Workshops © 1999, Cisco Systems, Inc. www.cisco.com

OSPF Route TypesOSPF Route Types

Intra-area Route

all routes inside an area

Inter-area Route

routes advertised from one areato another by an Area BorderRouter

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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14ISP/IXP Workshops © 1999, Cisco Systems, Inc. www.cisco.com

Inter-Area RouteInter-Area RouteSummarisationSummarisation

• Prefix or all subnets

• Prefix or all networks

• ‘Area range’ command

1.A 1.B 1.C

FDDIDual Ring

R1 (ABR)

R2

Network1

Next HopR1

Network1.A1.B1.C

Next HopR1R1R1

With summarisation

Withoutsummarisation

BackboneArea 0

Area 1

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15ISP/IXP Workshops © 1999, Cisco Systems, Inc. www.cisco.com

External RoutesExternal Routes

• Redistributed into OSPF

• Flooded unaltered throughout the AS

• OSPF supports two types of externalmetrics

Type 1 external metrics

Type 2 external metrics (Default)

RIPIGRPEIGRPBGPetc.

OSPF

Redistribute

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16ISP/IXP Workshops © 1999, Cisco Systems, Inc. www.cisco.com

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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17ISP/IXP Workshops © 1999, Cisco Systems, Inc. www.cisco.com

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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18ISP/IXP Workshops © 1999, Cisco Systems, Inc. www.cisco.com

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 separatelyfor each area

• LSA flooding is bounded by area

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19ISP/IXP Workshops © 1999, Cisco Systems, Inc. www.cisco.com

Protocol FunctionalityProtocol Functionality

• Bringing up adjacencies

• LSA types

• Area classification

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20ISP/IXP Workshops © 1999, Cisco Systems, Inc. www.cisco.com

The Hello ProtocolThe Hello Protocol

• Responsible for establishing and maintainingneighbour relationships

• Elects designated router on multi-accessnetworks

FDDIDual Ring

Hello

HelloHello

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21ISP/IXP Workshops © 1999, Cisco Systems, Inc. www.cisco.com

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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22ISP/IXP Workshops © 1999, Cisco Systems, Inc. www.cisco.com

Designated RouterDesignated Router

• One per multi-access network

Generates network links advertisements

Assists in database synchronization

Designated Router

Designated Router

BackupDesignated Router

BackupDesignated

Router

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23ISP/IXP Workshops © 1999, Cisco Systems, Inc. www.cisco.com

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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24ISP/IXP Workshops © 1999, Cisco Systems, Inc. www.cisco.com

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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25ISP/IXP Workshops © 1999, Cisco Systems, Inc. www.cisco.com

Neighbouring StatesNeighbouring States

• FullRouters are fully adjacent

Databases synchronised

Relationship to DR andBDR

DR BDR

Full

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26ISP/IXP Workshops © 1999, Cisco Systems, Inc. www.cisco.com

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 designatedrouter

• The neighbouring router is the backupdesignated router

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27ISP/IXP Workshops © 1999, Cisco Systems, Inc. www.cisco.com

LSAsLSAs Propagate Along Propagate AlongAdjacenciesAdjacencies

• LSAs acknowledged alongadjacencies

DR BDR

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28ISP/IXP Workshops © 1999, Cisco Systems, Inc. www.cisco.com

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 packetsHello - 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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29ISP/IXP Workshops © 1999, Cisco Systems, Inc. www.cisco.com

Different Types ofDifferent Types of LSAs LSAs

• Five distinct type of LSAs

Type 1 : Router LSA

Type 2 : Network LSA

Type 3 and 4: Summary LSA

Type 5 and 7: External LSA

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30ISP/IXP Workshops © 1999, Cisco Systems, Inc. www.cisco.com

Router LSA (Type 1)Router LSA (Type 1)

• Describes the state and cost of therouter’s links to the area

• All of the router’s 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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31ISP/IXP Workshops © 1999, Cisco Systems, Inc. www.cisco.com

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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32ISP/IXP Workshops © 1999, Cisco Systems, Inc. www.cisco.com

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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33ISP/IXP Workshops © 1999, Cisco Systems, Inc. www.cisco.com

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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34ISP/IXP Workshops © 1999, Cisco Systems, Inc. www.cisco.com

Not Summarised: Specific LinksNot Summarised: Specific Links

BackboneArea #0

External links

1.A

1.C

1.B

1.D

TokenRing

TokenRing

TokenRing

TokenRing

3.D

3.A

3.C

3.B

1.A1.B1.C1.D

3.A3.B3.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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35ISP/IXP Workshops © 1999, Cisco Systems, Inc. www.cisco.com

Summarised: Summary LinksSummarised: Summary Links

BackboneArea #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 not

propagate

1 3

2

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36ISP/IXP Workshops © 1999, Cisco Systems, Inc. www.cisco.com

BackboneArea #0

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.C

2.B

TokenRing

TokenRing

ASBR

Not Summarised: Specific LinksNot Summarised: Specific Links

2.A2.B2.C3.A3.B3.C3.D

1.A1.B1.C1.D3.A3.B3.C3.D

1.A1.B1.C1.D2.A2.B2.C

• Specific link LSA advertised in• Link state changes propagate in

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37ISP/IXP Workshops © 1999, Cisco Systems, Inc. www.cisco.com

Summarised: Summary LinksSummarised: Summary Links

BackboneArea #0

3.D

3.A

2.B

• Only summary LSA advertised in• Link state changes do not

propagate 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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38ISP/IXP Workshops © 1999, Cisco Systems, Inc. www.cisco.com

TokenRing

TokenRing

TokenRing

TokenRing

Regular Area (Not a Stub)Regular Area (Not a Stub)

From area 1’s viewpoint

• Summary networks from other areas injected• External networks injected, for example

network X.1ASBR

External Networks

1.A

1.C

1.B

1.DTokenRing

TokenRing

3.C

3.B

2.A

2,3

1,3

1,2X.1

X.1

X.1

X.1

2.D2.C

2.B

3.A

3.D

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39ISP/IXP Workshops © 1999, Cisco Systems, Inc. www.cisco.com

TokenRing

TokenRing

TokenRing

TokenRing

Normal Stub AreaNormal Stub Area

From area 1’s 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 stubarea 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

1,2X.1

X.1

X.1

X.1

2.D2.C

2.B

3.A

3.D

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40ISP/IXP Workshops © 1999, Cisco Systems, Inc. www.cisco.com

TokenRing

TokenRing

TokenRing

TokenRing

Totally Stubby AreaTotally Stubby Area

From area 1’s viewpoint

• Only a default network is injected into the areaRepresents 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

1,2X.1

X.1

X.1

X.1

2.D2.C

2.B

3.A

3.D

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41ISP/IXP Workshops © 1999, Cisco Systems, Inc. www.cisco.com

TokenRing

TokenRing

TokenRing

TokenRing

Not-So-Stubby AreaNot-So-Stubby Area

• Capable of importing external routes in a limitedfashion

• Type-7 LSA’s carry external information within anNSSA

• NSSA Border routers translate selected type-7LSAs 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

1,2X.1

X.1, X.2

X.1, X.2X.1

2.D2.C

2.B

3.A

3.DExternalNetworks

X.2

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42ISP/IXP Workshops © 1999, Cisco Systems, Inc. www.cisco.com

AddressingAddressing

Area 1network 131.108.0.0subnets 17-31range 255.255.240.0

Area 2network 131.108.0.0subnets 33-47range 255.255.240.0

Area 3network 131.108.0.0subnets 49-63range 255.255.240.0

Area 0network 192.117.49.0range 255.255.255.0

Assign contiguous ranges of subnets per area to facilitate summarisation

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43ISP/IXP Workshops © 1999, Cisco Systems, Inc. www.cisco.com

SummarySummary

• Scalable OSPF Network Design

Area hierarchy

Stub areas

Contiguous addressing

Route summarisation

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44ISP/IXP Workshops © 1999, Cisco Systems, Inc. www.cisco.com

Redistributing Routes intoRedistributing Routes intoOSPFOSPF

ROUTER OSPF <pid#x>

REDISTRIBUTE {protocol} <as#y>

<metric>

<metric-type (1 or 2)

<tag>

<subnets>

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45ISP/IXP Workshops © 1999, Cisco Systems, Inc. www.cisco.com

Router Sub-commandsRouter Sub-commands

• NETWORK <n.n.n.n> <mask> AREA <area-id>

• AREA <area-id> STUB {no-summary}

• AREA <area-id> AUTHENTICATION

• AREA <area-id> DEFAULT_COST <cost>

• AREA <area-id> VIRTUAL-LINK <router-id>...

• AREA <area-id> RANGE <address mask>

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46ISP/IXP Workshops © 1999, Cisco Systems, Inc. www.cisco.com

Interface SubcommandsInterface Subcommands

• IP OSPF COST <cost>

• IP OSPF PRIORITY <8-bit-number>

• IP OSPF HELLO-INTERVAL <number-of-seconds>

• IP OSPF DEAD-INTERVAL <number-of-seconds>

• IP OSPF AUTHENTICATION-KEY <8-bytes-of-password>

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