CA_Ex_S2M11_OSPF.ppt [Compatibility Mode]

8/3/2019 CA_Ex_S2M11_OSPF.ppt [Compatibility Mode]

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Chapter 11: OSPF

CCNA Ex loration version 4.0

1


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Objectives

Describe the background and basic features of OSPF Identify and apply the basic OSPF configuration

commands

Describe, modify and calculate the metric used byOSPF

Hc vin mng Bch Khoa - Website: www.bkacad.com 2

Describe the Designated Router/Backup DesignatedRouter (DR/BDR) election process in multiaccessnetworks

Describe the uses of additional configurationcommands in OSPF


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Introduction



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Introduction to OSPF



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Background of OSPF Began in 1987

1989 OSPFv1 released in RFC 1131This version was experimental & never deployed 1991 OSPFv2 released in RFC 1247 1998 OSPFv2 updated in RFC 2328

Background of OSPF


v pu s e n


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Single Area OSPF Concepts: OSPF terminology



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Single Area OSPF Concepts: OSPF terminology



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OSPF area



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OSPF Message Encapsulation



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OSPF Packet Types



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Hello Protocol



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1.Discover OSPF neighbors andestablish neighbor adjacencies.

2.Advertise parameters on which tworouters must agree to become

Hello Protocol


ne g ors.3.Elect the Designated Router (DR) and

Backup Designated Router (BDR) on

multiaccess networks like Ethernetand Frame Relay.


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OSPF Hello IntervalsUsually multicast (224.0.0.5)

Sent every 30 seconds for NBMAsegments

Hello Protocol


network

OSPF Dead Intervals

This is the time that must be expiredbefore the neighbor is considered down

Default time is 4 times the hello interval


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Hello Protocol



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Hello Protocol

Electing a DR and BDR

Designated Router (DR): DR isresponsible for updating all other


Backup Designated Router(BDR): This router takes over

DRs responsibilities if DR fails


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Purpose of a Link State Update(LSU):Used to deliver link state

advertisements

OSPF Link-state Updates


Purpose of a Link StateAdvertisement (LSA)Contains information about

neighbors & path costs


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OSPF Link-state Updates



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Extra: How routing information is maintained when a failure is occurs

Network 1 isdown

LSAs LSAs LSAs


1. When a failure occurs in the network, such as a neighbor becomes unreachable

2. Link-state protocols flood LSAs using a special multicast address throughout an area.

3. Each link-state router takes a copy of the LSA, updates its link-state/topological database.

4. The link-state router will then forward the LSA to all neighboring devices.

5. LSAs cause every router within the area to recalculate routes. Because LSAs need to be floodedthroughout an area

Note: all routers within that area need to recalculate their routing tables ==> the number of link-staterouters that can be in an area should be limited

Area x


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OSPF Algorithm



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Administrative Distance



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Purpose is to encrypt & authenticate routinginformation

This is an interface specific configuration Routers will only accept routing information

OSPF Authentication


from other routers that have beenconfigured with the same password orauthentication information

Note: Authentication does not encrypt therouter's routing table.


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Basic OSPF Configuration



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Lab Topology



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Addressing



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R1(config)#router ospf process-id Process ID:

A locally significant number between 1and 65535

Does not have to match other OSPF

The router ospf command


routers


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Router(config-router)# network network-address wildcard-maskarea area-id

wildcard mask - the inverse of the subnet mask.The network address along with the wildcard maskis used to specify the interface or range of

The network command


interfaces that will be enabled for OSPF using thisnetwork command.

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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The network command



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Router ID ?

OSPF Router ID



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Commands used to verify current router IDShow ip protocols

Show ip ospf

Show ip ospf interface

OSPF Router ID



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Router ID & Loopback addressesHighest loopback address will be used as router ID if

router-id command isnt used

OSPF Router ID

Host Mask


Advantage: loopback interface cannot fail OSPFstability

The OSPF router-id command: Introduced in IOS 12.0

Router(config)#router ospf process-id Router(config-router)#router-id ip-address

Modifying the Router ID Router#clear ip ospf process


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OSPF Router ID



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Duplicate Router IDs When two routers have the same router ID in an

OSPF, domain routing may not function properly.If the router ID is the same on two neighboring

OSPF Router ID


,

occur. When duplicate OSPF router IDs occur, IOS will

display a message similar to:

%OSPF-4-DUP_RTRID1: Detected router withduplicate router ID


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Verifying OSPF



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Verifying OSPF

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 meansthat the router and its neighbor have identical OSPF link-state databases. OSPF states are discussed in CCNP.


ea me - e amount o t me rema n ng t at t e routerwill wait to receive an OSPF Hello packet from theneighbor before declaring the neighbor down. This value isreset when the interface receives a Hello packet.

Address - The IP address of the neighbor's interface towhich this router is directly connected.

Interface - The interface on which this router has formedadjacency with the neighbor.


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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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Command Description

Show ip protocols

Displays OSPF process ID,router ID, networks router is

Verifying OSPF - Additional Commands

Verifying OSPF


a ver s ng a m n s ra ve

distance

Show ip ospf

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

calculated

Show ip ospf interfaceDisplays hello interval and deadinterval


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show ip protocols



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show ip ospf



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show ip ospf

A network that cycles between an up state and adown state is referred to as a flapping link.

A flapping link can cause OSPF routers in an areato constantly recalculate the SPF algorithm,preventing proper convergence.


,

seconds (5000 msecs) after receiving an LSUbefore running the SPF algorithm. This is knownas the SPF schedule delay.

In order to prevent a router from constantly

running the SPF algorithm, there is anadditional Hold Time of 10 seconds (10000msecs). The router waits 10 seconds afterrunning the SPF algorithm before rerunning the

algorithm again.


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show ip ospf interface



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show ip ospf database


E i i h i bl


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Note: OSPF does not automaticallysummarize at major network

boundaries

Examining the routing table



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The OSPF Metric


OSPF M t i


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OSPF Metric

OSPF uses cost as themetric for determining thebest route

Best route ~ lowest costCost

108/ bandwidth


Reference bandwidth defaults to 100Mbps

can be modified using

auto-cost reference-

bandwidth command

OSPF Metric


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COST of an OSPF route Is the accumulated value from one router to the

next

OSPF Metric


OSPF Metric


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Usually the actual speed of a link isdifferent than the default bandwidth

Bandwidth value does not actually affectthe speed of the link; it is used by somerouting protocols to compute the routing

OSPF Metric


me r c.

The show interface command will displayinterfaces bandwidth

Most serial link default to 1.544Mbps

OSPF Metric


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OSPF Metric


Modifying the Cost of a link


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Both sides of a serial link should beconfigured with the same bandwidth

Router(config-if)#bandwidthbandwidth kb



R1(config)#interface serial 0/0/0

R1(config-if)#ip ospf cost 1562



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Difference between bandwidthcommand & the ip ospf cost

commandIp ospf cost command



Sets cost to a specific valueBandwidth command

Link cost is calculated



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OSPF and Multiaccess Networks


Challenges in Multi-access Networks


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OSPF defines 5 network types:1. Point-to-point2. Broadcast Multiaccess3. Nonbroadcast Multiaccess (NBMA)4. Point-to-multipoint5. Virtual links

Challenges in Multi access Networks


OSPF in Multi-access Networks


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OSPF in Multi access Networks

2 challenges presented by multi-access networksMultiple adjacencies

Extensive LSA flooding




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Extensive flooding of LSAsFor 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 use ofDesignated router (DR)

Backup designated router (BDR) DR & BDR selection



Sending & Receiving LSADRothers send LSAs via multicast 224.0.0.6 to

DR & BDR

DR forward LSA via multicast address 224.0.0.5to all other routers



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DR/BDR Election Process


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DR/BDR elections DO NOT occur in point topoint networks




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DR/BDR elections will take place on multi-accessnetworks as shown below




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Criteria for getting elected DR/BDR

1.DR: Router with the highestOSPF interface priority.


. : ou er w e seconhighest OSPF interface priority.

3.If OSPF interface priorities areequal, the highest router ID isused to break the tie.



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Timing of DR/BDR ElectionOccurs as soon as 1st router has its

interface enabled on multi-access networkWhen a DR is elected it remains as the


DR until one of the following occurs

1. The DR fails.

2. The OSPF process on the DR fails.

3. The multiaccess interface on theDR fails.



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Manipulating the election process

1. Boot up the DR first, followed by theBDR, and then boot all other routers,


2. Shut down the interface on allrouters, followed by a no shutdown

on the DR, then the BDR, and thenall other routers.

OSPF Interface Priority


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Manipulating the DR/BDR electionprocess continued

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

OSPF Interface Priority


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More


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OSPF neighbor relationships progress through seven states:

not exchanged information


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not exchanged information

Send Hello packets to establish arelationship with neighbor routers.

There are 2 kinds of relationship

OSPF interfacescan be in one ofseven states.


Each router keeps a list ofadjacent neighbor, called theAdjacency databese.

Steps in the operation of OSPF


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OSPF routers progress through the following 5

distinct steps of operation:


.

2.Elect a DR and BDR, if necessary3.Discover routes4.Select the appropriate routes to use

5.Maintain routing information

1.Establish router adjacencies


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2.Elect a DR and BDR, if necessary


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3.Discover routes


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4.Select the appropriate routes to use


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5.Maintain Routing Information


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11

22

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More OSPF Configuration


Redistributing an OSPF Default Route


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Router connected to ISP Called an autonomous system border router

Used to propagate a default route

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

Example default route


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Fine-Tuning OSPF


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Since link speeds are getting faster it maybe necessary to change referencebandwidth values

- -


bandwidth commandExample:

R1(config-router)#auto-costreference-bandwidth 10000

Fine-Tuning OSPF


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Modifying OSPF timers


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Reason to modify timers Faster detection of network failures

Manually modifying Hello & Dead intervals Router(config-if)#ip ospf hello-


n erva secon s

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

Point to be made Hello & Dead intervals must be the same

between neighbors

Configuring Simple OSPF authentication


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The clear-text authentication Create a plain text password:

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


Router(config-router)#area {area-number}authentication

Or

Enable authentication on an interface:Router(config-if)# ip ospf authentication


Th d h i i


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The encrypted authentication Create a plain text password:

Router(config-if)#ip ospf message-digest-key {key-

id}md5 {password} Enable authentication on all interfaces:


ou er con g-rou er area area-num er

authentication message-digest Or Enable authentication on an interface:

Router(config-if)# ip ospf authentication message-digest

Note: key-idand passwordmust to match on all routers onthe same link


Th t d th ti ti


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The encrypted authentication


OSPF redistribution

Planning Redistribution


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Planning Redistribution

Locate the boundary router between two routing processes.

Determine which routing process is the core or backboneprocess

Determine which routing process is the edge or migration

process Select a method for injecting the required edge protocol routes

into the core.

92Hc vin mng Bach Khoa - Website: www.bkacad.com

Configuring Redistribution into OSPF

U hi d di ib i OSPF


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Use this command to redistribute routes into OSPF:

Router(config-router)# redistribute

protocol [process-id] [metric metric-value]

[metric-type type-value] [route-map map-


-

Default metric is 20 or 1.

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 ?

Autonomous system numberRtrA(config-router)# redistribute eigrp 100 ?

metric Metric for redistributed routes


me r c- ype - ex er or me r c ype or

redistributed routes

route-map Route map referencesubnets Consider subnets for redistribution into OSPF

tag Set tag for routes redistributed into OSPF



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Extra: E1-E2 routes


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Extra: RIPv1 versus OSPF


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Summary


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Documents

CA_Ex_S2M11_OSPF.ppt [Compatibility Mode]