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ObjectivesObjectives

Identify the key features of link-state routing Explain how link-state routing information ismaintained

Discuss the link-state routing algorithm

Examine the advantages and disadvantages of link-state routing

Compare and contrast link-state routing withdistance vector routing

Enable OSPF on a router Configure a loopback address to set router

priority

Change OSPF route preference by modifying thecost metric

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ObjectivesObjectives

Configure OSPF authentication Change OSPF timers Describe the steps to create and propagate a

default route Use show commands to verify OSPF operation Configure the OSPF routing process

Define key OSPF terms Describe the OSPF network types Describe the OSPF Hello protocol

Identify the basics steps in the operation of OSPF

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Table of ContentTable of Content

1 Link-state Routing Protocol2 OSPF Concepts3 OSPF Configuration

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LINK-STATE ROUTING PROTOCOLLINK-STATE ROUTING PROTOCOL

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Overview of link-state routingOverview of link-state routing

Protocol Description Characteristics

Distancevector

RIP v1 and RIPv2

InterriorGatewayRoutingProtocol (IGRP)

Copies routing table toneighbors

Updates frequently Slow converge RIP v1 and RIP v2 use hop

count as metric

View network from neighborsperspective Susceptible to routing loops Easy to configure and

administrate

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Overview of link-state routingOverview of link-state routing

Protocol Description Characteristics

Link-stateOpen ShortestPath First(OSPF)

Intermediate-system toIntermediate-system (IS-IS)

Use shortest path Updates are event triggered Fast to converge Send link-state packets to all

network routers Has common view of network Not as susceptible to routing loops Harder to configure Requires more memory and

processing power than distancevector

Consumes less bandwidth than

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Link-state routing protocol functionsLink-state routing protocol functions

Link-state routing protocols collect routinginformation from all other routers in the network.

Each router maintains its own complex database of topology information, the own view of the network.

Link-state routing protocols functions: Respond quickly to network changes Send triggered updates only when a network change has

occurred Send periodic updates known as link-state refreshes Use a hello mechanism to determine the reachability of

neighbors

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Link-state routing protocol featuresLink-state routing protocol features

Uses the hello information and Link-stateadvertisements (LSAs) it receives from otherrouters to build a database about thenetwork

A topological database

Uses the shortest path first (SPF) algorithm(Dijkstra algorithm) to calculate the shortestroute to each network

The resulting SPF tree

Stores this route information in its routing

table

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Hello mechanismHello mechanism

Perth1network

Perth2network

Perth3network

Perth4network

Hello Hello Hello Hello

HelloHello

Hello

P1 P2 P3

P4

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How routing information is maintainedHow routing information is maintained

When a failure occurs in the network, eachlink-state router take flowing action: Flood LSAs using a special multicast address

throughout an area. Copy of the LSA and updates its link-state, or

topological database. Then forward the LSA to all neighboring devices Recalculate their routing tables.

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Link-state routing algorithmsLink-state routing algorithms

They are known collectively as shortest pathfirst (SPF) protocols. They maintain a complex database of the

network topology. They are based on the Dijkstra algorithm.

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Link-state routing algorithmsLink-state routing algorithms

Cost=1

A B

DC

Net 20Net 10

Net 30 Net 40

Cost=1

Cost=1

Cost=1Cost=1

Router Destination Nexthop

Cost

A 20 B 1A 30 C 1

A 40 C 2A 40 B 2B 10 A 1B 30 C 1B 40 D 1B

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Advantages and disadvantages of link-state routingAdvantages and disadvantages of link-state routing

Advantages Disadvantages Fast convergence:

Changes are reportedimmediately by the source

affected Robustness against

routing loops Routers know the topology

Link-state packets aresequenced and aged

The link-state databasesizes can minimized withcareful network design

Significant demands onmemory and processingresources

Requires very strictnetwork design

Requires a knowledgeablenetwork administrator

Initial flooding can impedenetwork performance

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SINGLE AREA OSPF CONCEPTSSINGLE AREA OSPF CONCEPTS

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OSPF overviewOSPF overview

Open Shortest Path First (OSPF) is a link-staterouting protocol based on open standards.

The most recent description is RFC 2328. TheOpen in OSPF means that it is open to thepublic and is non-proprietary.

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OSPF terminologyOSPF terminology

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OSPF terminology:OSPF terminology: LinksLinks

TokenRing

Links

An interface on Router

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OSPF terminology:OSPF terminology: Link stateLink state

The status of a link between two routers. Alsoa routers interface and its relationship to itsneighboring routers.

Neighbors

TokenRing

Links

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OSPF terminology:OSPF terminology: AreaArea

A collection of networks and routers that havethe same area identification. Each router within an area has the same link-

state information.

A router within an area is an internal router

TokenRing

Area 1Area 0

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OSPF terminology:OSPF terminology: Link CostLink Cost

The value assigned to a link. Rather than hops,link-state protocols assign a cost to a link that isbased on the speed of the media.

Interface Output Cost .

Neighbors

TokenRing

Interfaces

Cost = 10

Cost = 6Cost = 1785

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OSPF terminology:OSPF terminology: Adjacency databaseAdjacency database

A listing of all the neighbors to which a routerhas established bi-directional communication.Not every pair of neighboring routers becomeadjacent

Adjacencydatabase

Neighbors

TokenRing

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OSPF terminology:OSPF terminology: Link-state databaseLink-state database

Also known as a topological database A list of link-state entries of all other routers in

the internetwork

Token

Ring

TopologicalDatabase

Adjacencydatabase

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OSPF terminology:OSPF terminology: Routing tableRouting table

The routing table (also known as forwardingdatabase) generated when an algorithm is run onthe link-state database.

Each routers routing table is unique

AdjacencyDatabase

Lists neighbors

Token

Ring

TopologicalDatabase

Lists all routes

RoutingTable

Lists best routes

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OSPF terminology:OSPF terminology: DR and BDR routerDR and BDR router

Designated router (DR) and backup designated router (BDR):

A router that is elected by all other routers on the same LAN torepresent all the routers.

Each network has a DR and BDR

TokenRing

DRDR

BDR

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Comparing OSPF with distance vector routing protocolsComparing OSPF with distance vector routing protocols

Distance vector OSPF

View network topologyfrom neighborsperspective

Adds distance vectorsfrom router to router

Frequent, periodicupdate: Slow

convergence Passes copies routing

tables to neighbor routers Use flat topology

Gets common view of entire networktopology

Calculates the shortestpath to other routers

Event-triggeredupdate: Fast to

convergence Passes link-state

routing updates toother routers

Allow hierarchical

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Shortest path algorithmShortest path algorithm

1

4

1 4

2

22

ABC

D

E F G

The best path is the lowest cost path.

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Shortest Path First Tree for node BShortest Path First Tree for node B

1

(1,B)

1 4

2

22

A

B

C

D

E F G

(3,C)

(4,E)

2

(5,E)

(6,A)

(4,B)

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OSPF network typesOSPF network types

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OSPF network types: Fourth typeOSPF network types: Fourth type

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DR and BDR receive LSAsDR and BDR receive LSAs

Hellos elect DR and BDR to present segment

Each router then forms adjacency with DR

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OSPF Hello ProtocolOSPF Hello Protocol

The rules that govern the exchange of OSPF hellopackets are called the Hello protocol.

Hello packets use : 224.0.0.5 (all routers). Hello packets are sent at regular intervals (default):

Multi access and Point-to-point: 10s NBMA : 30s

On multi-access networks the Hello protocol elects

a designated router (DR) and a backup designatedrouter (BDR). The hello packet carries information that all

neighbors must agree upon before an adjacency isformed, and link-state information is exchanged.

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OSPF packet header

For the hello packet the type field is set to 1.

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OSPF Hello Protocol -OSPF Hello Protocol - Hello headerHello header

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Steps in the operation of OSPF

5 steps of operation:1. Establish router adjacencies.2. Elect a DR and BDR (if necessary).3. Discover routes.4. Select the appropriate routes to

use.5. Maintain routing information.

d l

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3 2000, Cisco Systems, Inc. www.cisco.com 4-39

OSPF Operation in a Broadcast MultiaccessOSPF Operation in a Broadcast MultiaccessTopologyTopology

BroadcastMultiaccess

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Step 1: Establish router adjacenciesStep 1: Establish router adjacencies

First step in OSPF operation is to establishrouter adjacencies

RTB sends hello packets, advertising its ownrouter ID highest IP address:10.6.0.1(noloopback)

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Step 1: Establish router adjacencies (cont.)Step 1: Establish router adjacencies (cont.)

Router IDHello/dead intervalsNeighborsArea-IDRouter priorityDR IP address

BDR IP addressAuthentication passwordStub area flag

* *

* *

* Entry must match on neighboring routers

Hello

afadjfjorqpoeru39547439070713

Hello

AA

DD EE

CCBB

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Step 2: Electing the DR and BDR (if necessary).

P=1 P=0P=1

P=3 P=2

DRDR BDRBDR

Hello

The router with the highest priority value isthe DR. The router with the second highest priority

value is the BDR.

The default for the interface OSPF priority is

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Step 3: Discover routes

On difference network have differ discover process.

On multi-access network, the exchange of routing

information occurs between the DR or BDR and everyother router on the network.

Link partners on a point-to-point or point-to-multipoint network also engage in the exchangeprocess.

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Exchange ProcessExchange Process

Router BNeighbors List

172.16.5.1/24, int E1

Router BNeighbors List

172.16.5.1/24, int E1

172.16.5.1/24E0

I am router ID 172.16.5.2, and I see 172.16.5.1.

Router ANeighbors List

172.16.5.2/24, int E0

172.16.5.2/24E1

I am router ID 172.16.5.1 and I see no one.Down State

Init State

A B

Two-way State

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Step 3: Discover routes (cont.)Step 3: Discover routes (cont.)

DBD

afadjfjorqpoeru39547439070713

Here is a summary of my link-state database.

Here is a summary of my link-state database.DBD

afadjfjorqpoeru39547439070713

E0172.16.5.1

DRE0172.16.5.3

No, I will start exchange because I have ahigher router ID.

I will start exchange because I have router ID 172.16.5.1.Hello

afadjfjorqpoeru39547439070713

Hello

afadjfjorqpoeru39547439070713

Exstart StateExstart State

Exchange StateExchange State

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Step 3: Discover routes (cont.)Step 3: Discover routes (cont.)

Full StateFull State

I need the complete entry for network 172.16.6.0/24.

Here is the entry for network 172.16.6.0/24.

Thanks for the information!

LSR

afadjfjorqpoeru39547439070713

LSAck

afadjfjorqpoeru39547439070713

LSU

afadjfjorqpoeru39547439070713

Loading StateLoading State

E0172.16.5.1

E0172.16.5.3

LSAck

afadjfjorqpoeru39547439070713

Thanks for the information! LSAck

afadjfjorqpoeru39547439070713

DR

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Step 5: Maintaining Routing Information

Router A tells all OSPF DRs on 224.0.0.6

xxLSU1

Link-State ChangeDRDR

AA BB

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Step 5: Maintaining Routing Information

Router A tells all OSPF DRs on 224.0.0.6 DR tells all others on 224.0.0.5

LSU

2

xx

Link-State Change

LSU1

DRDR

AA BB

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Step 5: Maintaining Routing InformationStep 5: Maintaining Routing Information

I need to updatemy routing table.

4

LSU

3

LSU

2

xx

Link-State Change

LSU1

DRDR

AA BB

Router A tells all OSPF DRs on 224.0.0.6 DR tells all others on 224.0.0.5

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Step 5: Maintaining Routing Information (cont.)

No

Send LSUwith newerinformation to

source

Is seq. #higher?

No

Yes

Is seq. #the same?

Yes

Ignore LSAIs entry inlink-state

database?LSA

LSU

No

Run SPF to calculatenew routing table

Add to database

Flood LSA

Send LSAckto DR

End End

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Step 5: Maintaining Routing Information (cont.)

Yes

GotoA

A

No

Send LSUwith newerinformation to

source

Is seq. #higher?

No

Yes

Is seq. #the same?

Yes

Ignore LSAIs entry inlink-state

database?LSA

LSU

No

Run SPF to calculatenew routing table

Add to database

Flood LSA

Send LSAckto DR

End End

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OSPF Operation in a Point-to-PointOSPF Operation in a Point-to-Point

Point-to-Point Neighborship Router dynamically detects its neighboring

router using the Hello protocol

No election: Adjacency is automatic assoon as the two routers can communicate

OSPF packets are always sent as multicast224.0.0.5

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OSPF Operation in an NBMAOSPF Operation in an NBMA

NBMA Topology

Single interface interconnects multiple sites NBMA topologies support multiple routers

but without broadcasting capabilities

X.25Frame Relay

ATM

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SINGLE AREA OSPF ConfigurationSINGLE AREA OSPF Configuration

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interface Ethernet0ip address 10.64.0.2 255.255.255.0

!interface Serial0ip address 10.2.1.2 255.255.255.0

interface Ethernet0ip address 10.64.0.2 255.255.255.0

!interface Serial0ip address 10.2.1.2 255.255.255.0

interface Ethernet0ip address 10.64.0.1 255.255.255.0

!

interface Ethernet0ip address 10.64.0.1 255.255.255.0

!

router ospf 1

Basic OSPF ConfigurationBasic OSPF ConfigurationBasic OSPF ConfigurationBasic OSPF Configuration

Can Assign Network or

Interface Address.

Broadcast Network Point-to-Point Network E010.64.0.1

10.64.0.2E0

S010.2.1.2 10. 2.1.1

S1AA BB CC

network 10.0.0.0 0.255.255.255 area 0 router ospf 50network 10.2.1.2 0.0.0.0 area 0

network 10.64.0.2 0.0.0.0 area 0

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Configuring OSPF loopback addressConfiguring OSPF loopback address

Router ID: Number by which the router is known to OSPF

Default: The highest IP address on an active interface at themoment of OSPF process startup

Can be overridden by a loopback interface: Highest IP addressof any active loopback interface

! Create the loopback 0 interface

Router(configf)#Interface loopback 0Router(configf-if)#ip address 192.168.31.33 255.255.255.255

! C reate the loopback 0 interface

Router(configf)#Interface loopback 0Router(configf-if)#ip address 192.168.31.33 255.255.255.255

! Remove loopback 0 interface

Router(configf)#no Interface loopback 0

! Remove loopback 0 interface

Router(configf)#no Interface loopback 0

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Configuring OSPF router priorityConfiguring OSPF router priority

The router with the highest priority value is the DR. The default for the interface OSPF priority is 1. In case

of a tie, the routers router ID is used.

! Setting OSPF Priority

Router(configf)#Interface Fastethernet 0/0Router(configf-if)#ip ospf priority 50

! Setting OSPF Priority

Router(configf)#Interface Fastethernet 0/0Router(configf-if)#ip ospf priority 50

The priorities can be set to any value from 0 to 255 The command show ip ospf interface will display the interface

priority value as well as other key information.

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Modifying OSPF cost metricModifying OSPF cost metric

Cost is calculated using the formula10 8/bandwidth, where bandwidth is

expressed in bps. Bandwidth dividend is user configurable:

Interface subcommand: bandwidth 64

Interface subcommand: ip ospf cost

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Configuring OSPF authenticationConfiguring OSPF authentication

! Create a key that is used to generate the authentication data! in the OSPF packet header.

Router(config-if)#ip ospf authentication-key password

! C reate a key that is used to generate the authentication data! in the OSPF packet header.

Router(config-if)#ip ospf authentication-key password

After the password is configured, authentication must be enabled:

outer(config-router)#area area-number authentication

After the password is configured, authentication must be enabled:

outer(config-router)#area area-number authentication

The authentication key, known as a password, is a

shared secret between the routers. The password can be up to eight characters. The password is sent as plain text.

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Configuring OSPF authentication:Configuring OSPF authentication: with MD5with MD5

pecifies the type of message-digest hashing algorithm to usend key value

uter(config-if)#ip ospf message-digest-key key-id md5 encryption-typ

pe cifies the type of message-digest hashing algorithm to usend key value

uter(config-if)#ip ospf message-digest-key key-id md5 encryption-typ

! After that, authentication must be enabled:

Router(config-router)#area area-id authentication message-digest

! Af ter that, authentication must be enabled:

Router(config-router)#area area-id authentication message-digest

The value of encryption-type field is 0 means none

and 7 means proprietary. The key-id is an identifier (1 to 255) The key is an alphanumeric password up to sixteen

characters. Neighbor routers must use the same key identifier

with the same key value.

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Configuring OSPF timersConfiguring OSPF timers

! To configure the hello and dead intervals on an interface

Router(config-if)#ip ospf hello-interval secondsRouter(config-if)#ip ospf dead-interval seconds

! To configure the hello and dead intervals on an interface

Router(config-if)#ip ospf hello-interval secondsRouter(config-if)#ip ospf dead-interval seconds

OSPF networks, the default Hello interval is 10 seconds Dead interval is 40 seconds.

On nonbroadcast networks, the default Hello interval is 30 seconds Dead interval is 120 seconds.

These timers must be configured to matchthose of any neighboring router.

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OSPF, propagating a default routeOSPF, propagating a default route

!Configure a gateway of last resort

Router(config)#ip route 0.0.0.0 0.0.0.0 [interface | next-hop address

!Configure a gateway of last resort

Router(config)#ip route 0.0.0.0 0.0.0.0 [interface | next-hop address

! To propagate this route to all the routers in a normal OSPF area

Router(config-router)#default-information originate

! To propagate this route to all the routers in a normal OSPF area

Router(config-router)#default-information originate

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Common OSPF configuration issuesCommon OSPF configuration issues

Failure to establish a neighbor relationship is caused byany of the following reasons: Hellos are not sent from both neighbors. Hello and dead interval timers are not the same.

Interfaces are on different network types. Authentication passwords or keys are different.

In OSPF routing it is also important to ensure the following: All interfaces have the correct addresses and subnet

mask. network area statements have the correct wildcard

masks. network area statements put interfaces into the

correct area.

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Router#show ip ospf interface

Verifying OSPF OperationVerifying OSPF Operation

Displays area ID and adjacency information

Router#

show ip protocols

Verifies that OSPF is configuredRouter#

show ip route

Displays all the routes learned by the router

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Displays OSPF timers and statistics

Displays information about DR, BDR andneighbors

Displays the link-state database

Verifying OSPF Operation (cont.)Verifying OSPF Operation (cont.)

Router#

show ip ospf neighbor detail

Router#

show ip ospf database

Router#show ip ospf

f

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Allows you to clear the IP routing table

Router#

clear ip route *

Router#

debug ip ospf option

Displays router interaction during thehello, exchange, and floodingprocesses

Verifying OSPF Operation (cont.)Verifying OSPF Operation (cont.)Verifying OSPF Operation (cont.)Verifying OSPF Operation (cont.)

h f f

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show ip ospf interface

R2#sh ip ospf int e0Ethernet0 is up, line protocol is up

Internet Address 192.168.0.12/24, Area 0

Process ID 1, Router ID 192.168.0.12, Network Type BROADCAST, Cost: 10Transmit Delay is 1 sec, State DROTHER, Priority 1Designated Router (ID) 192.168.0.11, Interface address 192.168.0.11Backup Designated router (ID) 192.168.0.13, Interface address

192.168.0.13Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5

Hello due in 00:00:04

Neighbor Count is 3, Adjacent neighbor count is 2Adjacent with neighbor 192.168.0.13 (Backup Designated Router)Adjacent with neighbor 192.168.0.11 (Designated Router)

Suppress hello for 0 neighbor(s)

h i f i hb

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show ip ospf neighbor

Neighbor ID Pri State Dead Time Address Interface192.168.0.13 1 2WAY/DROTHER 00:00:31 192.168.0.13 Ethernet0192.168.0.14 1 FULL/BDR 00:00:38 192.168.0.14 Ethernet0192.168.0.11 1 2WAY/DROTHER 00:00:36 192.168.0.11 Ethernet0

192.168.0.12 1 FULL/DR 00:00:38 192.168.0.12 Ethernet0

OSPF over Ethernet - Multiaccess Network

Neighbor ID Pri State Dead Time Address Interface192.168.0.11 1 FULL/ - 00:00:39 10.1.1.2 Serial1

OSPF over HDLC - Point-to-Point Network

h i f d b

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R2# show ip ospf database

OSPF Router with ID (192.168.0.12) (Process ID 1)

Router Link States (Area 0)

Link ID ADV Router Age Seq# Checksum Link count192.168.0.10 192.168.0.10 817 0x80000003 0xFF56 1192.168.0.11 192.168.0.11 817 0x80000003 0xFD55 1192.168.0.12 192.168.0.12 816 0x80000003 0xFB54 1192.168.0.13 192.168.0.13 816 0x80000003 0xF953 1192.168.0.14 192.168.0.14 817 0x80000003 0xD990 1

Net Link States (Area 0)

Link ID ADV Router Age Seq# Checksum192.168.0.14 192.168.0.14 812 0x80000002 0x4AC8

show ip ospf database

SS

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SummarySummary

Link-state routing protocols collect routing information fromall other router in the network or within a defined area of the network. Link-state routing protocols perform thefollowing functions:

Respond quickly to network changes

Send triggered update only when a network change hasoccurred

Send periodic updates know as link-state refreshes

Use a hello mechanism to determine the reachability of neighbors

OSPF is link-state routing protocol based on open standards. OSPF routing uses the concept of area. Each router contains

a complete database of link-states in specific area

Q&AQ&A

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Q&AQ&A