Chapter 08 single area OSPF Fetel Academy

Introduction about single area OSPF

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UNIVERSITY OF SCIENCE, HOCHIMINH CITY, VIETNAM ELECTRONICS AND TELECOMMUNICATION FACULTY Networking

CHAPTER

Tnstructor: Nguyen Vĩ la

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Chapter 8

1 Characteristics of OSPF

2 Configuring Single-area OSPFv2

3 Configure Single-area OSPFv3

OSPF Area 0

|Cisco Networking Academy, Electronics and Telecommunications Faculty, University of Science, Ho Chi Minh City, Vietnam Là

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hapter 8: Objectives

After completing this chapter, you will be able to:

Explain the process by which link-state routers learn about other networks

Describe the types of packets used by Cisco IOS routers to establish and maintain

an OSPF network

Explain how Cisco IOS routers achieve convergence in an OSPF network

Configure an OSPF router ID

Configure single-area OSPF v2 in a small, routed IPv4 network

Explain how OSPF uses cost to determine best path

Verify single-area OSPFv2 in a small, routed network

Compare the characteristics and operations of OSPFv2 to OSPFv3

Configure single-area OSPFv3 in a small, routed network

Verify single-area OSPFV3 in a small, routed network

|Cisco Networking Academy, Electronics and Telecommunications Faculty, University of Science, Ho Chi Minh City, Vietnam ry

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Features of OSPF

updates

OSPF Features

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Components of OSPF

s» Data Structures

Adjacency Neighbor + List of all neighbor routers to which a router

Database Table has established bidirectional communication

+ This table is unique for each router

+ Can be viewed using the show ip ospf neighbor command

Link-state Topology Lists information about all other routers in the

(LSDB) + The database represents the network topology

+ Allrouters within an area have identical LSDB

+ Can be viewed using the show ip ospf

database command

Forwarding Routing + List of routes generated when an algorithm is

Database Table run on the link-state database

+ Each router’s routing table is unique and

contains information on how and where to

send packets to other routers

+ Can be viewed using the show ip route

command

|Cisco Networking Academy, Electronics and Telecommunications Faculty, University of Science, Ho Chi Minh City, Vietnam

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Components of OSPF

+ Routing Protocol Messages:

o OSPF exchanges messages to convey routing information

using five types of packets

Ded tees Dee PS 000cc)

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2 Single-area and Multiarea OSPF

Area 0 is also called the backbone area

gle-Area OSPF is useful in smaller networks with few routers

|Cisco Networking Academy, Electronics and Telecommunications Faculty, University of Science, Ho Chi Minh City, Vietnam ca

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Multiarea OSPF

Area 1 Area 0 Area 51

Implemented using a two-layer area hierarchy as all areas must connect to the backbone area (area 0)

Interconnecting routers are called Area Border Routers (ABRs)

Useful in larger network deployments to reduce processing and memory overhead

|Cisco Networking Academy, Electronics and Telecommunications Faculty, University of Science, Ho Chi Minh City, Vietnam 12

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Single-area and Multiarea OSPF Multiarea OSPF have these advantages:

o Smaller routing tables - Fewer routing table entries

o Reduced link-state update overhead - Minimizes

processing and memory requirements

o Reduced frequency of SPF calculations - Localizes the impact of a topology change within an area

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Link Change Impacts Local Area Only

Area 1 Area 0 Area 51

Link failure affects the local area only (area 51)

The ABR (R2) isolates the fault to area 51 only

Routers in areas 0 and 1 do not need to run the SPF algorithm

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Identify OSPF Features and Terminology

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OSPF Messages

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Encapsulating OSPF Messages

Data Link Frame OSPF Packet OSPF Packet Type- Header IP Packet Header Header Specific Database

Data Link Frame (Ethernet Fields shown here)

MAC Destination Address = Multicast: 01-00-5E-00-00-05 or 01-00-5E-00-00-06

MAC Source Address = Address of sending interface

IP Packet

IP Source Address = Address of sending interface

IP Destination Address = Multicast: 224.0.0.5 or 224.0.0.6

Protocol Field = 89 for OSPF

OSPF Packet Header Type code for OSPF Packet type Router ID and Area ID

OSPF Packet Types

0x01 Hello 0x02 Database

Description (DD) 0X03 Link State Request

0X04 Link State Update

0X05 Link State

Acknowledgment

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Types of OSPF Packets

Packet Name

adjacencies between them

synchronization between routers

records from router to router

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OSPF Hello Packets)

¢ Discover OSPF neighbors and establish neighbor adjacencies

e Advertise parameters on which two routers must agree to become neighbors

e Elect the Designated Router and Backup Designated Router

on multi-access networks such as Ethernet and Frame Relay

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Hello Packet

Data Link Frame IP Packet Header OSPF Packet Header OSPF Packet Type-Specific Data

Hello interval Option Router Priority

OSPF Hello Router Dead interval

Packatia) Designated Router (DR)

Backup Designated Router (BDR)

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Hello Packet

_OSPF Packet Type: Hello (1), DD (2), LS Request (3), LS

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Data Link Frame

Hello Interval Option Router Priority

Router Dead Interval Designated Router (DR}

List of Neighbor(s}

22

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Hello Packet

Version 1ype= Area from which the packet originated

List of Neighboris)

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Data Link Frame

Router Dead Interval Designated Router (DR}

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Hello Interval Option Router Priority

OSPF Hello Router Dead Interval

Packatia) Designated Router (DR)

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Hello Packet

_ Usedin DR/BDR election (discussed later) |

Hello Interval Router Priority OSPF Hello Router Dead interval

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Hello Packet

OSPF Hello ee an Router iD of the DR, ifany #

Packatia) Designatea Router (DR)

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Hello Packet

OSPF Hello Router ID of the BDR, if any |

List of Neighbor(s}

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Hello Packet

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Hello Packet Intervals

10.5.0.0/16

1020016@ fy 10.9.0.0/16 10.1.0.016 10.11.00/16

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Hello Packet Intervals

10.5.0.0/16

fy 10.9.0.0/16 10.1.0.016 10.11.00/16

104.00/16 SŠ

10800116

¢ The OSPF Dead interval is the period, expressed in seconds, that the router will wait to receive a Hello packet before declaring the neighbor “down.”

¢ Cisco uses a default of four times the Hello interval

= 40 seconds for multi-access and point-to-point links

= 120 seconds for NBMA networks

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Link-State Updates

1 Hello Discovers neighbors and builds adjacencies between them

2 DBD Checks for database synchronization between routers

3 LSR Requests specific link-state records from router to router

6 Multicast OSPF LSAs

i Defined for Not-So-Stubby Areas

8 External Attributes LSA for Border

Gateway Protocol (BGP) Opaque LSAs

v OSPFv3 renamed several of these LSAs and also contains two additional LSAs

LSA: Link-State Advertisements

|Cisco Networking Academy, Electronics and Telecommunications Faculty, University of Science, Ho Chi Minh City, Vietnam vn

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Identify the OSPF Packet Types

c

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OSPF Operation

nh ES

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OSPF Operational States

“ When an OSPF router is initially connected to a network,

it attempts to:

= Create adjacencies with neighbors

= Exchange routing information

= Calculate the best routes

= Reach convergence

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OSPF Operational States

Down State Down State

+ No Hello packets received =

Establish Down

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Establish * Hello packets are received from

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OSPF Operational States @

+ The master initiates the DBD packet exchange

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Synchronize + Routers exchange DBD packets

+ If additional router information is

Loading; otherwise, transition to Full

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+ LSRs and LSUs are used to gain

additional route information

Loading State + Routes are processed using the

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ees Routers have converged

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University of Science, Ho Chi Minh City, Vietnam C7

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Establish Neighbor Adjacencies

s* The OSPF router ID is used by the OSPF process to uniquely identify each router in the OSPF area

“+ A router ID is an IP address assigned to identify a specific

router among OSPF peers

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s* R2 receives the Hello packet from R1 and adds the R1 router

ID to its neighbor list

“* R2 then sends a Hello packet to R1

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s*1If the two adjacent neighbors are interconnected over a point-to-point link, then they immediately transition from the Two-Way state to the database synchronization phase

“If the routers are interconnected over a common Ethernet network, then a designated router DR and a BDR must be

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R2 has a default priority

of 1 and the highest

R1 has a default priority

of 1 and the second

highest router ID It will

be the BDR on this link

router ID It will be the DR

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>» Extensive flooding of LSAs

Broadcast Multiaccess Network

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OSPF DR and BDR

IỀ \djacency Adjacenct bs }

Adjacencies n(n-1)/2

10

45

190 4,950

a network would cause the number of adjacencies to grow exponentially

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OSPF DR and BDR

initialized or when there is a change in the topology

¢ This would lead to an excessive number of LSAs

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OSPF DR and BDR

e The solution to managing

the number of adjacencies

and the flooding of LSAs

on a multi-access network

is the Designated Router

(DR)

e On multi-access networks,

OSPF elects aDR to be

distribution point for LSAs sent and received

e A Backup Designated Router (BDR) is also elected in case the

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Synchronizing OSPF Database

s* The Hello packet was used to establish neighbor adjacencies, the other four types of OSPF packets are used during the process of exchanging and synchronizing LSDBs

1 172 16.5.0/24 2

ExStart State R2 has the highest router ID,

becoming the master, and therefore it

prepares to send a LSDB in a DBD packet

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s* When R1 receives the DBD, it performs the following actions:

o 1 It ACK the receipt of the DBD using the LSAck packet

o 2 R1 then sends DBD packets to R2

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Tdentify the OSPF States for Establishing Adjacency

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2 Configuring Single-Area OSPFv2

OSPF Router ID

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OSPF Netwo

s All interfaces on routers R1, R2, and R3 (except the loopback

on R2) are within the OSPF backbone area

Note: In this topology the loopback interface is used to simulate the WAN link to the Internet

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E5 Router OSPF Configuration Mode

e OSPF requires a Process ID

= Between 1 and 65,535 and chosen by the network

administrator

e The process ID is only locally significant

= Does not have to match on all OSPF routers

e For consistency, we will enable OSPF on all three routers using the same Process ID of 1

R1 (config-router) #

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