Presentation Ospf Ranjeetv0.2

1

Introduction to

OSPF Protocol Design

By:

Ranjeet SainiEngineering Test LeadAgnity Technologies, Inc.

March, 2008

2

Agenda

I. References & Standard

II. Terminology

III. OSPF Format

IV. OSPF Algorithm

3

References & Standard

1. RFC 2328 OSPF Version 2 April, 1998by John Moy

2. RFC 2370 The OSPF Opaque LSA Option April, 1998 by R. Coltun

3. Understanding TCP/IP December, 1995SynOptics Communications

4. RFC 1349 Type of Service in the Internet Protocol July, 1992

4


II. Terminology

III. OSPF Format

IV. OSPF Algorithm

5

Internet Routing

Fast SwitcHub-8mi30+201051100MTx/Rx

Full DuplexSelect/Link

PWR

Collision

StatusUtil%

Forward%

Filter%Demo

Diag

Full/Half

Config

1 2 3 4 5 6 7 8

90+705035201051Link

Rate %

SNMP

1X 2X 3X 4X 5X 6X 7X 8MDI-X-or-8MDI

10 M/ 100 M



PWR

Collision

StatusUtil%

Forward%

Filter%Demo

Diag

Full/Half

Config

1 2 3 4 5 6 7 8

90+705035201051Link

Rate %

SNMP


10 M/ 100 M



PWR

Collision

StatusUtil%

Forward%

Filter%Demo

Diag

Full/Half

Config

1 2 3 4 5 6 7 8

90+705035201051Link

Rate %

SNMP


10 M/ 100 M



PWR

Collision

StatusUtil%

Forward%

Filter%Demo

Diag

Full/Half

Config

1 2 3 4 5 6 7 8

90+705035201051Link

Rate %

SNMP


10 M/ 100 M

R1 R2

R3Fast SwitcHub-8mi

30+201051100MTx/Rx


PWR

Collision

StatusUtil%

Forward%

Filter%Demo

Diag

Full/Half

Config

1 2 3 4 5 6 7 8

90+705035201051Link

Rate %

SNMP


10 M/ 100 M



PWR

Collision

StatusUtil%

Forward%

Filter%Demo

Diag

Full/Half

Config

1 2 3 4 5 6 7 8

90+705035201051Link

Rate %

SNMP


10 M/ 100 M



PWR

Collision

StatusUtil%

Forward%

Filter%Demo

Diag

Full/Half

Config

1 2 3 4 5 6 7 8

90+705035201051Link

Rate %

SNMP


10 M/ 100 M



PWR

Collision

StatusUtil%

Forward%

Filter%Demo

Diag

Full/Half

Config

1 2 3 4 5 6 7 8

90+705035201051Link

Rate %

SNMP


10 M/ 100 M

R4 R5

R6



PWR

Collision

StatusUtil%

Forward%

Filter%Demo

Diag

Full/Half

Config

1 2 3 4 5 6 7 8

90+705035201051Link

Rate %

SNMP


10 M/ 100 M



PWR

Collision

StatusUtil%

Forward%

Filter%Demo

Diag

Full/Half

Config

1 2 3 4 5 6 7 8

90+705035201051Link

Rate %

SNMP


10 M/ 100 M



PWR

Collision

StatusUtil%

Forward%

Filter%Demo

Diag

Full/Half

Config

1 2 3 4 5 6 7 8

90+705035201051Link

Rate %

SNMP


10 M/ 100 M



PWR

Collision

StatusUtil%

Forward%

Filter%Demo

Diag

Full/Half

Config

1 2 3 4 5 6 7 8

90+705035201051Link

Rate %

SNMP


10 M/ 100 M

R7 R8

R9

Core System

Autonomous System #1 Autonomous System #3

Autonomous System #2

Area Border Router

Area Border Router

Area Border Router

6



PWR

Collision

StatusUtil%

Forward%

Filter%Demo

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Full/Half

Config

1 2 3 4 5 6 7 8

90+705035201051Link

Rate %

SNMP


10 M/ 100 M



PWR

Collision

StatusUtil%

Forward%

Filter%Demo

Diag

Full/Half

Config

1 2 3 4 5 6 7 8

90+705035201051Link

Rate %

SNMP


10 M/ 100 M



PWR

Collision

StatusUtil%

Forward%

Filter%Demo

Diag

Full/Half

Config

1 2 3 4 5 6 7 8

90+705035201051Link

Rate %

SNMP


10 M/ 100 M



PWR

Collision

StatusUtil%

Forward%

Filter%Demo

Diag

Full/Half

Config

1 2 3 4 5 6 7 8

90+705035201051Link

Rate %

SNMP


10 M/ 100 M

IGP1 IGP1

IGP1

Autonomous System1



PWR

Collision

StatusUtil%

Forward%

Filter%Demo

Diag

Full/Half

Config

1 2 3 4 5 6 7 8

90+705035201051Link

Rate %

SNMP


10 M/ 100 M



PWR

Collision

StatusUtil%

Forward%

Filter%Demo

Diag

Full/Half

Config

1 2 3 4 5 6 7 8

90+705035201051Link

Rate %

SNMP


10 M/ 100 M



PWR

Collision

StatusUtil%

Forward%

Filter%Demo

Diag

Full/Half

Config

1 2 3 4 5 6 7 8

90+705035201051Link

Rate %

SNMP


10 M/ 100 M



PWR

Collision

StatusUtil%

Forward%

Filter%Demo

Diag

Full/Half

Config

1 2 3 4 5 6 7 8

90+705035201051Link

Rate %

SNMP


10 M/ 100 M

IGP2

Autonomous System #2

IGP1

IGP2 IGP2

IGP2

EGP

Exterior Gateway Protocol

An External Links Advertisement originates for each destination outsideThe AS, and these advertisements are in turn flooded throughout the AS.

An External Links Advertisement is used for externally derived routingInformation obtained by another routing protocol such as EGP or BGP.

7

Autonomous Systems(AS) Each AS is a group of networks & routers administered by single authority using a common routing protocol.

Interior Gateway Protocol(IGP) Routers within single AS communicate using one of several dynamic routing protocols, known generically as an IGP.

Exterior Gateway Protocols(EGP) Communication between routers belonging to different AS requires additional protocol, so-called EGP.

Open Shortest Path First(OSPF) is an Interior Gateway Protocol(IGP) IP routing protocol.

Terminology

8

Configurable Metrics



PWR

Collision

StatusUtil%

Forward%

Filter%Demo

Diag

Full/Half

Config

1 2 3 4 5 6 7 8

90+705035201051Link

Rate %

SNMP

14X 5X 6X 7X 8MDI-X-or-8MDI

10 M/ 100 M



PWR

Collision

StatusUtil%

Forward%

Filter%Demo

Diag

Full/Half

Config

1 2 3 4 5 6 7 8

90+705035201051Link

Rate %

SNMP


10 M/ 100 M



PWR

Collision

StatusUtil%

Forward%

Filter%Demo

Diag

Full/Half

Config

1 2 3 4 5 6 7 8

90+705035201051Link

Rate %

SNMP


10 M/ 100 M

Hop = 1Cost= 10

Hop = 1Cost= 10

Hop = 1Cost= 100

A B

T1 T1

56Kb

Using RIP, traffic would be routed over slow 56K link since the hop countMetric is 1. Using OSPF as the IGP, traffic can be routed over faster T-1Link since total cost would be 20.

For single destination, there may be separate routing table entries for eachType Of Service. A metric for TOS 0 must always be specified.

9

Network Types

OSPF operates over below physical networks :

1) Point-to-point Network A network joining single pair of routers.

2) Broadcast Network A network with more than 2 attached routers, and the ability to address single physical messages to all of attached routers.

3) Non-broadcast Network A network with more than 2 attached router, but having nobroadcast capability such as X.25 public data network.

DR

BDR

10

Adjacencies

Fast SwitcHub-8mi30+20105

1100MTx/Rx


PWR

Collision

StatusUtil%

Forward%

Filter%Demo

Diag

Full/Half

Config

1 2 3 4 5 6 7 8

90+70503520

1051Link

Rate %

SNMP


10 M/ 100 M


1100MTx/Rx


PWR

Collision

StatusUtil%

Forward%

Filter%Demo

Diag

Full/Half

Config

1 2 3 4 5 6 7 8

90+70503520

1051Link

Rate %

SNMP


10 M/ 100 M


1100MTx/Rx


PWR

Collision

StatusUtil%

Forward%

Filter%Demo

Diag

Full/Half

Config

1 2 3 4 5 6 7 8

90+70503520

1051Link

Rate %

SNMP


10 M/ 100 M

Adjacency

Router Router

Router

An “adjacency” is a two-way communication between selected neighboring routers for the purpose of exchangingrouting information through link-state advertisements.

11



PWR

Collision

StatusUtil%

Forward%

Filter%Demo

Diag

Full/HalfConfig

1 2 3 4 5 6 7 8

90+705035201051Link

Rate %

SNMP

1X 2X 3X 4X 5X 6X 7X 8MDI-X-or-MDI

10 M/ 100 M


1100MTx/Rx


PWR

Collision

StatusUtil%

Forward%

Filter%

Demo

Diag

Full/HalfConfig

1 2 3 4 5 6 7 8

90+70503520

1051Link

Rate %

SNMP


10 M/ 100 M



PWR

Collision

StatusUtil%

Forward%

Filter%Demo

Diag

Full/HalfConfig

1 2 3 4 5 6 7 8

90+705035201051Link

Rate %

SNMP


10 M/ 100 M



PWR

Collision

StatusUtil%

Forward%

Filter%Demo

Diag

Full/HalfConfig

1 2 3 4 5 6 7 890+705035201051Link

Rate %

SNMP


10 M/ 100 M



PWR

Collision

StatusUtil%

Forward%

Filter%Demo

Diag

Full/HalfConfig

1 2 3 4 5 6 7 8

90+705035201051Link

Rate %

SNMP


10 M/ 100 M

N6

N7

N8

Router

Router

N9

N10

Router

Router

RouterArea 1 Area 2

AS 300

AS 100

AS 200

Area Border Router (ABR)

N1

N2

N3

N4

N5

Note : Each area has its own topological link-state database. All routers within an area contain Router LSAs & Network LSAs. Router is Autonomous System Boundary Router(ASBR).

Routing Area

12

Routing Area(Con’t)

OSPF allows collections of contiguous networks and hosts to be groupedtogether. This group, along with the routers having interfaces connectedto the networks in this group, is called an “area”.

A single area limits the boundary for Link-Stat Advertisement(LSA) flooding.the Shortest Path First(SPF) tree is computed on a per-area basis, and anyintra-area destinations are derived from the SPF tree.

All areas must have at least one route/interface connected to area 0.0.0.0(the backbone area).

An Area Border Router(ABR) is an OSPF router having interface connectedto multiple areas. ABRs must keep a distinct link-state database for eacharea, and run the SPF algorithm on distinct database.

13

Routing Area- Backbone Area

ABR

ABR

ABR

Area-1

Area-2

Area-3

Backbone Area

If an AS is divided into areas, the areas must be connected to each otherVia special area called the “Backbone Area”. Backbone consists of thoseNetworks not contained in any area.

All ABRs in an AS must belong to the Backbone Area. Backbone area isassigned an area_ID to 0.0.0.0

AS 300

14

Backbone Area(Con’t)

An ABR connected to the Backbone executes two copies of OSPF protocol:

1) Operates on the interface connected to local area and accept floodedadvertisements from other routers that are members of the area

2) Executes over the interface that connects to the backbone. This secondcopy won’t propagate flooded advertisements from the area acrossthe backbone. Instead, it sends Summary Link Advertisements overthe backbone so that attached area can learn about backbone reachability.

15

Stub Area

When an OSPF area within an AS has a single entry/exit router thatis used by all externally addressed traffic, it is possible to block theimport of the AS External Link Advertisements into the area :

~ No LSA type 4 & 5’s ~ ASBRs are not supported with stub area ~ Virtual links are not supported in stub area ~ The ABR must be configured as default router for stub area ~ The ASBR can be configured to be disabled for an area

Area0.0.0.0Area

0.0.0.0Stub

Area 51Stub

Area 51

OSPF Domain

AS 101RIP or

IGP

AS 101RIP or

IGP

ASBR 0.0.0.0 Default

16

Virtual Link

Router

ABRABR

Router

Router Router

ABR

ABR

AS 200

Backbone

Area 1 Area2

Area 3

Area 4

A virtual link can be configured to allow the connection of an ABR to backboneWhen the ABR and its are aren’t contiguous to the backbone.

In below figure, Area 1 cannot directly learn all inter-area from the other areasSince it lacks a direct connection to the backbone. Area 1 is connected to theBackbone by a virtual link between the ABRs in Area 2. All inter-area routes From the backbone are flooded over virtual link to the ABR in Area 1. The ABRIn Area 1 will summarize all intra-area routes for Area 1 over virtual link forTransmission on the backbone.

17

Virtual Link(Con’t)

Area0.0.0.7Area

0.0.0.7ABR ABR

Area 0.0.0.0Area

0.0.0.0Area

0.0.0.51Area

0.0.0.51

Virtual Link

Any physical arrangement of areas can be logically attached to the Backbone through a virtual link.

Virtual links allow summary-LSAs to be tunneled across a non-Backbone area to exchange the routing information.

18

Virtual Link(Con’t)

Area0.0.0.7Area

0.0.0.7Area

0.0.0.0Area

0.0.0.0

Area 51Area 51C

B

Area 52Area 52A

DData

DataDa

taVirtual link

Virtual link

The exchange of routing information continues to follow viavirtual link but the forwarding of data packets does not.

A data packets from router C destine for router D would gothrough routers A & B, but not through Area 0.

19

Routing Area greatly reduce the amount of routing informationtraffic that must be propagated throughout entire AS

Areas allow the development of a hierarchy of routing information,thus protecting each area from external routing information.

The area’s information is hidden from routers outside of the area.This “information hiding” technique is important from a security standpoint, since it prohibits other areas from identifying the physical topology of an area.

Area Routing Advantages

20


II. Terminology

III. OSPF Format

IV. OSPF Algorithm

21

0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7

VERS LEN Type Of Service Total Length

Identification Fragment OffsetFlags

Time To live = 1

Protocol= 89 Header Checksum

Source IP Address

Destination IP Address

Option Padding

D A T A

0

4

8

C

E

10

14

Total: 20 bytes

IPv4 Format

22

OSPF Assigned Class D Address

Multicast Class D addresses assigned for OSPF :

224.0.0.5 All OSPF routers must be capable of transmitting & receiving packets with this destination IP address

224.0.0.6 All OSPF Designated Routers must be capable of receiving packets with this destination address. This includes the Backup Designated Router.

IP Address MAC

224.0.0.5 01005E-000005

224.0.0.6 01005E-000006

Note : OSPF multicast addresses are used on both point-to-point links & multi-access networks, but does not use over non-broadcast networks or virtual links. To ensure that multicast OSPF messages won’t travel multiple hops, their IP TTL must be set to 1.

23

OSPF Packet Header

0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7

Version No. Type Packet Length

AuTypeHeader Checksum

Area ID

Authentication

Router ID

0

4

8

C

10

14

Type Description

1 Hello 2 Database Description 3 Link State Request 4 Link State Update 5 Link State Acknowledgment

OSPF Packet Header = 24 bytes

24

Network Mask

HelloInterval Option Rtr Pri

RouterDeadInterval

Designated Router

Backup Designated Router Neighbor :

:

Hello

Entry 1(24 bytes)

Entry n

Version No.Type= 1

Packet Length

AuTypeChecksum

Area ID

Authentication

Router ID

Network Mask

HolloInterval Option Rtr Pri

RouterDeadInterval

Designated Router

Backup Designated Router

Neighbor

D A T A

OSPF Hello Packet

Note: DR/BDR = 0 means no designated router

25

The router’s “Router Priority” used to determine the DesignatedRouter & Backup Designated Router.

The hello interval in which the transmitting router sends Hellopackets on given network.

The interval(in seconds) in which the transmitting router expectsto receive Hello packets from a neighbor before determining that the neighbor is down.

A list of routers from which Hello packets have been recentlyreceived.

The router’s current choice fro the Designated Router & BackupDesignated Router. A value of zero in these fields indicates thatone has not yet been selected.

Hello Message Contents

26

The OSPF Optional Capability

7 6 5 4 3 2 1 0- - DC EA N/P MC E -

AS-external-LSAs are flooded

Whether IP multicast datagrams are forwarded

The handling of Type-7 LSAs

The router’s willingness to receive & forwardExternal-attributes-LSAs

The router’s handling of demand circuits

27



PWR

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Config

1 2 3 4 5 6 7 8

90+705035201051Link

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SNMP


10 M/ 100 M

X 2X 3X

Router

Router Fast SwitcHub-8mi30+201051100MTx/Rx


PWR

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StatusUtil%

Forward%

Filter%Demo

Diag

Full/Half

Config

1 2 3 4 5 6 7 8

90+705035201051Link

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SNMP


10 M/ 100 M



PWR

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StatusUtil%

Forward%

Filter%Demo

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Full/Half

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1 2 3 4 5 6 7 8

90+705035201051Link

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10 M/ 100 M



PWR

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Filter%Demo

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Full/Half

Config

1 2 3 4 5 6 7 8

90+705035201051Link

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SNMP


10 M/ 100 M



PWR

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StatusUtil%

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Full/Half

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1 2 3 4 5 6 7 8

90+705035201051Link

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SNMP


10 M/ 100 M



PWR

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StatusUtil%

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Full/Half

Config

1 2 3 4 5 6 7 8

90+705035201051Link

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SNMP


10 M/ 100 M



PWR

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Diag

Full/Half

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1 2 3 4 5 6 7 8

90+705035201051Link

Rate %

SNMP


10 M/ 100 M

Router

Router

RouterDR

BDR

Multi-access Network

Each router on the network exchanges link-state information(forms an adjacency)Only with the Designated Router(DR) and the Backup Designated Router(BDR).Each neighbor exchanged information with a DR & BDR specified, the number ofExchanges is reduced to O(2n).

The router with the highest configured Router Priority is elected DR.The BDR is elected at the same time as the DR. The router with second highestRouter Priority is elected the BDR.

Designated Router & Backup Designated Router

28

Version No.Type= 2

Packet Length

AuTypeChecksum

Area ID

Authentication

Router ID

D A T A

OSPF Database Description Packet

DD Sequence number

Interface MTU Option 0 0 0 0 0 I M MS

:::

An LSA HeaderAuthentication

0

4

8

C

10

14

18

29

Link State Request Packet

Version No.Type= 3

Packet Length

AuTypeChecksum

Area ID

Authentication

Router ID

D A T A

Advertising Router

LS Type

Link State ID

:::

0

4

8

30

Link State Update Packet

Version No.Type= 24

Packet Length

AuTypeChecksum

Area ID

Authentication

Router ID

D A T A

# LSA

:::

LSAs

0

4

8

C

10

14

18

31

Link State Acknowledgment Packet

Version No.Type= 5

Packet Length

AuTypeChecksum

Area ID

Authentication

Router ID

D A T A :::

An LSA Header

0

4

8

C

10

14

18

32

IP-172.16.32.2RID-100.100.100.6

IP-172.16.32.1RID-100.100.100.4

Hello (Packet 15 & 17)

Hello (Packet 16 & 18)

DB Description (Packet 19 & 21)

DB Description (Packet 20)

Link State Request (Packet 22)

Link State Update (Packet 25)

(Adjacency Up)

Hello

Hello

Down DownInit

2-WayExStart ExStart

Exchange Exchange

Loading Loading

Full Full

Exchange

Full

Forming Adjacency

33

OSPF LSA Header

0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7

LS age Option LS type

LengthLS Checksum

Advertising Router

LS sequence number

Link State ID

0

4

8

C

10

Type Description

1 Router-LSAs 2 Network-LSAs 3 Summary-LSAs (IP network) 4 Summary-LSAs (ASBR) 5 AS-external-LSAsOSPF LSA Header = 20 bytes

34

Router-LSAs Format

LS age LS Type= 1Options

LengthChecksum

Advertising Router

LS sequence number

Link State ID

Link Data

Type # TOS Metric

Link State ID

0 V E B 0 No. of Links

:

:

D A T A

0

4

8

C

10

14

18Note: V for virtual link; E for AS boundary router B for Area Border router. Each router in an area originates a router-LSA.

35

Router-LSAs Format(Con’t)

Type Link ID Link Data

1 Neighboring router’s Router ID Interface’s MIB ifIndex value

2 IP address of Designated Router DR IP address

3 IP network / subnet number Network IP address

4 Neighboring router’s Router ID Network IP address

Type Description

1 Point-to-point connection to another router

2 Connection to a transit network

3 Connection to a stub network

4 Virtual link

36

LS age LS Type=2Options

LengthChecksum

Advertising Router

LS sequence number

Link State ID

Network-LSAs Format

Network Mask

Attached Router

0

4

8

C

10

14

18

Note: the distance from network to all attached routers is 0. Network-LSA is originated by the network’s DR.

37


LengthChecksum

Advertising Router

LS sequence number

Link State ID

D A T A

Summary-LSAs(IP network) Format

TOS TOS metric

Network Mask

0 metric

:::

Note: Link State ID= 0.0.0.0 & Network mask= 0.0.0.0 if default summary route. Metric is the cost of this route.

0

4

8

38


LengthChecksum

Advertising Router

LS sequence number

Link State ID

D A T A

Summary-LSAs(ASBR) Format

TOS TOS metric

Network Mask

0 metric

:::

0

4

8

39

LS age LS Type= 5Options

LengthChecksum

Advertising Router

LS sequence number

Link State ID

D A T A

As-external-LSAs Format

Forwarding address

Network Mask

E 0 metric

:::

External Route Tag

Forwarding address

E TOS TOS metric

0

4

8

C

10

14

40

Shortest Path Tree


1100MTx/Rx


PWR

Collision

StatusUtil%

Forward%

Filter%Demo

Diag

Full/Half

Config

1 2 3 4 5 6 7 8

90+70503520

1051Link

Rate %

SNMP


10 M/ 100 M


1100MTx/Rx

Full Duplex

Select/Link

PWR

Collision

StatusUtil%

Forward%

Filter%Demo

Diag

Full/Half

Config

1 2 3 4 5 6 7 8

90+70503520

1051Link

Rate %

SNMP


10 M/ 100 M


1100MTx/Rx

Full Duplex

Select/Link

PWR

Collision

StatusUtil%

Forward%

Filter%Demo

Diag

Full/Half

Config

1 2 3 4 5 6 7 8

90+70503520

1051Link

Rate %

SNMP


10 M/ 100 M

Router3

Router2

Router1

Network2

Network3

Network4

Network1

Cost = 30

Cost = 20

41

Link State Database

R1 R2 R3

(N1-0) (N2-0) (N3-0)

(N2-0) (N3-0) (N4-0)

(R2-20) (R1-20) (R2-30) (R3-30)

From the Link-State Database, each router builds a shortest path treeusing itself as the root.

Each node of the tree shows the shortest, or best cost path tothe vertex from the root.

Each router then build its routing table from the shortest path tree.

42

R1

N1 N2R2

20

N3

R3

N4

30

Destination Next Hop Metric

N1 Direct 0

N2 Direct 0

N3 R2 20

N4 R2 50


1100MTx/Rx


PWR

Collision

StatusUtil%

Forward%

Filter%Demo

Diag

Full/Half

Config

1 2 3 4 5 6 7 8

90+70503520

1051Link

Rate %

SNMP


10 M/ 100 M


1100MTx/Rx


PWR

Collision

StatusUtil%

Forward%

Filter%Demo

Diag

Full/Half

Config

1 2 3 4 5 6 7 8

90+70503520

1051Link

Rate %

SNMP


10 M/ 100 M


1100MTx/Rx

Full Duplex

Select/Link

PWR

Collision

StatusUtil%

Forward%

Filter%Demo

Diag

Full/Half

Config

1 2 3 4 5 6 7 8

90+70503520

1051Link

Rate %

SNMP


10 M/ 100 M

Network1

Network2

Network3

Network4

Router1

Router2

Router3

Cost = 20

Cost = 30Routing Table for R1

Shortest Path Tree & Routing Table

43


N1 R1 20

N2 Direct 0

N3 Direct 0

N4 R3 30


1100MTx/Rx


PWR

Collision

StatusUtil%

Forward%

Filter%Demo

Diag

Full/Half

Config

1 2 3 4 5 6 7 8

90+70503520

1051Link

Rate %

SNMP


10 M/ 100 M


1100MTx/Rx


PWR

Collision

StatusUtil%

Forward%

Filter%Demo

Diag

Full/Half

Config

1 2 3 4 5 6 7 8

90+70503520

1051Link

Rate %

SNMP


10 M/ 100 M


1100MTx/Rx

Full Duplex

Select/Link

PWR

Collision

StatusUtil%

Forward%

Filter%Demo

Diag

Full/Half

Config

1 2 3 4 5 6 7 8

90+70503520

1051Link

Rate %

SNMP


10 M/ 100 M

Network1

Network2

Network3

Network4

Router1

Router2

Router3

Cost = 20


Shortest Path Tree & Routing Table(Con’t)

R2

N1

20

R1R3

N4

30

N2N3

44


N1 R2 50

N2 R2 30

N3 Direct 0

N4 Direct 0


1100MTx/Rx


PWR

Collision

StatusUtil%

Forward%

Filter%Demo

Diag

Full/Half

Config

1 2 3 4 5 6 7 8

90+70503520

1051Link

Rate %

SNMP


10 M/ 100 M


1100MTx/Rx


PWR

Collision

StatusUtil%

Forward%

Filter%Demo

Diag

Full/Half

Config

1 2 3 4 5 6 7 8

90+70503520

1051Link

Rate %

SNMP


10 M/ 100 M


1100MTx/Rx

Full Duplex

Select/Link

PWR

Collision

StatusUtil%

Forward%

Filter%Demo

Diag

Full/Half

Config

1 2 3 4 5 6 7 8

90+70503520

1051Link

Rate %

SNMP


10 M/ 100 M

Network1

Network2

Network3

Network4

Router1

Router2

Router3

Cost = 20


Shortest Path Tree & Routing Table(Con’t)

N3

N1

N2

N4

R3

30

R1

R2

20

45

• When an intra-area route has been added, deleted or modified

• When an inter-area route has been added, deleted or modified

• When a router has an interface that becomes active in an area

• When a router’s virtual link changes

• When an external route changes

• When a router that was an ASBR is no longer an ASBR

When Router re-calculate SPF tree?

46


II. Terminology

III. OSPF Format

IV. OSPF Algorithm

47

Features of Link-State Algorithm

All routers maintain identical routing tables.

The database of each router describes complete topology ofthe router’s domain. The router’s domain may be theentire AS, or an area within the AS.

Each router uses the database to calculate a set of shortestpaths to all destinations. The routing table is built fromthese calculation.

OSPF uses a Link-State Routing Algorithm.

48



PWR

Collision

StatusUtil%

Forward%

Filter%Demo

Diag

Full/HalfConfig

1 2 3 4 5 6 7 8

90+705035201051Link

Rate %

SNMP


10 M/ 100 M


1100MTx/Rx


PWR

Collision

StatusUtil%

Forward%

Filter%

Demo

Diag

Full/HalfConfig

1 2 3 4 5 6 7 8

90+70503520

1051Link

Rate %

SNMP


10 M/ 100 M



PWR

Collision

StatusUtil%

Forward%

Filter%Demo

Diag

Full/HalfConfig

1 2 3 4 5 6 7 8

90+705035201051Link

Rate %

SNMP


10 M/ 100 M



PWR

Collision

StatusUtil%

Forward%

Filter%Demo

Diag

Full/HalfConfig

1 2 3 4 5 6 7 890+705035201051Link

Rate %

SNMP


10 M/ 100 M



PWR

Collision

StatusUtil%

Forward%

Filter%Demo

Diag

Full/HalfConfig

1 2 3 4 5 6 7 8

90+705035201051Link

Rate %

SNMP


10 M/ 100 M

N6

N7

N8

Router

ASBR

N9

N10

Router

Router

ASBRArea 1 Area 2

AS 300

AS 100

AS 200

Example of Link-State Algorithm

Area Border Router (ABR)

N1

N2

N3

N4

N5

49

Basic Operation of Link-State Algorithm

1) Exchange of Routing Information- Each router periodically sends out a description of its connections to its neighbors.- Routers are neighbors if they are directly connected via a common network.- A router sends the LSA to each of its neighbors. The LSA includes a listing of all interfaces & configured “cost” of each link and each configured cost- TOS pairing.

2) Routing Area- The LSA is flooded throughout the router’s domain. The router’s domain may be entire AS, or limited area within the AS.- Areas are configured by assigning an area_ID for each router interface. If the area_ID is identical for all ports on a router, then the router is contained in a single area.

3) Link-State Database- Each router in the domain maintains an identical, synchronized copy of a database composed of this link-state information.- Router belonging to multiple areas maintain a separate Link_State database

for each area.

50

4) Shortest Path Tree- Each router runs an algorithm on the database used to create a shortest-path tree. A different shortest-path tree is constructed for each TOS support.- The shortest-path tree contains the shortest path to every router and every network that other routers can reach. The router performing the calculation places itself at the root of each tree.

5) Routing Table- The resulting shortest-path trees determine total cost to the destination network and next hop router. The shortest-path trees are used as the basis of creating the routing table. A different routing table is created for each TOS.

6) Optional TOS Support- OSPF allows the network administrator to configure OSPF routers to calculate/use only single routing table(TOS 0 table).- A router desiring to calculate/use single table informs its peers by resetting the TOS-capable bit in the option field of the router’s links advertisement. If a route cannot be found that uses a non-zero TOS value, the traffic is forwarded along the TOS 0 route.

Basic Operation of Link-State Algorithm(Con’t)

51

How to Forward datagrams

The forwarding process uses routing tables to route datagrams.

1) The destination network number is extracted from an incomingdatagram.

2) The TOS field is examined for information pointing to

3) The datagram is forwarded towards its final destination.Datagram having same final destination may be routed alongdifferent paths based on the TOS requested by the source station.

52

Initial Link-State Database Synchronization

A pair of routers attempting to become adjacent send a summary of their Link-State databases to one another. This summary is called a “Database Description Packet”.

The Database Description Packet consists of a list of abbreviated link-stateAdvertisements(LSA).

Based on the Database Description Packet received from its neighbor, eachrouter builds a list of requests for LSAs, required to update its own database.

A router builds this list by checking its link-state database for a copy of eachLSA received in the summary. If the router doesn’t have a particular LSA in its link-state database, or determines that its neighbor has a more recent version of A LSA, that LSA is added to the request list.

Each router sends this list in a Link-State Request packet to its neighbor.

Each router responds to a received Link-State Request packet with a Link-State Update packet containing the requested LSAs. The neighbors becomeFully adjacent when they have received all requested LSAs.

Once the routers become fully adjacent, they run the SPF algorithm on the data-Base, add OSPF routes to their routing tables, and periodically exchange LSAs.

53

Maintain Link-State Database Synchronization

Flooding ProcedureWhen an LSA is flooded, it is passed from a router to an adjacent router untilIt has been distributed throughout the routing domain.Each router determines, individually, whether the LSA should be passed toAn adjacent neighbor. More details are described in Section 13, 13.3 & 13.4 ofRFC 2328.

Reliable UpdatesReliability is accomplished by requiring that both the receipt and transfer ofan LSA be acknowledged by adjacent router. In the absence of such anacknowledgment, the source router retransmits the LSA until it is acknowledged.

- Each router originates a router-LSA. - Area Border routers originate a single summary-LSA for each known inter- area destination. - AS Boundary routers originate a single As-external-LSA for each known AS external destination.

(Ten events can cause new instance of an LSA to be originated)

54

Link-State AgeAn LSA’s age field is periodically incremented while residing in a router’s link-state database. It is possible for an LSA to reach an age where it is no longer usedin the flooding procedure, and must be flushed from the link-state database. IfThere’s a change in the link-state database, a new shortest-path tree is constructedand the routing table is updated.

Link-State Sequence NumbersA common event is for an LSA to be replaced by the receipt of more recent LSA from its adjacent neighbor. Each LSA contains 32-bit sequence number usedby OSPF routers to detect old, or duplicate LSAs. A linearly ordered sequence number is used for LSA identification. All routers keep their link-state databases synchronized by ageing LSAs in their database, and updating withnew incoming LSAs.

Maintain LS Database Synchronization(Con’t)

Documents

Presentation Ospf Ranjeetv0.2