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Open Shortest Path FirstOpen Shortest Path FirstOSPFOSPF

OSPF OverviewOSPF Overview OSPF OperationOSPF Operation

By Grace Deng Oct.16.2003By Grace Deng Oct.16.2003

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OSPF OverviewOSPF OverviewHistoryHistory

Development began 1987 by IETF Development began 1987 by IETF Goal—a link state protocol more efficient Goal—a link state protocol more efficient

and scaleable than RIPand scaleable than RIP Latest revision is RFC 2328Latest revision is RFC 2328

April 1998April 1998

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OSPF OverviewOSPF OverviewOSPF versus RIPOSPF versus RIP

OSPFOSPF Link stateLink state Efficient routing updates Efficient routing updates

(sends changes only)(sends changes only) No hop count limitNo hop count limit Fast ConvergenceFast Convergence

Supports VLSMSupports VLSM

Path selection based on Path selection based on bandwidthbandwidth

RIPRIP Distance vector Distance vector Copies entire routing Copies entire routing

tabletable Hop count limit of 15Hop count limit of 15 Hold-down timers to Hold-down timers to

prevent routing loopsprevent routing loops Does not advertise sub-Does not advertise sub-

net masksnet masks Uses only hop count as Uses only hop count as

metricmetric

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OSPF OverviewOSPF Overview Concepts Concepts

OSPF is a Link-State Routing ProtocolOSPF is a Link-State Routing Protocol Uses IP as transport, IP protocol 89Uses IP as transport, IP protocol 89 Uses Uses multicast addressesmulticast addresses in neighbor in neighbor

maintenance and flooding of LSAsmaintenance and flooding of LSAs 224.0.0.5 – All OSPF Routers224.0.0.5 – All OSPF Routers 224.0.0.6 – All DRouters224.0.0.6 – All DRouters

Employs Dijkstra’s Shortest Path First (SPF) Employs Dijkstra’s Shortest Path First (SPF) algorithm to calculate the path treealgorithm to calculate the path tree

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OSPF OverviewOSPF Overview Concepts – (cont.) Concepts – (cont.)

Uses Metrics—path cost Uses Metrics—path cost Typically faster convergence than DVRPsTypically faster convergence than DVRPs Support for CIDR, VLSM, Authentication, Support for CIDR, VLSM, Authentication,

Multi-path and IP unnumberedMulti-path and IP unnumbered Relatively low steady state bandwidth Relatively low steady state bandwidth

requirementsrequirements

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OSPF OverviewOSPF Overview TerminologyTerminology

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OSPF OverviewOSPF Overview Terminology Terminology

Link Link Link stateLink state Link State (LS) or topological database Link State (LS) or topological database AreaArea OSPF Metric CostOSPF Metric Cost Routing tableRouting table Adjacencies databaseAdjacencies database

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OSPF Overview OSPF Overview Topology/Link State DatabaseTopology/Link State Database

A router has A router has a separatea separate Link State (LS) or Link State (LS) or topological database for topological database for each areaeach area to which to which it belongsit belongs

All routers belonging to the All routers belonging to the samesame areaarea should have should have identical databasesidentical databases

SPF calculation is performed SPF calculation is performed independently independently for each areafor each area

LSA flooding is bounded by areaLSA flooding is bounded by area

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OSPF Overview OSPF Overview AreasAreas

OSPF uses a 2 level hierarchical modelOSPF uses a 2 level hierarchical model Areas labeled with a 32-bit numberAreas labeled with a 32-bit number

Can be defined using single decimal or IP Can be defined using single decimal or IP address format valueaddress format value

(i.e. (i.e. Area 0.0.0.0Area 0.0.0.0 or or Area 0Area 0)) Area 0 reserved for the backbone areaArea 0 reserved for the backbone area All areas must connect to area 0All areas must connect to area 0

101010

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OSPF Overview OSPF Overview OSPF MetricOSPF Metric

Cost applied on all router link paths Cost applied on all router link paths 16-bit positive number 1–65,53516-bit positive number 1–65,535 The lower the more desirableThe lower the more desirable Relevant going out an interface onlyRelevant going out an interface only Route decisions made on total cost of path Route decisions made on total cost of path

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OSPF Overview OSPF Overview OSPF Packet TypesOSPF Packet Types

OSPF Packetformat

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OSPF Packet Types OSPF Packet Types (cont.)(cont.)

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OSPF Overview OSPF Overview Router IDRouter ID

Routers are identified by a unique 32-bit IDRouters are identified by a unique 32-bit ID RID: highest IP address configured on any RID: highest IP address configured on any

active active loopback interfaceloopback interface RID: if no loopback exists, highest IP address RID: if no loopback exists, highest IP address

configured on any active configured on any active physical interfacephysical interface RID can be configured withRID can be configured with

router-id <ip address>router-id <ip address>

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OSPF Overview OSPF Overview OSPF Hello PacketsOSPF Hello Packets

Multicast 224.0.0.5 on all router interfacesMulticast 224.0.0.5 on all router interfaces Hello interval 10 sec. LAN, 30 sec. NBMAHello interval 10 sec. LAN, 30 sec. NBMA Used to form adjacencies between routersUsed to form adjacencies between routers

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OSPF Overview OSPF Overview Database Descriptor Packets (DDP)Database Descriptor Packets (DDP)

Contain link state database headersContain link state database headers Describe the current LS databaseDescribe the current LS database ExchangeExchange stage stage

••••••

DD seq=x+1,S

DD seq=x+n,S

DD seq=x+n,M

DD seq=x+1,M

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OSPF Overview OSPF Overview Link State Request & Update PacketsLink State Request & Update Packets

Request for specific parts of databaseRequest for specific parts of database Send only database updates requestedSend only database updates requested LoadingLoading Stage, labeled Stage, labeled FullFull when complete when complete

Link State Update

Link State Request

Link State Request

Link State Update

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OSPF OperationOSPF Operation

Network changes generate link-state Network changes generate link-state advertisements (LSA)advertisements (LSA) Cost change to an interfaceCost change to an interface Link being added or deleted from topologyLink being added or deleted from topology

All routers exchange LSAs to build and All routers exchange LSAs to build and maintain a consistent databasemaintain a consistent database

The protocol remains relatively quiet during The protocol remains relatively quiet during steady-state conditions.steady-state conditions.

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OSPF Operation OSPF Operation Steps to OSPF OperationSteps to OSPF Operation

1. Establishing router adjacencies1. Establishing router adjacencies

2. Electing DR and BDR2. Electing DR and BDR

3. Discovering Routes3. Discovering Routes

4. Choosing Routes4. Choosing Routes

5. Maintaining Routing Information5. Maintaining Routing Information

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OSPF OperationOSPF Operation OSPF StatesOSPF States

OSPF router interfaces can be in one of OSPF router interfaces can be in one of seven states:seven states:

Down StateDown State Init StateInit State Two-way StateTwo-way State ExStart State ExStart State Exchange StateExchange State Loading StateLoading State Full Adjacency StateFull Adjacency State

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OSPF OperationOSPF Operation Steps to OSPF Operation with OSPF StatesSteps to OSPF Operation with OSPF States

1. Establishing router adjacencies1. Establishing router adjacencies Down StateDown State Init StateInit State Two-way StateTwo-way State (ExStart State unless DR/BDR election (ExStart State unless DR/BDR election

needed)needed)

2. Electing DR and BDR2. Electing DR and BDR ExStart State with DR and BDRExStart State with DR and BDR Two-way State with all other routersTwo-way State with all other routers

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3. Discovering Routes3. Discovering Routes ExStart StateExStart State Exchange StateExchange State Loading StateLoading State Full StateFull State

4. Choosing Routes4. Choosing Routes

5. Maintaining Routing Information5. Maintaining Routing Information

OSPF OperationOSPF Operation Steps to OSPF Operation with OSPF StatesSteps to OSPF Operation with OSPF States

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Initially, an OSPF router interface is in Initially, an OSPF router interface is in the the down statedown state.not exchanged .not exchanged information with any neighbor.information with any neighbor.

OSPF OperationOSPF Operation 1. Establishing Adjacencies (1)1. Establishing Adjacencies (1)

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Init StateInit State Init StateInit State - OSPF routers send Type 1 - OSPF routers send Type 1

Hello packets at regular intervals (10 Hello packets at regular intervals (10 sec.) to establish neighbors.sec.) to establish neighbors.

When a router receives its first Hello When a router receives its first Hello packet, it enters the packet, it enters the init stateinit state, meaning , meaning the router is ready to take the the router is ready to take the relationship to the next level.relationship to the next level.

OSPF OperationOSPF Operation 1. Establishing Adjacencies 1. Establishing Adjacencies (2)(2)

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From From init stateinit state to the to the two-way statetwo-way state RTB receives Hello packets from RTA and RTB receives Hello packets from RTA and

RTC (its neighbors), and sees its own RTC (its neighbors), and sees its own Router ID (10.6.0.1) in the Neighbor ID field.Router ID (10.6.0.1) in the Neighbor ID field.

RTB declares takes the relationship to a RTB declares takes the relationship to a new level, and declares a new level, and declares a two-way statetwo-way state between itself and RTA, and itself and RTC.between itself and RTA, and itself and RTC.

OSPF OperationOSPF Operation 1. Establishing Adjacencies (3)1. Establishing Adjacencies (3)

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Two-way stateTwo-way state to to ExStart stateExStart state?? RTB now decides who to establish a full adjacency with RTB now decides who to establish a full adjacency with

depending upon the type of network that the particular depending upon the type of network that the particular interfaces resides on.interfaces resides on.

If the interface is on a If the interface is on a point-to-pointpoint-to-point link, the routers link, the routers becomes adjacent with its sole link partner (aka “soul becomes adjacent with its sole link partner (aka “soul mates”), and take the relationship to the next level by mates”), and take the relationship to the next level by entering the entering the ExStart stateExStart state..

If the interface is on a If the interface is on a multi-accessmulti-access link (Ethernet, link (Ethernet, Frame Relay, …) RTB must enter an election process Frame Relay, …) RTB must enter an election process to see who it will establish a full adjacency with, and to see who it will establish a full adjacency with, and remains in the remains in the two-way statetwo-way state. (Next!). (Next!)

OSPF OperationOSPF Operation 1. Establishing Adjacencies (4)1. Establishing Adjacencies (4)

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OSPF Operation OSPF Operation Designated RouterDesignated Router

Reduce OSPF traffic on multiaccess linksReduce OSPF traffic on multiaccess links Routers form FULL adjacencies with DR/BDRRouters form FULL adjacencies with DR/BDR

Store and distribute neighbors LSDBsStore and distribute neighbors LSDBs Backup DR for redundancyBackup DR for redundancy OSPF priority used in DR selectionOSPF priority used in DR selection

Range 1–255 default 1, 0 for non-candidate. Range 1–255 default 1, 0 for non-candidate. Priority carried in Hello packetPriority carried in Hello packet

ip ospf priority <value>ip ospf priority <value>

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OSPF Operation OSPF Operation Function of DR/BDRFunction of DR/BDR

DR

BDR

Link

chan

ge Flood Link change224.0.0.5

AllOSPFRouters

224.0.0.6AllDRrouters

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OSPF OperationOSPF Operation 2.Electing a DR and BDR (1)2.Electing a DR and BDR (1)

On On point-to-point point-to-point links links adjacencies are established adjacencies are established with all neighbors, because with all neighbors, because there is only one neighbor.there is only one neighbor.

OnOn multi-access multi-access networks,OSPF elects a DR networks,OSPF elects a DR and BDR to limit the number and BDR to limit the number of adjacencies.of adjacencies. Reduce routing update Reduce routing update

traffictraffic

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DRDR - Designated Router- Designated Router BDRBDR – Backup Designated Router – Backup Designated Router DR’s serve as collection points for Link DR’s serve as collection points for Link

State Advertisements (LSAs)State Advertisements (LSAs) A BDR back ups the DR.A BDR back ups the DR. If the IP network is If the IP network is multi-accessmulti-access, the , the

OSPF routers will elect 1 DR and 1 BDROSPF routers will elect 1 DR and 1 BDR (unless there is only 1 router on the (unless there is only 1 router on the network).network).

OSPF OperationOSPF Operation 2.Electing a DR and BDR (2)2.Electing a DR and BDR (2)

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The formation of an adjacency between The formation of an adjacency between every attached router would create many every attached router would create many unncessary LSA (Link State unncessary LSA (Link State Advertisements), n(n-1)/2 adjacencies.Advertisements), n(n-1)/2 adjacencies.

Flooding on the network itself would be Flooding on the network itself would be chaotic.chaotic.

To prevent this problem, a To prevent this problem, a Designated Designated RouterRouter is elected on multi-access networks. is elected on multi-access networks.

OSPF OperationOSPF Operation 2.Electing a DR and BDR (3)2.Electing a DR and BDR (3)

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All other routers, “DRother”, establish All other routers, “DRother”, establish adjacencies with only the DR and BDR.adjacencies with only the DR and BDR.

DRother routers multicast LSAs to only the DR DRother routers multicast LSAs to only the DR and BDRand BDR (224.0.0.6 - all DR routers)(224.0.0.6 - all DR routers)

DR sends LSA to all adjacent neighbors DR sends LSA to all adjacent neighbors (224.0.0.5 - all OSPF routers)(224.0.0.5 - all OSPF routers)

OSPF OperationOSPF Operation 2.Electing a DR and BDR (4)2.Electing a DR and BDR (4)

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Once a DR is established, a new router that enters Once a DR is established, a new router that enters the network with a higher priority or router id will the network with a higher priority or router id will NOT become the DR or BDR. (Bug in early IOS NOT become the DR or BDR. (Bug in early IOS 12.0)12.0)

If DR fails, BDR takes over as DR and selection If DR fails, BDR takes over as DR and selection process for new BDR begins.process for new BDR begins.

State of the relationshipState of the relationship DRothers enterDRothers enter ExStart stateExStart state with DR and BDR with DR and BDR

andand two-way statetwo-way state with all other routerswith all other routers

OSPF OperationOSPF Operation 2.Electing a DR and BDR (5)2.Electing a DR and BDR (5)

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DR - SummaryDR ElectionDR Election Router with the highest interface priority Router with the highest interface priority

(priority = 0 cannot become DR or BDR)(priority = 0 cannot become DR or BDR)

Router with the highest router ID. Router with the highest router ID. Loopback address used firstLoopback address used first IP Address on active interface used IP Address on active interface used

secondsecond BDR is the second highestBDR is the second highest

OSPF OperationOSPF Operation 2.Electing a DR and BDR (6)2.Electing a DR and BDR (6)

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DR - SummaryAdjacencies and multicastingAdjacencies and multicasting All other routers, DRother, establish All other routers, DRother, establish

adjacencies with only the DR and BDR.adjacencies with only the DR and BDR. All routers continue to multicast Hello All routers continue to multicast Hello

packets to AllSPFRouters (224.0.0.5) so they packets to AllSPFRouters (224.0.0.5) so they can track neighbors.can track neighbors.

But updates (LSAs) are multicast to DR and But updates (LSAs) are multicast to DR and BDR only (224.0.0.6 - AllDRrouters) and in BDR only (224.0.0.6 - AllDRrouters) and in turn turn

DR floods updates (LSAs) to all adjacent DR floods updates (LSAs) to all adjacent neighbors (224.0.0.5 - AllSPFRrouters)neighbors (224.0.0.5 - AllSPFRrouters)

OSPF OperationOSPF Operation 2.Electing a DR and BDR (7)2.Electing a DR and BDR (7)

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BDR-summary Listens, but doesn’t act.Listens, but doesn’t act. If LSA is sent, BDR sets a timer.If LSA is sent, BDR sets a timer. If timer expires before it sees the reply from If timer expires before it sees the reply from

the DR, it becomes the DR and takes over the DR, it becomes the DR and takes over the update process.the update process.

The process for a new BDR begins.The process for a new BDR begins.

OSPF OperationOSPF Operation 2.Electing a DR and BDR (8)2.Electing a DR and BDR (8)

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OSPF OperationOSPF Operation 3. Discovering Routes and reaching Full State3. Discovering Routes and reaching Full State

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Link state databaseLink state database Created with Link State Packets (LSPs) from Created with Link State Packets (LSPs) from

each routereach router TENT databaseTENT database

Tentative triples (ID, path cost, direction)Tentative triples (ID, path cost, direction) PATH databasePATH database

Best path triples (ID, path cost, direction)Best path triples (ID, path cost, direction) Forwarding databaseForwarding database

The The Routing TableRouting Table

Dijkstra - Dijkstra - Shortest Path First (SPF) Shortest Path First (SPF) AlgorithmAlgorithm

OSPF OperationOSPF Operation 4. Choosing routes (1)4. Choosing routes (1)

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All routers exchange Link State Packets (LSPs)All routers exchange Link State Packets (LSPs) Each router starts with itself as rootEach router starts with itself as root Tent is built from LSPsTent is built from LSPs Path is created by examining and comparing Path is created by examining and comparing

TENT triplesTENT triples Once path is final the forwarding table is Once path is final the forwarding table is

populatedpopulated

Dijkstra (SPF) Overview (Cont.)Dijkstra (SPF) Overview (Cont.)

OSPF OperationOSPF Operation 4. Choosing routes (2)4. Choosing routes (2)

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B C D E F

B/4

G/2

G

A/4

C/1

A

B/1

D/4

E/2

C/4

E/1

C/2

D/1

F/2

E/2

G/2

A/2

F/2

B A C

D

E F G

4 2

2 2

2 1 4

1

Lowest cost best

Link State Packet (LSP) Link State Packet (LSP) DataData

OSPF OperationOSPF Operation 4. Choosing routes (3)4. Choosing routes (3)

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....

OSPF Operation OSPF Operation 5. Maintaining routes5. Maintaining routes

Router 2, Area 1

Old Routing Table New Routing Table

Link State Table

LSA

Dijkstra Algorithm

ACK

Every router in Every router in area receives the area receives the new LSA via new LSA via floodingflooding

Each router Each router computes computes shortest path shortest path routing table routing table when a link when a link changes State.changes State.

Router 1, Area 1

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Issues with large OSPF netsIssues with large OSPF nets

Large routing table Large routing table Large link-state tableLarge link-state table Frequent SPF calculationsFrequent SPF calculations

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referencereference RFC 1403RFC 1403, "BGP OSPF Interaction", K. Varadhan, 1993., "BGP OSPF Interaction", K. Varadhan, 1993. RFC 1584RFC 1584, "Multicast Extensions to OSPF", J. Moy, March 1994., "Multicast Extensions to OSPF", J. Moy, March 1994. RFC 1850RFC 1850, "OSPF Version 2 Management Information Base", F. , "OSPF Version 2 Management Information Base", F.

Baker and R. Coltun, Nov 1995.Baker and R. Coltun, Nov 1995. RFC 2328RFC 2328, "OSPF Version 2", J. Moy, April 1998, also STD 54. , "OSPF Version 2", J. Moy, April 1998, also STD 54. RFC 2370RFC 2370, "The OSPF Opaque LSA Option", R. Coltun, July 1998., "The OSPF Opaque LSA Option", R. Coltun, July 1998. http://www2.rad.com/networks/1995/ospf/ospf.htmhttp://www2.rad.com/networks/1995/ospf/ospf.htm, “OSPF”, B. , “OSPF”, B.

Daniel, B. Omer, R. Carmel.Daniel, B. Omer, R. Carmel. Internetworking with TCP/IP (Vol I) - Comer Internetworking with TCP/IP (Vol I) - Comer www.et.fnt.hvu.nl/docenten/cuiterwijk/ccnp/guideswww.et.fnt.hvu.nl/docenten/cuiterwijk/ccnp/guides,“The Technology ,“The Technology

Innovation Centre Brimingham”.Innovation Centre Brimingham”.