Exploration Routing Chapter 11

5/25/2018 Exploration Routing Chapter 11

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2007 Cisco Systems, Inc. All rights reserved. Cisco Public

ITE PC v4.0

Chapter 1 1

OSPF

Routing Protocols andConceptsChapter 11


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Chapter 1 2 2007 Cisco Systems, Inc. All rights reserved. Cisco Public

Objectives

Describe the background and basic features of OSPF

Identify and apply the basic OSPF configurationcommands

Describe, modify and calculate the metric used byOSPF

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

Background of OSPF

Began in 1987

1989 OSPFv1 released in RFC 1131

This version was experimental & never deployed

1991 OSPFv2 released in RFC 1247 1998 OSPFv2 updatedin RFC 2328

1999 OSPFv3 published in RFC 2740


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Introduction to OSPFOSPF Message Encapsulation

OSPF packet type

There exist 5 types

OSPF packet header

Contains- Router ID andarea IDand Type code forOSPF packet type

IP packet header

Contains- Source IPaddress, Destination IPaddress, & Protocol fieldset to 89


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OSPF Message Encapsulation

Data link frame header

Contains - Source MAC address and DestinationMAC address


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OSPF Packet Types


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Introduction to OSPFHello Protocol

OSPF Hello PacketPurpose of Hello Packet

Discover OSPF neighbors & establish adjacencies

Advertise guidelines on which routers must agree to becomeneighbors

Used by multi-access networks to elect a designated router anda backup designated router


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Hello Packets continued

Contents of a Hello Packet

router ID of transmitting router

OSPF Hello Intervals

Usually multicast (224.0.0.5)

Sent every 30 seconds for NBMAsegments

OSPF Dead Intervals

This is the time that must transpire

before the neighbor is considered

down

Default time is 4 times

the hello interval


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Hello protocol packets contain information that is usedin electing

-Designated Router (DR)

DR is responsible for updating all other OSPF routers

-Backup Designated Router (BDR)

This router takes over DRs responsibilities if DR fails


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Introduction to OSPFOSPF 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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Introduction to OSPFOSPF Algorithm

OSPF routers build &maintain link-statedatabase containing LSAreceived from other routers

Information found indatabase is utilized uponexecution of Dijkstra SPFalgorithm

SPF algorithm used tocreate SPF tree

SPF tree used to populaterouting table


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Administrative Distance

Default Administrative Distance for OSPF is 110


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Introduction to OSPF OSPF Authentication

Purpose is to encrypt & authenticate routinginformation

This is an interface specific configuration

Routers will only accept routing information from

other routers that have been configured with thesame password or authentication information


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Basic OSPF Configuration

Lab Topology

Topology used for this chapter

Discontiguous IP addressing scheme

Since OSPF is a classless routing protocol the subnet mask isconfigured in


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The router ospf command

To enable OSPF on a router use the followingcommand

R1(config)#router ospf process- id

Process id A locally significant number between 1and 65535

-this means it does not have to match other OSPFrouters


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Basic OSPF Configuration OSPF network command

-Requires entering: network address

wildcard mask - the inverse of the subnetmask

area-id - area-id refers to the OSPF area.

OSPF area is a group of routers thatshare link state information

-Example: Router(config-router)#networknetwork-addresswildcard-ask areaarea-id


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Router IDThis is an IP address used to identify a router

3 criteria for deriving the router ID

Use IP address configured with OSPF router-id command

-Takes precedence over loopback and physical interfaceaddresses

If router-id command not used then router chooses highestIP address of any loopback interfaces

If no loopback interfaces are configured then the highest IPaddress on any active interface is used


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OSPF Router ID Commands used to verify current router ID

Show ip protocols

Show ip ospf

Show ip ospf interface


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OSPF Router ID

Router ID & Loopback addresses

-Highest loopback address will be used as router ID if router-idcommand isnt used

-Advantage of using loopback addressthe loopback interface cannot fail OSPF stability

The OSPF router-id command

Introduced in IOS 12.0

Command syntaxRouter(config)#router ospfprocess-id

Router(config-router)#router-idip-address

Modifying the Router ID

Use the command Router#clear ipospf process


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Basic OSPF ConfigurationVerifying OSPF

Use the show ip ospf command to verify & troubleshoot OSPF networks

Command will display the following:

Neighbor adjacency

-No adjacency indicated by -Neighboring routers Router ID is not displayed

A state of fullis not displayed

-Consequence of no adjacency-

No link state information exchangedInaccurate SPF trees & routing tables


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Command Description

Show ip protocols

DisplaysOSPF process ID,

router ID, networks router isadvertising & administrativedistance

Show ip ospf

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

Show ip ospf interfaceDisplayshello interval and deadinterval

Verifying OSPF - Additional Commands


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Examining the routing table

Use the show ip route command to display the routingtable

-An O at the beginning of a route indicates that the routersource is OSPF

-Note OSPF does not automatically summarize at majornetwork boundaries


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OSPF Metric

OSPF uses costas the metric for determining thebest route

-The best route will have the lowest cost

-Cost is based on bandwidth of an interface

Cost is calculated using the formula

108/ bandwidth

-Reference bandwidth

defaults to 100Mbpscan be modified using

auto-cost reference-bandwidth command


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OSPF Metric COST of an OSPF route

Is the accumulated value from one router to the next


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OSPF Metric Usually the actual speed of a link is differentthan the

default bandwidthThis makes it imperative that the bandwidth value reflectslinks actual speed

Reason: so routing table has best path information

The show interfacecommand will display interfaces

bandwidth-Most serial link default to 1.544Mbps


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Basic OSPF ConfigurationModifying the Cost of a link

Both sides of a serial link should be configured with thesame bandwidth

Commands used to modify bandwidth value

Bandwidthcommand

Example: Router(config-if)#bandwidthbandwidth-kbpsip ospf costcommandallows you to directly specifyinterface cost

-Example:R1(config)#interface serial 0/0/0

R1(config-if)#ip ospf cost 1562


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Modifying the Cost of the link Difference between bandwidth command & the ip ospf

cost command

Ip ospf costcommand

Sets cost to a specific value

Bandwidthcommand

Link cost is calculated


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OSPF and Multiaccess NetworksChallenges in Multiaccess Networks

OSPF defines five network types:

Point-to-point

Broadcast Multiaccess

Nonbroadcast Multiaccess (NBMA)

Point-to-multipoint

Virtual links


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OSPF in Multiaccess Networks

2 challenges presented by multiaccess networksMultiple adjacencies

Extensive LSA flooding


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Extensive flooding of LSAsFor every LSA sent out there must be an acknowledgement ofreceipt sent back to transmitting router.

consequence: lots of bandwidth consumed and chaotic traffic


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Solution to LSA flooding issueis the use of

Designated router (DR)

Backup designated router (BDR)

DR & BDR selectionRouters are elected to send &receive LSA

Sending & Receiving LSA

DRothers send LSAs viamulticast 224.0.0.6 to DR & BDR

DR forward LSA via multicastaddress 224.0.0.5 to all otherrouters


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DR/BDR Election Process

DR/BDR elections DO NOToccur in point to pointnetworks


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DR/BDR elections will take place on multiaccessnetworksas shown below


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Criteria for getting elected DR/BDR1. DR:Router with the highestOSPF

interface priority.

2. BDR: Router with the second highestOSPF interface priority.

3. If OSPF interface priorities are equal, thehighest router IDis used to break the tie.


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Timing of DR/BDR Election

Occurs as soon as 1strouter has its interface enabled onmultiaccess network

When a DR is elected it remains as the DR until one of thefollowing occurs

-The DR fails.

-The OSPF process on the DR fails.

-The multiaccess interface on the DR fails.


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Manipulating the election process

-If you want to influence the election of DR & BDR then do oneof 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 noshutdownon the DR, then the BDR, and then all otherrouters.


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OSPF in Multiaccess NetworksOSPF Interface Priority

Manipulating the DR/BDR election process continued

Use the ip ospf priority interfacecommand.

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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More OSPF Configuration

Redistributing an OSPF Default Route

Topology includes a link to ISP

Router connected to ISP

Called an autonomous system border routerUsed 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 originatecommand

Example of default-information originate command

R1(config-router)#default-information originate


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Fine-Tuning OSPF

Since link speeds are gettingfaster it may be necessary tochange reference bandwidthvalues

Do this using the auto-costreference-bandwidthcommand

Example:

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


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Fine-Tuning OSPF

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 betweenneighbors


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Summary

RFC 2328 describes OSPF link state concepts and

operations

OSPF Characteristics

A commonly deployed link state routing protocol

Employs DRs & BDRs on multi-access networks

DRs & BDRs are elected

DR & BDRs are used to transmit and receive LSAs

Uses 5 packet types:

1: HELLO

2: DATABASEDESCRIPTION

3: LINKSTATE REQUEST

4: LINK STATE UPDATE

5: LINKSTATEACKNOWLEDGEMENT


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Summary

OSPF CharacteristicsMetric = cost

Lowest cost = best path

Configuration

Enable OSPF on a router using the following command

R1(config)#router ospf process- id

use the network command to define which interfaces willparticipate in a given OSPF process

Router(config-router)#networknetwork-addresswildcard-mask areaarea-id


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Summary

Verifying OSPF configuration

Use the following commands

show ip protocol

show ip route

show ip ospf interface

show ip ospf neighbor


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Exploration Routing Chapter 11