03 Introduction to OSPF v2

8/3/2019 03 Introduction to OSPF v2

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Campus Networking Workshop

Introduction to OSPF


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Agenda

Basic Elements of OSPF

OSPF in Service Provider Networks

Best Common Practices in OSPF Network Aggregation

OSPF Command Reference


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Basic Elements of OSPF


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OSPF

Open ShortestPath First

Link State or SPFtechnology

Developed by the IETFsOSPF working group (RFC1247)

Designed for TCP/IP Fast Convergence

Variable length netmasks Non-contiguous subnets No need for periodic updates

Route authentication OSPF is defined in RFC2328


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Link-State

Topology information is

stored in a DB separate

from the routing table

AA

BB

CC

22

1313

1313

QQ

ZZ

XX

Xs Link-state

ZZ

XX

YYQQ

Zs Link-state

Qs Link-state


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Link-State Routing

Neighbor discovery

Construct a Link State Packet (LSP)

Distribute the LSP

Link State Announcement LSA

Route calculation

If a link fails

Flood new LSPs All routers recalculate their routing tables


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FDDI

Dual Ring

Low Bandwidth Utilization

Only propagate changes

Use Multicast in multi-access networks

R1

LSA

X

LSA


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FDDIDual Ring

FDDIDual Ring

Using the Optimal PathThe optimal path is determined by adding thecosts of the interfaces : Cost = 10^8/(Bandwidth)

N1

N2 N3

N4

N5R1

R2

R3

R4

Cost = 1 Cost = 1

Cost = 10

Cost = 10


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Fast Convergence

Detection plus LSA/SPF

XR1 R3

R2

N2

Alternate Path

Primary Path

N1


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Fast Convergence

Finding a new path

Flood LSAs in the area

Based in acknowledgements(Ack)

Synchronized topology DB

Each router calculates its routing

table for each destination network

LSA

XR1

N1


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Uses IP Multicast to

Send/Receive changes

Multi-Access networks

All routers must accept packets sent to the AllSPFRouters

(224.0.0.5) address

All DR and BDR routers must accept packets sent to the

AllDRouters (224.0.0.6) address

Hello packets are sent to the AllSPFRouters address

(Unicast for point-to-point and virtual links)


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OSPF Areas

Group of contiguousnodes/networks

Per area topology DB Invisible outside the area

Reduces routing traffic

Backbone Area is contiguous All others areas must connect to

the backbone

Virtual Links

Area 1

Area 4

Area 0

Backbone Area

Area 2 Area 3


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Router Classification

Internal Router (IR)

Area Border Router(ABR)

Backbone Router (BR)

Autonomous SystemBorder Router (ASBR)

Area 1

IR/BR

Area 0

Area 2 Area 3

IR

ABR/BR

To another AS

ASBR


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OSPF Route Types

Intra-Area Route All routes within an area

Inter-Area Route Routes announced from area to

another by an ABR

External Route Routes imported into OSPF from

another protocol or Static routes

Area 0Area 2 Area 3

ABR

To Another AS

ASBR


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Inter-Area Route Summarization

Prefix or all subnets

Prefix or all networks

Area range command

1.A 1.B 1.C

FDDI

Dual Ring

R1 (ABR)

R2

Network

1

Next Hop

R1

Network

1.A

1.B

1.C

Next Hop

R1

R1

R1

With

Summarization

Without

Summarization

Backbone

Area 0

Area 1


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External Routes

Redistributed into OSPF

Flooded without changes throughout the AS

OSPF supports two type of external metrics

Type 1 Type 2 (Default)

RIP

IGRP

EIGRP

BGP

etc.

OSPF

Redistribute


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External Routes

Type 1 external metric: metrics are added to

the internal link cost

Network

N1

N1

Type 1

11

10

Next Hop

R2

R3

Cost = 10

To N1

External Cost = 1

To N1

External Cost = 2R2

R3

R1

Cost = 8

Selected

Route


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External Routes Type 2 external metric: metrics are compared

without adding the internal link cost

Network

N1

N1

Type 2

1

2

Next Hop

R2

R3

Cost = 10

To N1

External Cost = 1

To N1

External Cost = 2R2

R3

R1

Cost = 8

Selected Route


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Topology/Links-State DB

A router has a separate DB for each area it

belongs

All routers within an area have an identical DB SPF calculation is done separately for each area

LSA flooding is limited to the particular area


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Protocol Functionality

Bringing up adjacencies

LSA Types

Area Classification


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The Hello Protocol

Responsible to establish and maintain neighbor

relationships

Elects designated router in multi-access

networks

FDDIDual Ring

Hello

HelloHello


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The Hello Packet

Router Priority

Hello Interval

Router dead interval Network mask

Options: T-bit, E-bit

List of neighbors

FDDI

Dual Ring

Hello

HelloHello


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Designated Router (DR)

y One per multi-access network

Generates network links advertisements

Assists in DB synchronization

DesignatedRouter

Designated

Router

Backup

Designated

Router

BackupDesignated

Router


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Designated Router by Priority

Configured priority (per interface)

Otherwise determined by the highest router ID

The router ID is the loopback interface address, inconfigured otherwise is the highest IP address

144.254.3.5

R2 Router ID = 131.108.3.3

131.108.3.2 131.108.3.3

R1 Router ID = 144.254.3.5

DR


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Neighbor States

2-way

The router sees itself in other Hello packets

DR is selected from neighbors in state 2-way or greater

DR BDR

2-way


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Neighbor States

Full Routers are fully adjacent

DB is synchronized Relationship to the DR and

BDR

DR BDR

Full


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When to Become Adjacent

Underlying network is point-to-point

Underlying network type is virtual link

The router itself is the DR

The router itself is the BDR

The neighboring router is the DR

The neighboring router is the BDR


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LSAs Propagate Along Adjacencies

LSAs acknowledged along adjacencies

DR BDR


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Routing Protocol Packets

Share a common protocol header Routing protocol packets are sent with a TOS of 0 Five types of OSPF routing protocol packets

Hello packet type 1 DB Description packet type 2 Link-state request packet type 3 Link-state update packet type 4 Link-state Acknowledgment packet type 5


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Different Types of LSAs

Five LSA types

Type 1 : Router LSA

Type 2 : Network LSA Type 3 y 4: Summary LSA

Type 5 y 7: External LSA


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Router LSA (Type 1)

Describes the state and cost of the routers link to

the area All the routers links in an area must be described

in a single LSA

Flooded throughout the particular area and not

beyond Router indicates whether it is an ASBR, ABR, or

the end point of a virtual link


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Network LSA (Type 2)

Generated for every transit broadcast or NBMA

network

Describes all the routers attached to thenetwork

Only the DR originates this type of LSA

Flooded throughout the area and not beyond


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Summary LSA (Type 3 y 4)

Describes a destination outside the area but

still within the AS

Flooded throughout a single area

Originated by an ABR

Only intra-area routes are advertised into

the backbone (Area 0) Type 4 is the information about the ASBR


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External LSA (Type 5)

Defines routes to destinations outside the AS

Default route is also sent as external

Two Types of external LSA: E1: Considers the total cost of to the external destination

E2: Considers only the cost of the outgoing interface to theexternal destination


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Not Summarized: Specific Link

Specific link LSA advertised out Link state changes propagate out

Backbone

Area #0

External Links

1.A

1.C

1.B

1.D

TokenRing

TokenRing

TokenRing

TokenRing

3.D

3.A

3.C

3.B

1.A1.B1.C1.D

3.A3.B3.C3.D

2.A2.B2.C

2.A

2.C

2.B

TokenRing

TokenRing

ASBR


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Summarized: Summary Links

Only Summary LSA advertised out

Link State changes do not propagate

Backbone

Area #0

ASBR

External Links

1.A

1.C

1.B

1.D

TokenRing

TokenRing

TokenRing

TokenRing

3.D

3.A

3.C

3.B

2.A

2.B

TokenRing

TokenRing

1 3

2


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Backboen

Area #0

External Links

1.A

1.C

1.B

1.D

TokenRing

TokenRing

TokenRing

TokenRing

3.D

3.A

3.C

3.B

2.A

2.C

2.B

TokenRing

TokenRing

ASBR

Not Summarized: Specific Links

Specific Link LSA advertised in Links state changes propagate in

2.A

2.B

2.C3.A

3.B

3.C

3.D

1.A1.B

1.C

1.D

3.A

3.B

3.C

3.D

1.A

1.B

1.C

1.D2.A

2.B

2.C


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ASBR

External Links

1.A

1.C

1.B

1.DTokenRing

TokenRing

TokenRing

TokenRing

3.C

3.B

2.ATokenRing

TokenRing

2,3

1,3

1,2

Summarized: Summary Links

Specific Link LSA advertised in Link state changes propagate in

Backbone

Area #0

3.D

3.A

2.B


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TokenRing

TokenRing

TokenRing

TokenRing

Regular Area (Not a stub)

From area 1s point of view

Summary networks from other areas injected

External networks injected, for examplenetwork X.1

ASBRExternal Networks

1.A

1.C

1.B

1.DTokenRing

TokenRing

3.C

3.B

2.A

2,3

1,3

1,2

X.1

X.1

X.1X.1

2.D

2.C

2.B

3.A

3.D


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TokenRing

TokenRing

TokenRing

TokenRing

Normal Stub Area

From area 1s point of view Summary networks from other areas injected

Default route injected into the area represent externallinks

Default path to closest ABR

Define all routers in the area as stub area x stub command ASBRExternal Networks

1.A

1.C

1.B

1.DTokenRing

TokenRing

3.C

3.B

2.A

2,3 & Default

1,3

1,2

X.1

X.1

X.1

X.1

2.D

2.C

2.B

3.A

3.D


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TokenRing

TokenRing

TokenRing

TokenRing

Totally Stubby Area

From area 1s point of view

Only a default network is injected into the area Represents external networks and all inter-area routes

Default route to the closest ABR

Define all routers in the area as totally stubby area x stub no-summary command

ASBR

External Networks

1.A

1.C

1.B

1.DTokenRing

TokenRing

3.C

3.B

2.A

Default 2&3

1,3

1,2

X.1

X.1

X.1

X.1

2.D

2.C

2.B

3.A

3.D


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TokenRing

TokenRing

TokenRing

TokenRing

Not-So-Stubby Area

Capable of importing external routes in a limitedfashion

Type-7 LSAs carry external information within anNSSA

NSSA border routers translate selected type-7

LSAs into type -5 external network LSAsASBR

External Networks

1.A

1.C

1.B

1.DTokenRing

TokenRing

3.C

3.B

2.A

Default 2&3

1,3

1,2

X.1

X.1, X.2

X.1, X.2X.1

2.D

2.C

2.B

3.A

3.D

External

NetworksX.2


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Addressing

Area 1

Network 131.108.0.0

Subnets 17-31

Range 255.255.240.0

Area 2

Network 131.108.0.0

Subnets 33-47

Range 255.255.240.0

area 3

Network 131.108.0.0

Subnets 49-63

Range 255.255.240.0

Area 0

Network 192.117.49.0

Range 255.255.255.0

Try to assign contiguous subnet ranges to facilitate summarization


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Summary

Scalable OSPF Network Design

Area hierarchy

Stub areas

Contiguous addressing

Route summarization


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OSPF Design Service

Provider Networks


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Backbone

Router

OSPF Areas and Rules

Backbone area(0) must exist

All other areasmust have

connection tobackbone

Backbone mustbe contiguous

Do not partitionarea (0)

Area 1

Area 4

Area 0

Area 2 Area 3

Ruteador

Interno

Area

Border

Router

Autonomous

System (AS)

Border Router

Internet


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OSPF Design

Figure out your addressing first OSPFand addressing go together The objective is to maintain a small link-state

DB Create address hierarchy to match the

network topology

Separate blocks for infrastructure, customer

interfaces, customers, etc.


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OSPF Design

Examine the physical topology

Is it meshed or hub-and-spoke (star)

Try to use as Stubby an area as possible

It reduces overhead and LSA counts

Push the creation of a backbone

Reduces mesh and promotes hierarchy


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OSPF Design

One SPF per area, flooding done per area

Try not to overload the ABRs

Different types of areas do different flooding

Normal areas

Stub areas

Totally stubby (stub no-summary)

Not so stubby areas (NSSA)


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OSPF Design

Redundancy Dual links out of each area using metrics (cost) for traffic

engineering Too much redundancy

Dual links to backbone in stub areas must be the same otherwise sub-optimal routing will result

Too much redundancy in the backbone area without

good summarization will affect convergence in the area0


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OSPF for ISPs

OSPF features you should consider:

OSPF logging neighbor changes OSPF reference cost

OSPF router ID command

OSPF Process Clear/Restart


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OSPF Best Common

Practices Adding Networks


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OSPF Network Aggregation

BCP Individual OSPF networkstatement for each infrastructurelink Have separate IP address blocks for

infrastructure and customer links

Use IP unnumbered interfaces orBGP to carry /30 to customers

OSPF should only carryinfrastructure routes in an ISPsnetwork

OC12c

OC12c

Customer Connections

OC48

ISP Backbone


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OSPF Adding Networks

Redistribute connected subnet

Works for all connected interfaces on the

router but sends networks as external types-

2s which are not summarized router ospf 100

redistribute connected subnets

Not recommended


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Specific network statements

Each interface requires an OSPF network

statement. Interfaces that should not betbroadcasting Hello packets need a passive-

interface statement router ospf 100

network 192.168.1.1 0.0.0.3 area 51 network 192.168.1.5 0.0.0.3 area 51

passive interface Serial 1/0


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Network statements - wildcard mask Every interface covered by a wildcard mask used

in the OSPF network statement. Interfaces thatshould not be broadcasting Hello packets need apassive-interface statement ordefault passive-interface should be used router ospf 100

network 192.168.1.0 0.0.0.255 area 51 default passive-interface default

no passive interface POS 4/0


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The key theme when selecting which

method to use is to keep the links-state

DB as small as possible

Increases stability

Reduces the amount of information in the

LSAs

Speeds up convergence time


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OSPF Useful

Features

OSPF Logging Neighbor


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OSPF Logging Neighbor

Changes

The router will generate a log messagewhenever an OSPF neighbor changes state

Syntax: [no] ospf log[no] ospf log--adjacencyadjacency--changeschanges

A typical log message: %OSPF%OSPF--55--ADJCHG: Process 1, NbrADJCHG: Process 1, Nbr

223.127.255.223 on Ethernet0 from LOADING to223.127.255.223 on Ethernet0 from LOADING to

FULL, Loading DoneFULL, Loading Done


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Number of State Changes

The number of state transitions is available viaSNMP (ospfNbrEvents) and the CLI:

show ip ospf neighbor [type number]show ip ospf neighbor [type number][neighbor[neighbor--id] [detail]id] [detail] Detail(Optional) Displays all neighbors given in detail (list

all neighbors).When specified, neighbor state transitioncounters are displayed per interface or neighbor ID


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State Changes (Cont.)

To reset OSPF related statistics, use the clear ipclear ip

ospf countersospf counters EXEC command.

clear ip ospf counters [neighbor [] [neighbor--id]]id]]


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OSPF Cost: Reference Bandwidth

Bandwidth used in metric calculation

Cost = 10^8/BW

Not useful for BW > 100 Mbps but can be

changed Syntax:

ospf autoospf auto--cost referencecost reference--bandwidth

Default reference bandwidth is still100Mbps for

backward compatibility


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OSPF Router ID

If the loopback interface exists and has an IPaddress, that is used as the router ID in routingprotocols - stability!

If the loopback interface does not exist, or has no IPaddress, the router ID is the highest IP addressconfigured danger!

Subcommand to manually set the OSPF router ID :

router-id


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OSPF Clear/Restart

clear ip ospf [pid] redistributionThis command can clear redistribution based on OSPF routingprocess ID. If no PID is given, it assumes all OSPF processes

clear ip ospf [pid] countersThis command clear counters based on OPSF routing processID. If no PID is given, it assumes all OSPF processes

clear ip ospf [pid] processThis command will restart the specified OSPF process. If no PIDis given, it assumes all OSPF processes. It attempts to keep theold router-id, except in cases where a new router-id wasconfigured, or an old user configured router-id was removed. Itrequires user confirmation because it will cause network churn.


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OSPF Command Summary


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Redistributing Routes into OSPF

ROUTER OSPF

REDISTRIBUTE {protocol}


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OSPF Router Sub-Commands

NETWORK AREA

AREA STUB {no-summary}

AREA AUTHENTICATION AREA DEFAULT_COST

AREA VIRTUAL-LINK ...

AREA RANGE


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Interface Sub-Commands

IP OSPF COST

IP OSPF PRIORITY

IP OSPF HELLO-INTERVAL IP OSPF DEAD-INTERVAL

IP OSPF AUTHENTICATION-KEY

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03 Introduction to OSPF v2

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