Cnap 3 02 Single Area OSPF

7/27/2019 Cnap 3 02 Single Area OSPF

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CNAP Semester 3: Switching Basics and

Intermediate Routing

Module 2

Single Area OSPF


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Objectives

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

maintained

Discuss the link-state routing algorithm

Examine the advantages and disadvantages of link-state routing

Compare and contrast link-state routing with

distance 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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Objectives

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 Content

1 Link-state Routing Protocol2 Single Area OSPF Concepts

3 Single Area OSPF Configuration


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LINK-STATE ROUTING PROTOCOL


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Overview of link-state routing

Protocol Description Characteristics

Distancevector

RIP v1 and RIPv2

InterriorGateway RoutingProtocol (IGRP)

Copies routing table toneighbors

Updates frequently

Slow convergeRIP v1 and RIP v2 use hopcount as metric

View network from neighbors

perspectiveSusceptible to routing loops

Easy to configure andadministrate

Consumes a lot of bandwidth


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Overview of link-state routing

Protocol Description Characteristics

Link-state Open ShortestPath First(OSPF)

Intermediate-system to

Intermediate-system (IS-IS)

Use shortest path

Updates are event triggered

Fast to converge

Send link-state packets to all networkrouters

Has common view of network

Not as susceptible to routing loops

Harder to configure Requires more memory andprocessing power than distancevector

Consumes less bandwidth thandistance vector


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Link-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 hasoccurred

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 features

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

A topological database

Uses the shortest path first (SPF) algorithm(Dijkstra algorithm) to calculate the shortest route

to each network The resulting SPF tree

Stores this route information in its routing table


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Hello mechanism

Perth1 network Perth2 network

Perth3 network

Perth4 network

Hello Hello Hello Hello

HelloHello

Hello

P1 P2 P3

P4


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Link-state advertisements (LSAs).

Network 1 Network 2

Network 3

Perth4network

Hello Hello Hello Hello

HelloHello

Hello

P1 P2 P3

P4

Neighbors P3 and P4 onnetwork 3

Neighbors P2 on network1


Neighbors P2 on network2



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How routing information is maintained

When a failure occurs in the network, each link-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 algorithms

They are known collectively as shortest path first(SPF) protocols.

They maintain a complex database of the networktopology.

They are based on the Dijkstra algorithm.


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Link-state routing algorithms

Cost=1

A B

DC

Net 20Net 10

Net 30 Net 40

Cost=1

Cost=1

Cost=1

Cost=1

Router Destination Nexthop

Cost

A 20 B 1

A 30 C 1

A 40 C 2

A 40 B 2

B 10 A 1

B 30 C 1

B 40 D 1B


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Advantages and disadvantages of link-state routing

Advantages Disadvantages

Fast convergence: Changesare reported immediately bythe source affected

Robustness against routingloops

Routers know the topology

Link-state packets aresequenced and aged

The link-state database sizescan minimized with careful

network design

Significant demands onmemory and processingresources

Requires very strict networkdesign

Requires a knowledgeablenetwork administrator

Initial flooding can impedenetwork performance


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SINGLE AREA OSPF CONCEPTS


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OSPF overview

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

The most recent description is RFC 2328. The Openin OSPF means that it is open to the public and is

non-proprietary.


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OSPF terminology


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OSPF terminology: Links

TokenRing

Links

An interface on Router


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OSPF terminology: Link state

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

Neighbors

TokenRing

Links


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OSPF terminology: Area

A collection of networks and routers that have thesame area identification.

Each router within an area has the same link-stateinformation.

A router within an area is an internal router

TokenRing

Area 1

Area 0


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OSPF AreasExample

Area 0

Area 1

Area 2

Area 3


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OSPF terminology: Link Cost

The value assigned to a link. Rather thanhops, link-state protocols assign a cost to alink that is based on the speed of the media.

Interface Output Cost.

Neighbors

TokenRing

Interfaces

Cost = 10

Cost = 6Cost = 1785


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OSPF terminology: Adjacency database

A listing of all the neighbors to which a router hasestablished bi-directional communication. Not everypair of neighboring routers become adjacent

Adjacencydatabase

Neighbors

TokenRing


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OSPF terminology: Link-state database

Also known as a topological database

A list of link-state entries of all other routers in theinternetwork

Token

Ring

TopologicalDatabase

Adjacencydatabase


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OSPF terminology: Routing table

The routing table (also known as forwarding

database) 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: DR and BDR router

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

A router that is elected by all other routers on thesame LAN to represent all the routers.

Each network has a DR and BDR

TokenRing

DR

BDR


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Comparing OSPF with distance vector routing protocols

Distance vector OSPF

View network topology fromneighbors perspective

Adds distance vectors from

router to router Frequent, periodic update:

Slow convergence

Passes copies routing tables

to neighbor routers

Use flat topology

Gets common view ofentire network topology

Calculates the shortest

path to other routers Event-triggered update:

Fast to convergence

Passes link-state routing

updates to other routers

Allow hierarchical designfor large internetworks

Support for VLSM


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Shortest 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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OSPF network types


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OSPF network types: Fourth type


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DR and BDR receive LSAs

Hellos elect DR and BDR to present segment

Each router then forms adjacency with DR and BDR


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OSPF 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 allneighbors must agree upon before an adjacency is

formed, 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 - Hello 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.


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OSPF Topologies

Point-to-Point

BroadcastMultiaccess

NBMA X.25Frame Relay

OSPF O ti i


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

OSPF Operation in a

Broadcast Multiaccess Topology

BroadcastMultiaccess


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Step 1: Establish router adjacencies

First step in OSPF operation is to establish routeradjacencies

RTB sends hello packets, advertising its own router

ID highest IP address:10.6.0.1(no loopback)


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Step 1: Establish router adjacencies (cont.)

Router IDHello/dead intervals

NeighborsArea-IDRouter priorityDR IP addressBDR IP addressAuthentication passwordStub area flag

**

**

* Entry must match on neighboring routers

Hello

afadjfjorqpoeru39547439070713

Hello

A

D E

CB


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

P=1 P=0P=1

P=3 P=2

DR BDR

Hello

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

The default for the interface OSPF priority is 1. In

case of a tie (ha nhau), the routers router ID is used.


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Step 3: Discover routes

On difference network have differ discover process.

On multi-access network, the exchange ofrouting information occurs between the DR orBDR and every other router on the network.

Link partners on a point-to-point or point-to-multipoint network also engage (tham gia) in the

exchange process.


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Exchange Process

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.)

DBD


Here is a summary of my link-state database.

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


E0

172.16.5.1

DR

E0

172.16.5.3

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


Hello


Exstart State

Exchange State


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Step 3: Discover routes (cont.)

Full 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


LSAck


LSU


Loading State

E0

172.16.5.1

E0

172.16.5.3

LSAck


Thanks for the information!LSAck


DR


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Step 4: Choosing Routes

Topology TableNet Cost Out Interface10.2.2.0 6 To010.3.3.0 7 To010.3.3.0 10 E0

This is the best route to 10.3.3.0.

TokenRing

Cost=10

Cost=6

FDDI

Cost=1

A B C

10.1.1.0/24 10.2.2.0/24 10.3.3.0/24

10.4.4.0/24


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Step 5: Maintaining Routing Information

Router A tells all OSPF DRs on 224.0.0.6

xxLSU1

Link-State ChangeDR

AB


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DR tells all others on 224.0.0.5

LSU

2

xx

Link-State Change

LSU1

DR

AB


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LSU

3


LSU

2

xx

Link-State Change

LSU1

DR

AB




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I need to updatemy routing table.

4

LSU

3

LSU

2

xx

Link-State Change

LSU1

DR

AB




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Step 5: Maintaining Routing Information (cont.)

Is entry inlink-statedatabase?

LSA

LSU

No

Run SPF to calculatenew routing table

Add to database

Flood LSA

End

Send LSAckto DR


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Yes

Is seq. #the same?

Yes

Ignore LSAIs entry inlink-statedatabase?

LSA

LSU

No


Add to database

Flood LSA

Send LSAckto DR

End


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No

Send LSUwith newer

information tosource

Is seq. #higher?

No

Yes

Is seq. #the same?

Yes


LSA

LSU

No


Add to database

Flood LSA

Send LSAckto DR

End End


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Yes

GotoA

A

No

Send LSUwith newer

information tosource

Is seq. #higher?

No

Yes

Is seq. #the same?

Yes


LSA

LSU

No


Add to database

Flood LSA

Send LSAckto DR

End End


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OSPF 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 as soon asthe two routers can communicate

OSPF packets are always sent as multicast224.0.0.5


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OSPF Operation in an NBMA

NBMA Topology

Single interface interconnects multiple sites NBMA topologies support multiple routers but

without broadcasting capabilities

X.25

Frame Relay

ATM


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SINGLE 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 10.64.0.1 255.255.255.0!

router ospf 1

Basic OSPF Configuration

Can Assign Network or

Interface Address.

Broadcast Network Point-to-Point NetworkE0

10.64.0.1

10.64.0.2

E0

S0

10.2.1.2 10. 2.1.1

S1A B C

network 10.0.0.0 0.255.255.255 area 0 router ospf 50

network 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 address

Router ID:

Number by which the router is known to OSPF

Default: The highest IP address on an activeinterface at the moment of OSPF process startup

Can be overridden by a loopback interface: HighestIP address of 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

! Remove loopback 0 interface

Router(configf)#no Interface loopback 0


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Configuring 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

The priorities can be set to any value from 0 to 255

The command show ip ospf interface will displaythe interface priority value as well as other keyinformation.


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Modifying OSPF cost metric

Cost is calculated using the formula 108/bandwidth,where bandwidth is expressed in bps.

Bandwidth dividend is user configurable: Interface subcommand: bandwidth 64

Interface subcommand: ip ospf cost

Relevant going out an interface only

Medium Cost

56 kbps serial link 1785

T1 (1.544 Mbps serial link) 64

E1 (2.048 Mbps serial link) 48

Ethernet 10

Fast Ethernet/FDDI 1

C fi i OS i i


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Configuring 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

! After the password is configured, authentication must be enabled:

Router(config-router)#area area-numberauthentication

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.

C fi i OSPF h i i i h MD5


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Configuring OSPF authentication: with MD5

! Specifies the type of message-digest hashing algorithm to use! and key value

Router(config-if)#ip ospf message-digest-key key-idmd5 encryption-typekey

! After that, authentication must be enabled:

Router(config-router)#area area-idauthentication 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.

C fi i OSPF i


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Configuring 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

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 match those ofany neighboring router.

OSPF i d f l


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OSPF, 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]

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

Router(config-router)#default-information originate

C OSPF fi ti i


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Common OSPF configuration issues

Failure to establish a neighbor relationship is caused by any

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 correctarea.

V if i OSPF O ti


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Router#

show ip ospf interface

Verifying OSPF Operation

Displays area ID and adjacency information

Router#

show ip protocols

Verifies that OSPF is configured

Router#

show ip route

Displays all the routes learned by the router

V if i OSPF O ti ( t )


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Displays OSPF timers and statistics

Displays information about DR, BDR and neighbors

Displays the link-state database

Verifying OSPF Operation (cont.)

Router#

show ip ospf neighbor detail

Router#

show ip ospf database

Router#

show ip ospf

V if i OSPF O ti ( t )


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Allows you to clear the IP routing table

Router#

clear ip route *

Router#

debug ip ospf opt ion

Displays router interaction during the hello,exchange, and flooding processes

Verifying OSPF Operation (cont.)

h i f i t f


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show ip ospf interface

R2#sh ip ospf int e0

Ethernet0 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 1

Designated Router (ID) 192.168.0.11, Interface address 192.168.0.11

Backup Designated router (ID) 192.168.0.13, Interface address

192.168.0.13

Timer 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 Interface

192.168.0.13 1 2WAY/DROTHER 00:00:31 192.168.0.13 Ethernet0

192.168.0.14 1 FULL/BDR 00:00:38 192.168.0.14 Ethernet0

192.168.0.11 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

sho ip ospf database


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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 count

192.168.0.10 192.168.0.10 817 0x80000003 0xFF56 1

192.168.0.11 192.168.0.11 817 0x80000003 0xFD55 1

192.168.0.12 192.168.0.12 816 0x80000003 0xFB54 1

192.168.0.13 192.168.0.13 816 0x80000003 0xF953 1

192.168.0.14 192.168.0.14 817 0x80000003 0xD990 1

Net Link States (Area 0)

Link ID ADV Router Age Seq# Checksum

192.168.0.14 192.168.0.14 812 0x80000002 0x4AC8

show ip ospf database

Summary


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Summary

Link-state routing protocols collect routing information fromall other router in the network or within a defined area ofthe 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 ofneighbors

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&A


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Q&A


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Documents

Cnap 3 02 Single Area OSPF