Introduction to OSPF - wiki.apnictraining.net · Introduction to OSPF ISP Training Workshops 1. ... pThere is ONE designated router per multi- ... LSA for one specific OSPF area type)

Introduction to OSPFISP Training Workshops

1

OSPFp Open Shortest Path

Firstp Link state or SPF

technologyp Developed by OSPF

working group of IETF (RFC 1247)

p OSPFv2 standard described in RFC2328

p Designed for:n TCP/IP environmentn Fast convergencen Variable-length subnet

masksn Discontiguous subnetsn Incremental updatesn Route authentication

p Runs on IP, Protocol 89

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Link State

3

Topology Information is kept in a Database separate from the Routing Table

ABC

21313

QZX

Z

X

YQ

Z’s Link StateQ’s Link State

X’s Link State

Link State Routingp Neighbour discoveryp Constructing a Link State Packet (LSP)p Distribute the LSP

n (Link State Announcement – LSA)

p Compute routesp On network failure

n New LSPs floodedn All routers recompute routing table

4

Low Bandwidth Utilisation

p Only changes propagatedp Uses multicast on multi-access broadcast

networks5

LSA

X

LSA

R1

Fast Convergencep Detection Plus LSA/SPF

n Known as the Dijkstra Algorithm

6

X N2

Alternate Path

Primary Path

N1

R2

R1 R3

Fast Convergencep Finding a new

routen LSA flooded

throughout arean Acknowledgement

basedn Topology database

synchronisedn Each router derives

routing table to destination network

7

LSA

N1R1 X

OSPF Areasp Area is a group of

contiguous hosts and networksn Reduces routing

trafficp Per area topology

databasen Invisible outside the

areap Backbone area

MUST be contiguousn All other areas must

be connected to the backbone

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Area 1

Area 2 Area 3

R1 R2

R3R6

Area 4

R5 R4R7R8

RaRd

RbRcArea 0

Backbone Area

Virtual Links between OSPF Areas

p Virtual Link is used when it is not possible to physically connect the area to the backbone

p ISPs avoid designs which require virtual linksn Increases complexityn Decreases reliability and

scalability

9

Area 1R3

R6

Area 4R5 R4

R7R8

RaRd

RbRcArea 0

Backbone Area

Classification of Routers

p Internal Router (IR)p Area Border Router (ABR)p Backbone Router (BR)p Autonomous System

Border Router (ASBR)

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R1 R2

R3

R5 R4

Rd Ra

RbRc

IR

ABR/BR

IR/BRASBR

To other AS

IR

Area 1

Area 0

Area 2 Area 3

OSPF Route Types

p Intra-area Routen all routes inside an area

p Inter-area Routen routes advertised from

one area to another by an Area Border Router

p External Routen routes imported into

OSPF from other protocol or static routes

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R1 R2

R3

R5 R4

Rd Ra

RbRc

IR

ABR/BR

ASBR

To other AS

IR

Area 1

Area 0

Area 2 Area 3

External Routesp Prefixes which are redistributed into OSPF from

other protocolsp Flooded unaltered throughout the AS

n Recommendation: Avoid redistribution!!p OSPF supports two types of external metrics

n Type 1 external metricsn Type 2 external metrics (Cisco IOS default)

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RIPEIGRPBGPStaticConnectedetc.

OSPF

RedistributeR2

External Routesp Type 1 external metric: metrics are added

to the summarised internal link cost

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NetworkN1N1

Type 11110

Next HopR2R3

Cost = 10to N1

External Cost = 1

to N1 External Cost = 2Cost = 8

Selected Route

R3

R1

R2

External Routesp Type 2 external metric: metrics are

compared without adding to the internal link cost

14

Cost = 10to N1

External Cost = 1

to N1 External Cost = 2Cost = 8

Selected Route

R3

R1

R2

NetworkN1N1

Type 112

Next HopR2R3

Topology/Link State Databasep A router has a separate LS database for each

area to which it belongsp All routers belonging to the same area have

identical databasep SPF calculation is performed separately for each

areap LSA flooding is bounded by areap Recommendation:

n Limit the number of areas a router participates in!!n 1 to 3 is fine (typical ISP design)n >3 can overload the CPU depending on the area

topology complexity

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The Hello Protocolp Responsible for

establishing and maintaining neighbour relationships

p Elects designated router on multi-access networks

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Hello

HelloHello

The Hello Packetp Contains:

n Router priorityn Hello interval n Router dead

intervaln Network maskn List of neighboursn DR and BDRn Options: E-bit,

MC-bit,… (see A.2 of RFC2328)

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Hello

HelloHello

Designated Routerp There is ONE designated router per multi-

access networkn Generates network link advertisementsn Assists in database synchronization

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Designated Router

Designated Router

BackupDesignated Router

BackupDesignated

Router

Designated Router by Priorityp Configured priority (per interface)

n ISPs configure high priority on the routers they want as DR/BDR

p Else determined by highest router IDn Router ID is 32 bit integern Derived from the loopback interface address, if

configured, otherwise the highest IP address

19144.254.3.5

R2 Router ID = 131.108.3.3

131.108.3.2 131.108.3.3

R1 Router ID = 144.254.3.5

DR R2R1

Neighbouring Statesp Full

n Routers are fully adjacentn Databases synchronisedn Relationship to DR and BDR

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FullDR BDR

Neighbouring Statesp 2-way

n Router sees itself in other Hello packetsn DR selected from neighbours in state 2-way or

greater

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2-way

DR BDR

When to Become Adjacentp Underlying network is point to pointp Underlying network type is virtual linkp The router itself is the designated router

or the backup designated routerp The neighbouring router is the designated

router or the backup designated router

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LSAs Propagate Along Adjacencies

p LSAs acknowledged along adjacencies

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DR BDR

Broadcast Networksp IP Multicast used for Sending and

Receiving Updatesn All routers must accept packets sent to

AllSPFRouters (224.0.0.5)n All DR and BDR routers must accept packets

sent to AllDRouters (224.0.0.6)p Hello packets sent to AllSPFRouters

(Unicast on point-to-point and virtual links)

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Routing Protocol Packetsp Share a common protocol headerp Routing protocol packets are sent with type of

service (TOS) of 0p Five types of OSPF routing protocol packets

n Hello – packet type 1n Database description – packet type 2n Link-state request – packet type 3n Link-state update – packet type 4n Link-state acknowledgement – packet type 5

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Different Types of LSAsp Six distinct type of LSAs

n Type 1 : Router LSAn Type 2 : Network LSAn Type 3 & 4: Summary LSAn Type 5 & 7: External LSA (Type 7 is for NSSA)n Type 6: Group membership LSAn Type 9, 10 & 11: Opaque LSA (9: Link-Local, 10: Area)

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Router LSA (Type 1)p Describes the state and cost of the

router’s links to the areap All of the router’s links in an area must be

described in a single LSAp Flooded throughout the particular area

and no morep Router indicates whether it is an ASBR,

ABR, or end point of virtual link

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Network LSA (Type 2)p Generated for every transit broadcast and

NBMA networkp Describes all the routers attached to the

networkp Only the designated router originates this

LSAp Flooded throughout the area and no more

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Summary LSA (Type 3 and 4)p Describes the destination outside the area

but still in the ASp Flooded throughout a single areap Originated by an ABRp Only inter-area routes are advertised into

the backbonep Type 4 is the information about the ASBR

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External LSA (Type 5 and 7)p Defines routes to destination external to

the ASp Default route is also sent as externalp Two types of external LSA:

n E1: Consider the total cost up to the external destination

n E2: Considers only the cost of the outgoing interface to the external destination

p (Type 7 LSAs used to describe external LSA for one specific OSPF area type)

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Inter-Area Route Summarisationp Prefix or all subnetsp Prefix or all networksp ‘Area range’ command

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1.A 1.B 1.C

(ABR)Network1

Next HopR1

Network1.A1.B1.C

Next HopR1R1R1

With summarisation

Withoutsummarisation

BackboneArea 0

Area 1R1

R2

No Summarisationp Specific Link LSA advertised out of each areap Link state changes propagated out of each area

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3.A3.B

3.C 3.D2.A2.B

2.C 2.D

1.A1.B

1.C 1.D

1.A1.B1.C1.D Area 0

2.A2.B2.C2.D

3.A3.B3.C3.D

With Summarisationp Only summary LSA advertised out of each areap Link state changes do not propagate out of the area

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3.A3.B

3.C 3.D2.A2.B

2.C 2.D

1.A1.B

1.C 1.D

1Area 0

2

3

No Summarisationp Specific Link LSA advertised in to each areap Link state changes propagated in to each area

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3.A3.B

3.C 3.D2.A2.B

2.C 2.D

1.A1.B

1.C 1.D

2.A 2.B2.C 2.D3.A 3.B3.C 3.D Area 0

1.A 1.B1.C 1.D3.A 3.B3.C 3.D

1.A 1.B1.C 1.D2.A 2.B2.C 2.D

With Summarisationp Only summary link LSA advertised in to each areap Link state changes do not propagate in to each area

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3.A3.B

3.C 3.D2.A2.B

2.C 2.D

1.A1.B

1.C 1.D

23 Area 0

13

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Types of Areas

p Regularp Stubp Totally Stubbyp Not-So-Stubbyp Only “regular” areas are useful for ISPs

n Other area types handle redistribution of other routing protocols into OSPF – ISPs don’t redistribute anything into OSPF

p The next slides describing the different area types are provided for information only

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Regular Area (Not a Stub)p From Area 1’s point of view, summary networks from other

areas are injected, as are external networks such as X.1

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3.A3.B

3.C 3.D2.A2.B

2.C 2.D

1.A1.B

1.C 1.D

23 Area 0

13

12

ASBRExternal networks

X.1

X.1

X.1

X.1

X.1

X.1

X.1

Normal Stub Areap Summary networks, default route injectedp Command is area x stub

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3.A3.B

3.C 3.D2.A2.B

2.C 2.D

1.A1.B

1.C 1.D

23 Area 0

13

12


X.1

X.1

Default

X.1

X.1

Default

Default

Totally Stubby Areap Only a default route injected

n Default path to closest area border routerp Command is area x stub no-summary

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3.A3.B

3.C 3.D2.A2.B

2.C 2.D

1.A1.B

1.C 1.D

Area 01 3

1 2


X.1

X.1

Default

X.1

X.1

Default

DefaultTotally Stubby Area

Not-So-Stubby Areap Capable of importing routes in a limited fashionp Type-7 LSA’s carry external information within an NSSAp NSSA Border routers translate selected type-7 LSAs into type-5 external

network LSAs

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3.A3.B

3.C 3.D2.A2.B

2.C 2.D

1.A1.B

1.C 1.D

Area 01 3

1 2


X.1

X.1

Default

X.1

X.1

Default X.2

Default X.2

Not-So-Stubby Area

External networks

X.2

X.2

X.2

ISP Use of Areasp ISP networks use:

n Backbone arean Regular area

p Backbone arean No partitioning

p Regular arean Summarisation of point to point link addresses used

within areasn Loopback addresses allowed out of regular areas without

summarisation (otherwise iBGP won’t work)

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Addressing for Areas

p Assign contiguous ranges of subnets per area to facilitate summarisation

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Area 1network 192.168.1.64range 255.255.255.192




Summaryp Fundamentals of Scalable OSPF Network

Designn Area hierarchyn DR/BDR selectionn Contiguous intra-area addressingn Route summarisationn Infrastructure prefixes only

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Introduction to OSPFISP Training Workshops

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Introduction to OSPF - wiki.apnictraining.net · Introduction to OSPF ISP Training Workshops 1. ... pThere is ONE designated router per multi- ... LSA for one specific OSPF area type)