Chapter 11 - OSPF CCNA docx

Học viện mạng Bách Khoa - Website: www.bkacad.com 2Objectives • Describe the background and basic features of OSPF • Identify and apply the basic OSPF configuration commands • Describe,

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Chapter 11 - OSPF

CCNA Exploration 4.0

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Học viện mạng Bách Khoa - Website: www.bkacad.com 2

Objectives

• Describe the background and basic features of OSPF

• Identify and apply the basic OSPF configuration

commands

• Describe, modify and calculate the metric used by

OSPF

• Describe the Designated Router/Backup Designated

Router (DR/BDR) election process in multiaccess

networks

• Describe the uses of additional configuration

commands in OSPF

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Introduction

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OSPF Message Encapsulation

• OSPF packet type

– There exist 5 types.

• OSPF packet header

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Introduction to OSPF

OSPF Message Encapsulation

• Data link frame header

– Source MAC address

– Destination MAC address

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OSPF Packet Types

OSPF PACKET TYPE DESCRIPTION

1: HELLO Discovers neighbors & builds

ADJACENCY W/ NEIGHBORS

2: DATABASE DESCRIPTION Checks for database synchronization

between routers

3: LINK STATE REQUEST REQUESTS SPECIFIC LINK STATE

DATABASE information from router to router

4: LINK STATE UPDATE TRANSPORTS LINK STATE records

5: LINK STATE

ACKNOWLEDGEMENT

ACKNOWLEDGES RECEIPT OF other packet types

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Hello Protocol

• OSPF Hello Packet

 Discover OSPF neighbors & establish adjacencies

 Advertise guidelines on which routers must agree to become neighbors

 Used by multi-access networks to elect a designated

router(DR) and a backup designated router(BDR)

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OSPF Hello Packet

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• Hello Packets continued

– Router ID of transmitting router

– Sent every 10 seconds on

multiaccess & point to point serial link

• OSPF Hello Intervals

– Usually multicast (224.0.0.5)

– Sent every 30 seconds for NBMA

segments

• OSPF Dead Intervals

– This is the time that must be expired before

the neighbor is considered down

– Default time is 4 times the hello interval

OSPF timers

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DR and BDR

• Hello protocol packets contain information that is used in

electing

– Designated Router (DR)

• DR is responsible for updating all other OSPF routers

– Backup Designated Router (BDR)

• This router takes over DR’s responsibilities if DR fails

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OSPF Link-state Updates

• Purpose of a Link State Update (LSU)

– Used to deliver link state advertisements

• Purpose of a Link State Advertisement (LSA)

– Contains information about neighbors & path costs

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OSPF Algorithm

• OSPF routers build &

maintain link-state database

containing LSA received

from other routers

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OSPF Administrative Distance

• Default Administrative Distance for OSPF is 110

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• OSPF Authentication

– Purpose is to encrypt & authenticate routing information

– This is an interface specific configuration

– Routers will only accept routing information from other

routers that have been configured with the same

password or authentication informa.tion

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Basic OSPF configuration

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Example

Lab Topology

• Topology used for this chapter

– Discontiguous IP addressing scheme

– Since OSPF is a classless routing protocol the subnet

mask is configured in

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Addressing

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Enable OSPF

The router ospf command

• To enable OSPF on a router use the following command

– R1(config)#router ospf process-id

– Process id

• A locally significant number between 1 and 65535

• Does not have to match other OSPF routers

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Basic OSPF Configuration

• RA(config-router)#network network-address

wildcard-mask area area-id

– wildcard mask - the inverse of the subnet mask

– area-id

• Area-id refers to the OSPF area

• OSPF area is a group of routers that share link state information

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OSPF Router ID

Find Router ID

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OSPF Router ID

• Router ID & Loopback addresses

– Highest loopback address will be used as router ID if

router-id command isn’t used

– Advantage: loopback interface cannot fail  OSPF stability

• The OSPF router-id command

– Introduced in IOS 12.0

– Command syntax

• Router(config)#router ospf process-id.

• Router(config-router)#router-id ip-address.

• Modifying the Router ID

– Use the command Router#clear ip ospf process.

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Show ip ospf neighbor command

• Neighbor ID - The router ID of the neighboring router.

• Pri - The OSPF priority of the interface This is discussed in a later

section.

• State - The OSPF state of the interface FULL state means that the

router and its neighbor have identical OSPF link-state databases

OSPF states are discussed in CCNP.

• Dead Time - The amount of time remaining that the router will wait to

receive an OSPF Hello packet from the neighbor before declaring the neighbor down This value is reset when the interface receives a Hello packet.

• Address - The IP address of the neighbor's interface to which this

router is directly connected.

• Interface - The interface on which this router has formed adjacency

with the neighbor.

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• Neighboring router’s Router ID is not displayed

• A state of full is not displayed.

– Consequence of no adjacency

• No link state information exchanged

• Inaccurate SPF trees & routing tables

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Reasons of no adjacency

• Two routers may not form an OSPF adjacency if:

– The subnet masks do not match, causing the routers to be on separate networks

– OSPF Hello or Dead Timers do not match

– OSPF Network Types do not match

– There is a missing or incorrect OSPF network command

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Show ip protocols

Displays OSPF process ID , router ID,

networks router is advertising &

administrative distance

Show ip ospf

Displays OSPF process ID, router ID, OSPF area information & the last time SPF algorithm calculated

Show ip ospf interface Displays hello interval and dead interval

Verifying OSPF - Additional Commands

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Show ip protocols

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Show ip ospf

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Show ip ospf interface

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Show ip route - Examining the routing table

• Use the show ip route command to display the routing table

– “O’ at the beginning of a route indicates that the router

source is OSPF

– Note OSPF does not automatically summarize at major

network boundaries

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OSPF Metric

• OSPF uses cost as the metric for determining the best route

– Best route ~ lowest cost

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reference-Học viện mạng Bách Khoa - Website: www.bkacad.com 33

• COST of an OSPF route

– Is the accumulated value from one router to the next

OSPF Metric

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• Usually the actual speed of a link is different than the

default bandwidth

– This makes it imperative that the bandwidth value

reflects link’s actual speed

• Reason: so routing table has best path information

• The show interface command will display interface’s

bandwidth

– Most serial link default to 1.544Mbps

OSPF Metric

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Modify OSPF Cost value

Modifying the Cost of a link

• Both sides of a serial link should be configured with the

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Modifying the Cost of the link

• Difference between bandwidth command & the ip ospf cost

command:

– Ip ospf cost command

• Sets cost to a specific value

• Link cost is calculated

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OSPF and Multi-access Networks

Challenges in Multi-access Networks

• OSPF defines five network types:

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OSPF in Multi-access Networks

• 2 challenges presented by multi-access networks

– Multiple adjacencies

– Extensive LSA flooding

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• Extensive flooding of LSAs

– For every LSA sent out there must be an acknowledgement

of receipt sent back to transmitting router

 lots of bandwidth consumed and chaotic traffic

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• Solution to LSA flooding issue is the

• Sending & Receiving LSA

– DRothers send LSAs via

multicast 224.0.0.6 to DR & BDR

– DR forward LSA via multicast

address 224.0.0.5 to all other

routers

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DR/BDR Election Process

• DR/BDR elections DO NOT occur in point to point networks

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• DR/BDR elections will take place on multi-access

networks as shown below.

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• Criteria for getting elected DR/BDR

– DR: Router with the highest OSPF

interface priority

– BDR: Router with the second highest

OSPF interface priority

– If OSPF interface priorities are equal,

the highest router ID is used to break

the tie

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– The OSPF process on the DR fails

– The multiaccess interface on the DR fails

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DR/BDR Election – Refer to 11.4.2.3

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• Manipulating the election process

– If you want to influence the election of DR & BDR then do

one of the following

• Boot up the DR first, followed by the BDR, and then boot all other routers,

OR

• Shut down the interface on all routers, followed by a no shutdown on the DR, then the BDR, and then all other routers

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Manipulating the DR/BDR election

OSPF Interface Priority

• Manipulating the DR/BDR election process continued

– Use the ip ospf priority interface command

– Example:Router(config-if)#ip ospf priority {0 - 255}

 Priority number range 0 to 255

– 0 means the router cannot become the DR or BDR

– 1 is the default priority value

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Redistributing an OSPF Default Route

• Topology includes a link to ISP

– Router connected to ISP

– Example of static default route

– R1(config)#ip route 0.0.0.0 0.0.0.0 loopback 1

– Requires the use of the default-information originate command

– Example of default-information originate command

– R1(config-router)#default-information originate

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Example default route

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Fine-Tuning OSPF

• Since link speeds are getting

faster it may be necessary to

change reference bandwidth

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Modifying OSPF timers

• Modifying OSPF timers

– Reason to modify timers

• Faster detection of network failures

– Manually modifying Hello & Dead intervals

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

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

– Point to be made

• Hello & Dead intervals must be the same between neighbors

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Học viện mạng Cisco Bách Khoa - Website: www.ciscobachkhoa.com

• By default , a router trusts that routing information is coming from a router that

should be sending the information

• To guarantee this trust, routers in a specific area can be configured to authenticate

each other.

• Each OSPF interface can present an authentication key for use by routers sending OSPF information to other routers on the segment.

• The authentication key, known as a password , is a shared secret between the

routers This key is used to generate the authentication data in the OSPF packet header The password can be up to eight characters.

Configuring OSPF authentication

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

• After the password is configured, authentication must be

enabled:

• Router(config-router)#area area-number authentication

• Note: With simple authentication, the password is sent as plain text This means that it can be easily decoded if a packet sniffer captures an OSPF packet

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•To send encrypted authentication information and to ensure greater security, the message-digest

The authentication type identifies which authentication,

if any, is being used.

In the case of message-digest authentication, the

authentication data field contains the key-id and the

length of the message digest that is appended to the

packet

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OSPF redistribution

• Planning Redistribution

• Locate the boundary router between two routing processes

• Determine which routing process is the core or backbone

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Configuring Redistribution into OSPF

• Use this command to redistribute routes into OSPF:

Router(config-router)# redistribute

map-tag] [subnets] [tag tag-value]

• Default metric is 20

• Default metric type is 2

• Subnets do not redistribute by default

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RtrA(config)# router ospf 1

RtrA(config-router)# redistribute eigrp ?

<1-65535> Autonomous system number

RtrA(config-router)# redistribute eigrp 100 ?

metric Metric for redistributed routes

metric-type OSPF/IS-IS exterior metric type for

redistributed routes

route-map Route map reference

subnets Consider subnets for redistribution into OSPF

tag Set tag for routes redistributed into OSPF

…

<cr>

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Tiêu đề	Chapter 11 - OSPF CCNA Exploration 4.0
Trường học	Học viện mạng Bách Khoa
Chuyên ngành	Networking
Thể loại	lecture notes

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