1 module 4: implementing ospf. 2 lessons ospf ospf areas and hierarchical routing ospf operation...

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Module 4: Module 4: Implementing OSPFImplementing OSPF

2

LessonsLessons

• OSPF

• OSPF Areas and Hierarchical Routing

• OSPF Operation

• OSPF Routing Tables

• Designing an OSPF Network

3

Lesson 4-1: Lesson 4-1: OSPFOSPF

4

RIP vs. OSPFRIP vs. OSPF

• RIP Problems– Limited metric– Next-hop view– Limited network

diameter– Slow convergence– Non-hierarchical

routing

• OSPF Solutions– Arbitrary, 16-bit metric– Complete map– Theoretical unlimited

network diameter– Fast convergence– Hierarchical routing

structure

5

RIP vs. OSPFRIP vs. OSPF

Router A

Router B Router C

Router D

Router E

10

100

10

10

OSPF

10

10Net E

RIP

Router A

Router D

1

Net E (3 hops)

Figure 4-1

6

OSPF FeaturesOSPF Features

• Authentication

• Classless Routing

• Arbitrary Metric

• Hierarchical Routing Structure

• Equal-Load Balancing

• Multicast/Unicast Packets

7

OSPF and Link State RoutingOSPF and Link State Routing

• Neighbor Discovery/Maintenance

• Virtual Links

• Link State Advertisements (LSAs)

• Link State Database (LSDB)

• Adjacencies

• Route Generation Algorithm

• Routing Tables

8

Adjacent RoutersAdjacent Routers

Router A Router B Router C

Adjacent Adjacent

Figure 4-2

9

OSPF Sub-ProtocolsOSPF Sub-ProtocolsSUB-PROTOCOL PHASE FUNCTION

Hello Protocol Phase 1 The hello protocol specifies the format andexchange of Hello packets.

Exchange Protocol Phase 2 The exchange protocol specifies the format andexchange of Database Description (DD) packetsand Link State Request (LSRequest) packets.OSPF routers use DD packets to describe theirLSDB to other routers. OSPF routers useLSRequest packets to request LSAs that may bemissing from their LSDB.

Flooding Protocol Phase 3 The flooding protocol specifies the format andexchange of LSA packets and Link StateAcknowledgement (LSAck) packets. An OSPFrouter uses LSAck packets to acknowledge eachLSA it receives.

Table 4-1

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Non-linear Phases of Non-linear Phases of OSPF OperationOSPF Operation

Phase 1

T0 T1 T2

Phase 2

Phase 3

Phase 1

Figure 4-3

11

RIP vs. OSPFRIP vs. OSPFCOMPARISON RIP OSPF

Routing Method Distance vector / routingby rumor

Link state / comprehensive map of thenetwork topology

Routing structure Non-hierarchical (flat) Hierarchical

Metric Hop-count Arbitrary 16-bit number

Network Diameter 15-hop limit No theoretical limit

Update Method Broadcast or multicast Multicast or unicast

Convergence Slow Fast

Bandwidth Usage Constant High-to-low (the synchronization process isbandwidth intensive; after synchronization,bandwidth usage is low)

ProcessorIntensive?

No Yes

Memory Intensive? No Yes

Table 4-2

12

Lesson 4-2: Lesson 4-2: OSPF AreasOSPF Areas

and Hierarchical Routing and Hierarchical Routing

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ConceptsConcepts

• Types of OSPF Areas

• Types of OSPF Routers

• Types of Traffic

• Route Summarization

• Area Partitioning

• Virtual Links

14

OSPF Areas, OSPF Areas, Routers, Routers,

and Trafficand Traffic

Area 2(Stub Area)

Intra-Area Traffic

Area BorderRouters (ABR)

Area 1(Normal Area)

Inter-Area Traffic

Area 0(Backbone Area)

Intra-Area Traffic

External Traffic

Internal Routers

AutonomousSystem Boundary

Router (ASBR)

BackboneRouters

Area BorderRouter (ABR)

Figure 4-4

15

Route SummarizationRoute Summarization

ABR

172.1.1.0/24

Area 1

172.1.3.0/24

172.1.4.0/24172.1.2.0/24

172.1.5.0/24Sum m arized

Route:172.1.1.0/16

Figure 4-5

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Sample AreaSample Area

Area X

Router XABR ABR

Figure 4-6

17

Router X Fails and Area X Router X Fails and Area X is Partitionedis Partitioned

Router X

Area XArea X

ABR

xABR

Figure 4-7

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Connecting an Area Partition Connecting an Area Partition to the Backbone through a to the Backbone through a

Normal AreaNormal Area

Area 1(Partitioned)

Area 1(Partitioned)

Virtu

al Lin

kArea 2

(Normal Area)


ABR ABR

Figure 4-8

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Connecting a Partitioned Connecting a Partitioned Backbone through a Backbone through a

Normal AreaNormal Area



Area 1(Normal Area)

ABR

Virtual Link

ABR

Figure 4-9

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Lesson 4-3: Lesson 4-3: OSPF OperationOSPF Operation

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Format of an OSPF PacketFormat of an OSPF Packet

Layer 2Header

IP HeaderProtocol ID = 89

OSPF Header24 bytes

Type Specific Data

Figure 4-10

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Types of OSPF PacketsTypes of OSPF Packets

Table 4-3

Type Number Type/Name Abbreviation

Type 1 Hello —

Type 2 Database Description DD

Type 3 Link State Request LSRequest

Type 4 Link State Update LSUpdate/LSA

Type 5 Link State Acknowledgement LSAck

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Function of Hello PacketsFunction of Hello Packets

• Neighbor Discovery / Maintenance– announce the presence of a router– act as keepalives to verify the

continued participation of a router

• Election Process– determine the designated router (DR)

and backup designated router (BDR) of a multi-access network segment

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Only Router A Forwards LSAs Only Router A Forwards LSAs from the Multi-Access Networkfrom the Multi-Access Network

Nei

ghbo

rsA

djac

ency

Nei

gh

bo

rs

Neighbors

Adjacency

Router A(DR)

Router B

Router C

Link toInternetw ork

LSAs

Link toInternetw orkNo LSAsX

Link toInternetw ork

No LSAsXFigure 4-11

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Exchange Protocol:Exchange Protocol:Phase 2 OperationPhase 2 Operation

• Exchange DD Packets– establish master-slave relationship

• Exchange DD Packets– exchange information about LSDBs to

build a link state request list

• Transmit LSRequest Packets– request specific LSAs that are missing

from the LSDB of a router

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OSPF LSA Packet FormatOSPF LSA Packet Format

Layer 2Header

IP HeaderProtocol ID = 89

OSPF Header24 bytes

Type SpecificData

LSA Header20 bytesType = 4

Figure 4-12

27

Flooding Protocol ConceptsFlooding Protocol Concepts

• 6 Types of LSAs

• Sequencing of LSAs

• Aging of LSAs

• Guaranteed Delivery of LSAs

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Types of LSAsTypes of LSAs

Table 4-4

TYPE TYPE CODE FUNCTION

Router LSA1

Every router in an OSPF AS originates a router LSA. The router LSAdescribes the router and the interfaces on the router. Router LSAs arenot forwarded across area boundaries.

Network LSA2

Only the DR of a multi-access network originates a network LSA. TheLSA describes the network and identifies the other routers on thenetwork. Network LSAs are not forwarded across area boundaries.

NetworkSummary LSA 3

Only ABRs originate network summary LSAs. Network summary LSAsprovide the cost from the ABR to ABR that originated the LSA to adestination.

ASBRSummary LSA

4

Only ABRs originate ASBR summary LSAs. ASBR summary LSAsprovide the cost from the ABR that originated the LSA to a specificASBR. Combined with AS external LSAs, ASBR summary LSAsprovide reachability information for destinations external to the OSPFAS.

AS ExternalLSA 5

Only ASBRs originate AS external LSAs. AS external LSAs provide thecost from the ASBR that originated the LSA to a destination externalto the OSPF AS.

NSSAExternal LSA 7

Normal AS external LSAs cannot be flooded not-so-stubby areas(NSSA). NSSA external LSAs are similar to AS external LSAs and canbe flooded through a stub area. NSSAs are discussed in Appendix C.

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Sequence NumbersSequence Numbers

Figure 4-13

LSA From Router XSequence Number = 99

LSDB

LSA from Router XSequence Number = 100

Router C Router D

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Link State AcknowledgementsLink State Acknowledgements

• Implicit Acknowledgement– router sends back a copy of the LSA

in an LSUpdate packet

• Explicit Acknowledgement– router sends back an LSAck packet

that contains the 20-byte header of the LSA

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Lesson 4-4: Lesson 4-4: OSPF Routing TablesOSPF Routing Tables

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OSPF MetricsOSPF Metrics

• OSPF uses a 16-bit, arbitrary metric

• Metrics usually based on bandwidth

• Metric of a route = sum of all outgoing interfaces to the destination

• OSPF provides equal-load balancing

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OSPF Path TypesOSPF Path Types

• Intra-Area Paths

• Inter-Area Paths

• E1 Paths

• E2 Paths

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Dijkstra AlgorithmDijkstra Algorithm

• Also called the Shortest Path First (SPF) algorithm

• Converts the LSDB of a router into a shortest path tree (the router is the root of the tree)

• Run twice:– first to create the internal routing table– second to create the standard routing table

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Sample NetworkSample Network

Figure 4-14

Router BRouter A

Router GRouter E

Router C

Router F

ABR ABR

Network 4 Network 3

Network 2

Network 1

Network 5 Network 6

Router HASBR

Router DASBR

Known Destinations

Netw ork 8 (5)Netw ork 9 (10)

Netw ork 10 (10)Router D/ASBR (15)

Known Destinations

Netw ork 8 (10)Netw ork 9 (5)

Netw ork 10 (15)Router D/ASBR (10) Known Destinations

Netw ork 11 (10)/E1Netw ork 12 (5)/E1Netw rok 13 (5)/E1

Known Destinations

Netw ork 12 (5)/E2Netw ork 14 (10)/E1Netw ork 15 (10)/E2

1

10

1

5

1

5

10

10

1 1

1

1

1

1

Network 7

1

Router J Router KRouter I

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Shortest Path Tree of Router J Shortest Path Tree of Router J as Determined by the First SPF as Determined by the First SPF

CalculationCalculation

Figure 4-15Router J Router KRouter I

Router HASBR

Router A Router BABR ABR

Router C

Router GRouter E Router F

Router DASBR

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Shortest Path Tree of Router J Shortest Path Tree of Router J as Determined by the Second as Determined by the Second

SPF CalculationSPF Calculation

Figure 4-16Router J Router KRouter I

Router HASBR

Router AABR

Router B Router CABR

Router GRouter E Router F

Router DASBR

Netw ork 2

Netw ork 7Netw ork 9Netw ork 8

Netw ork 10

Netw ork 11Netw ork 12Netw ork 13

Netw ork 14Netw ork 15

Netw ork 4 Netw ork 3

Netw ork 1

Netw ork 5 Netw ork 6

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Lesson 4-5: Lesson 4-5: Designing an OSPF NetworkDesigning an OSPF Network

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OSPF Routing ConceptsOSPF Routing Concepts

• Hierarchical Routing Structure– three-tiered model

• Route Summarization– summarize at area boundaries– conserve bandwidth and router

resources

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Figure 4-17

Intra-AreaRouters

Area BorderRouters

BackboneRouters

Area BorderRouters

Intra-AreaRouters

Access Routers(3 rd Tier)

DistributionRouters(2nd Tier)

Core Routers(1st Tier)

DistributionRouters(2nd Tier)

Access Routers(3 rd Tier)

Three-TieredApproach toNetw orking

OSPFEquivalent

No

mo

re t

han

six

ro

ute

r h

op

s b

etw

een

en

dp

oin

ts

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Route SummarizationRoute Summarization

Figure 4-18

ABR

172.1.1.0/24

Area 1

172.1.3.0/24

172.1.4.0/24172.1.2.0/24

172.1.5.0/24Netw ork

Summary LSA172.1.1.0/16

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Topology ConsiderationsTopology Considerations

• Minimum/Maximum Routers per OSPF Network

• Minimum/Maximum Routers per OSPF Area

• Maximum Number of OSPF Areas

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Scalability ConsiderationsScalability Considerations

• Plan for growth

• Ensure routers have the appropriate memory and processing power

• Place routers appropriately in the network

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Area Design ConsiderationsArea Design Considerations

• Designing the Backbone Area

• Designing Stub Areas– normal stub areas– not-so-stubby areas

• Avoiding Partitions and Virtual Links

• Providing for Route Summarization

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Commonly Configurable Commonly Configurable OSPF ParametersOSPF Parameters

Table 4-5

PARAMETER DESCRIPTION DEFAULT VALUE

Router Type Describes the router as an intra-area router, an ABR, or an ASBR. —

Path Type The assigned path type of external LSA forwarded by ASBRs. 1

Authentication Type

Describes the type of authentication OSPF packets will use(usually authentication type is per area)

0(None)

Area ID The ID of the area to which the interface of a router belongs. —*

Area Type Identifies the type of area to which the router interface of a router isattached (backbone, normal, NSSA, or stub).

—

InterfaceType

Identifies the type of router interface (point-to-point, point-to-multipoint, virtual link, multi-access broadcast, or multi-access non-broadcast).

—

SummaryRoutes

ABRs and ASBRs are configured with summary routing informationby which they create and flood LSAs.

—

* A router should usually not be a member of more than two areas** Usually auto-detected / auto-assigned by the router based on the type of link

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Commonly Configurable Commonly Configurable OSPF Parameters (cont.)OSPF Parameters (cont.)

Table 4-5

PARAMETER DESCRIPTION DEFAULT VALUE

Cost Specifies the cost (metric) of the interface of a router. 1**

Key Sets the key for authentication —

Priority Sets the router priority of an interface (used to determine theDR/BDR of a multi-access network).

1

Hello Interval The interval between transmission of Hello packets from a routerinterface (in seconds)

10

Delay Indicates the number of seconds required to forward an LSA acrossthe interface (this time is added to the age value of the LSA).

1

LSARetransmission

Determines the amount of time (in seconds) between standard LSAupdates.

1800

Router DeadInterval

Determines the amount of time (in seconds) a router will wait toreceive a Hello packet from a neighbor before the router considersthe neighbor dead.

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* A router should usually not be a member of more than two areas** Usually auto-detected / auto-assigned by the router based on the type of link

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Module 4 Lab ExerciseModule 4 Lab Exercise

• Designing OSPF Solutions

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Lab Exercise 4-1:Lab Exercise 4-1:Designing OSPF SolutionsDesigning OSPF Solutions

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Lab Exercise 4-1: OverviewLab Exercise 4-1: Overview

• Setup– discussion lab

• Format– answer questions as a group– share proposed solutions with class– discuss the different strengths and

weaknesses of each proposed solution

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Module 4:Module 4:Review Questions...Review Questions...

Summary...Summary...

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1. Which of the following are link state 1. Which of the following are link state

routing protocols?routing protocols?

A. Link Service Protocol

B. Open Shortest Path First (OSPF)

C. Routing Information Protocol (RIP)

D. Intermediate System to Intermediate System (IS-IS)

(Choose three.)(Choose three.)

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1. Which of the following are link state 1. Which of the following are link state

routing protocols?routing protocols?

A. Link Service Protocol

B. Open Shortest Path First (OSPF)

C. Routing Information Protocol (RIP)

D. Intermediate System to Intermediate System (IS-IS)


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2. What piece of information regarding 2. What piece of information regarding network topology is available to a link network topology is available to a link state router, but not to a distance vector state router, but not to a distance vector

router?router?

A. the network ID of all reachable destinations

B. the next hop along the path to each destination

C. the distance (or metric) from the router to the destination

D. the status of the links between the router and any router in the network

(Choose one.)(Choose one.)

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2. What piece of information regarding 2. What piece of information regarding network topology is available to a link network topology is available to a link state router, but not to a distance vector state router, but not to a distance vector

router?router?

A. the network ID of all reachable destinations

B. the next hop along the path to each destination

C. the distance (or metric) from the router to the destination

D. the status of the links between the router and any router in the network


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3. Why do link state routers require more 3. Why do link state routers require more processing power than distance vector processing power than distance vector

routers?routers? A. because link state routers use the Dijkstra algorithm

to compute paths

B. because link state routers maintain more complex routing tables than distance vector routers

C. because link state routers can make connections to more destination networks and other routers than distance vector routers

D. because link state routers utilize a flooding process that requires them to transmit information about themselves and their links to every other router in their routing domains


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3. Why do link state routers require more 3. Why do link state routers require more processing power than distance vector processing power than distance vector

routers?routers? A. because link state routers use the Dijkstra algorithm

to compute paths

B. because link state routers maintain more complex routing tables than distance vector routers

C. because link state routers can make connections to more destination networks and other routers than distance vector routers

D. because link state routers utilize a flooding process that requires them to transmit information about themselves and their links to every other router in their routing domains


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4. In terms of link state routing protocols, 4. In terms of link state routing protocols,

what are neighboring routers? what are neighboring routers?

A. routers that share a common link

B. routers located on adjacent subnets of an IP network

C. routers that communicate with low latency because of their physical proximity

D. link state routers that can communicate without routing their packets through any distance vector routers


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4. In terms of link state routing protocols, 4. In terms of link state routing protocols,

what are neighboring routers? what are neighboring routers?

A. routers that share a common link

B. routers located on adjacent subnets of an IP network

C. routers that communicate with low latency because of their physical proximity

D. link state routers that can communicate without routing their packets through any distance vector routers


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5. What feature of link state routing 5. What feature of link state routing protocols enables link state routing protocols enables link state routing domains to converge more quickly than domains to converge more quickly than

distance vector routing domains?distance vector routing domains? A. Hello packets

B. Dijkstra algorithm

C. link state database

D. link state flooding


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5. What feature of link state routing 5. What feature of link state routing protocols enables link state routing protocols enables link state routing domains to converge more quickly than domains to converge more quickly than

distance vector routing domains?distance vector routing domains? A. Hello packets

B. Dijkstra algorithm

C. link state database

D. link state flooding


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6.6. What are the advantages of the What are the advantages of the hierarchical routing structure used by hierarchical routing structure used by

link state routers?link state routers? A. reduces amount of time necessary to build

adjacencies

B. reduces load on router memory, router processors, and network bandwidth

C. reduces number of fields required in routers' link state databases (LSDBs)

D. reduces number of link state advertisements (LSAs) that must be flooded to a routing domain

(Choose two.)(Choose two.)

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6.6. What are the advantages of the What are the advantages of the hierarchical routing structure used by hierarchical routing structure used by

link state routers?link state routers? A. reduces amount of time necessary to build

adjacencies

B. reduces load on router memory, router processors, and network bandwidth

C. reduces number of fields required in routers' link state databases (LSDBs)

D. reduces number of link state advertisements (LSAs) that must be flooded to a routing domain


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7. What are the three categories of OSPF 7. What are the three categories of OSPF

design considerations?design considerations? A. topology considerations

B. reliability considerations

C. scalability considerations

D. bandwidth considerations

E. availability considerations

F. area design considerations


64

7. What are the three categories of OSPF 7. What are the three categories of OSPF

design considerations?design considerations? A. topology considerations

B. reliability considerations

C. scalability considerations

D. bandwidth considerations

E. availability considerations

F. area design considerations


65

8. What is the Internet Engineering Task 8. What is the Internet Engineering Task Force (IETF) recommendation for the Force (IETF) recommendation for the maximum number of routers in an maximum number of routers in an OSPF network?OSPF network?

A. 200

B. 700

C. 1000

D. 1200


66

8. What is the Internet Engineering Task 8. What is the Internet Engineering Task Force (IETF) recommendation for the Force (IETF) recommendation for the maximum number of routers in an maximum number of routers in an OSPF network?OSPF network?

A. 200

B. 700

C. 1000

D. 1200


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9. In an OSPF network, what is the best 9. In an OSPF network, what is the best location for routers that are relatively location for routers that are relatively

low in memory and processing power?low in memory and processing power? A. backbone

B. stub areas

C. ISP interfaces

D. subnet interfaces


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9. In an OSPF network, what is the best 9. In an OSPF network, what is the best location for routers that are relatively location for routers that are relatively

low in memory and processing power?low in memory and processing power? A. backbone

B. stub areas

C. ISP interfaces

D. subnet interfaces


69

10. Why should virtual links be reserved 10. Why should virtual links be reserved for emergencies in an OSPF network for emergencies in an OSPF network and not used as a permanent part of the and not used as a permanent part of the

network's topology?network's topology? A. Virtual links are prone to errors.

B. Virtual links require extra bandwidth.

C. Virtual links are difficult to configure.

D. Virtual links are slower than physical links.

E. Virtual links place increased loads on routers.


70

10. Why should virtual links be reserved 10. Why should virtual links be reserved for emergencies in an OSPF network for emergencies in an OSPF network and not used as a permanent part of the and not used as a permanent part of the

network's topology?network's topology? A. Virtual links are prone to errors.

B. Virtual links require extra bandwidth.

C. Virtual links are difficult to configure.

D. Virtual links are slower than physical links.

E. Virtual links place increased loads on routers.


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AssumptionsAssumptions

• You understand the You understand the distance vector algorithm.distance vector algorithm.

• You know RIP.You know RIP.– Routing tablesRouting tables– Next-hop routerNext-hop router– Convergence processConvergence process

Module 4: Implementing OSPFModule 4: Implementing OSPF

1 module 4: implementing ospf. 2 lessons ospf ospf areas and hierarchical routing ospf operation...

Documents

ospf networklesson

ospf packetfigure

ospf operationformat

exchange of hello packets

exchange of lsa packets

ospf subprotocolstable

types of ospf packetstable

exchange protocolphase