Introduction to OSPF - Internet Societyws.edu.isoc.org/workshops/2004/CEDIA2/material/b2-1up.pdf ·...

transcript

Introduction to OSPFIntroduction to OSPFIntroduction to OSPF

ISP/IXP WorkshopsISP/IXP WorkshopsISP/IXP Workshops

OSPFOSPFOSPF

• Dynamic Routing Protocol

• Link State technology

• Runs over IP, protocol 89

• Designed by IETF for TCP/IP

• Supports VLSM

• Multi-vendor

• Fast rerouting

OSPF (continued)OSPF (continued)OSPF (continued)

• Minimises routing protocol traffic

• Low bandwidth requirements

• Supports different types of areas

• Route summarisation and authentication

• Cisco’s implementation is fully compliantwith the specification OSPF v2.

Link StateLink State

Topology Information IsKept in a DatabaseSeparate from the

Routing Table

AABBCC

2213131313

QQZZXX

X’s Link State

Z’s Link State

Q’s Link State

Link State RoutingLink State Routing

• Neighbour discovery

• Constructing an LSP

• Distribute LSP

• Compute routes

• On network failureNew LSPs flooded

All routers recompute routing tables

FDDIDual Ring

Low Bandwidth UtilisationLow Bandwidth Utilisation

• Only changes propagated

• Multicast on multi-access broadcastnetworks

FDDI Dual Ring

Optimal Path UtilisationOptimal Path Utilisation

Cost = 1 Cost = 1

Cost = 10

The optimal path is determined by thesum of the interface costs

Fast ConvergenceFast Convergence

• Detection Plus LSA/SPF

XR1 R3

Alternate Path

Primary Path

Fast ConvergenceFast Convergence

• Finding a new route

LSA flooded throughout area

Acknowledgement based

Topology databasesynchronised

Each router derives routingtable to destination networks

Utilises IP Multicast forUtilises IP Multicast forSending/Receiving UpdatesSending/Receiving Updates

• Broadcast networksDR and BDR —> AllSPFRouters (224.0.0.5)

All other routers —> AllDRRouters (224.0.0.6)

• Hello packets sent to AllSPFRouters(Unicast on point-to-point and virtuallinks)

OSPF AreasOSPF Areas

• Group of contiguoushosts and networks

• Per area topologicaldatabase

Invisible outside the area

Reduction in routing traffic

• Backbone areacontiguous

All other areas must beconnected to the backbone

• Virtual Links

Area 1Area 4

Area 0Backbone Area

Area 2 Area 3

Classification of RoutersClassification of Routers

• Internal Router (IR)

• Area Border Router(ABR)

• Backbone Router (BR)

• Autonomous SystemBorder Router (ASBR)

Area 1

IR/BRArea 0

Area 2 Area 3

ABR/BR

To other AS

OSPF Route TypesOSPF Route Types

Intra-area Route

all routes inside an area

Inter-area Route

routes advertised from one areato another by an Area BorderRouter

External Route

routes imported into OSPF fromother protocol or static routes

Area 0Area 2 Area 3

To other AS

Inter-Area RouteInter-Area RouteSummarisationSummarisation

• Prefix or all subnets

• Prefix or all networks

• ‘Area range’ command

1.A 1.B 1.C

FDDIDual Ring

R1 (ABR)

Network1

Next HopR1

Network1.A1.B1.C

Next HopR1R1R1

With summarisation

Withoutsummarisation

BackboneArea 0

Area 1

External RoutesExternal Routes

• Redistributed into OSPF

• Flooded unaltered throughout the AS

• OSPF supports two types of externalmetrics

Type 1 external metrics

Type 2 external metrics (Default)

RIPIGRPEIGRPBGPetc.

Redistribute

• Type 1 external metric: metrics are addedto the summarised internal link cost

NetworkN1N1

Type 11110

Next HopR2R3

Cost = 10

to N1External Cost = 1

to N1External Cost = 2R2

Cost = 8

Selected Route

• Type 2 external metric: metrics are comparedwithout adding to the internal link cost

NetworkN1N1

Type 212

Next HopR2R3

Cost = 10

to N1External Cost = 1

to N1External Cost = 2R2

Cost = 8

Selected Route

Topology/Link State DatabaseTopology/Link State Database

• A router has a separate LS databasefor each area to which it belongs

• All routers belonging to the samearea have identical database

• SPF calculation is performed separatelyfor each area

• LSA flooding is bounded by area

Protocol FunctionalityProtocol Functionality

• Bringing up adjacencies

• LSA types

• Area classification

The Hello ProtocolThe Hello Protocol

• Responsible for establishing and maintainingneighbour relationships

• Elects designated router on multi-accessnetworks

FDDIDual Ring

HelloHello

The Hello PacketThe Hello Packet

• Router priority

• Hello interval

• Router deadinterval

• Network mask

• Options: T-bit, E-bit

• List of neighbours

FDDIDual Ring

HelloHello

Designated RouterDesignated Router

• One per multi-access network

Generates network links advertisements

Assists in database synchronization

Designated Router

BackupDesignated Router

BackupDesignated

Router

Designated Router by PriorityDesignated Router by Priority

• Configured priority (per interface)

• Else determined by highest router ID

Router ID is the loopback interface address, ifconfigured, otherwise 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

Neighbouring StatesNeighbouring States

• 2-way

Router sees itself in other Hello packets

DR selected from neighbours in state2-way or greater

DR BDR

Neighbouring StatesNeighbouring States

• FullRouters are fully adjacent

Databases synchronised

Relationship to DR andBDR

DR BDR

When to Become AdjacentWhen to Become Adjacent

• Underlying network is point to point

• Underlying network type is virtual link

• The router itself is the designated router

• The router itself is the backup designatedrouter

• The neighbouring router is the designatedrouter

• The neighbouring router is the backupdesignated router

LSAsLSAs Propagate Along Propagate AlongAdjacenciesAdjacencies

• LSAs acknowledged alongadjacencies

DR BDR

Routing Protocol PacketsRouting Protocol Packets

• Share a common protocol header

• Routing protocol packets are sent with typeof service (TOS) of 0

• Five types of OSPF routing protocol packetsHello - packet type 1

Database description - packet type 2

Link-state request - packet type 3

Link-state update - packet type 4

Link-state acknowledgement - packet type 5

Different Types ofDifferent Types of LSAs LSAs

• Five distinct type of LSAs

Type 1 : Router LSA

Type 2 : Network LSA

Type 3 and 4: Summary LSA

Type 5 and 7: External LSA

Router LSA (Type 1)Router LSA (Type 1)

• Describes the state and cost of therouter’s links to the area

• All of the router’s links in an area must bedescribed in a single LSA

• Flooded throughout the particular areaand no more

• Router indicates whether it is an ASBR,ABR, or end point of virtual link

Network LSA (Type 2)Network LSA (Type 2)

• Generated for every transit broadcastand NBMA network

• Describes all the routers attached to thenetwork

• Only the designated router originatesthis LSA

• Flooded throughout the area and nomore

Summary LSA (Type 3 and 4)Summary LSA (Type 3 and 4)

• Describes the destination outside thearea but still in the AS

• Flooded throughout a single area

• Originated by an ABR

• Only intra-area routes are advertisedinto the backbone

• Type 4 is the information about theASBR

External LSA (Type 5)External LSA (Type 5)

• Defines routes to destination externalto the AS

• Default route is also sent as external

• Two types of external LSA:E1: Consider the total cost up to the externaldestination

E2: Considers only the cost of the outgoinginterface to the external destination

Not Summarised: Specific LinksNot Summarised: Specific Links

BackboneArea #0

External links

TokenRing

1.A1.B1.C1.D

3.A3.B3.C3.D

2.A2.B2.C

TokenRing

• Specific link LSA advertised out• Link state changes propagate out ASBR

Summarised: Summary LinksSummarised: Summary Links

BackboneArea #0

External links

TokenRing

• Only summary LSA advertised out• Link state changes do not

propagate

BackboneArea #0

External links

TokenRing

Not Summarised: Specific LinksNot Summarised: Specific Links

2.A2.B2.C3.A3.B3.C3.D

1.A1.B1.C1.D3.A3.B3.C3.D

1.A1.B1.C1.D2.A2.B2.C

• Specific link LSA advertised in• Link state changes propagate in

Summarised: Summary LinksSummarised: Summary Links

BackboneArea #0

• Only summary LSA advertised in• Link state changes do not

propagate ASBR

External links

TokenRing

Regular Area (Not a Stub)Regular Area (Not a Stub)

From area 1’s viewpoint

• Summary networks from other areas injected• External networks injected, for example

network X.1ASBR

External Networks

1.DTokenRing

TokenRing

1,2X.1

2.D2.C

TokenRing

Normal Stub AreaNormal Stub Area

• Summary networks from other areas injected

• Default network injected into the area - represents external links

• Default path to closest area border router

• Define all routers in the area as stubarea x stub command ASBR

External Networks

1.DTokenRing

TokenRing

2,3 & Default

1,2X.1

2.D2.C

TokenRing

Totally Stubby AreaTotally Stubby Area

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

• Default path to closest area border router

• Define all routers in the area as totally stubbyarea x stub no-summary command ASBR

External Networks

1.DTokenRing

TokenRing

Default 2&3

1,2X.1

2.D2.C

TokenRing

Not-So-Stubby AreaNot-So-Stubby Area

• Capable of importing external routes in a limitedfashion

• Type-7 LSA’s carry external information within anNSSA

• NSSA Border routers translate selected type-7LSAs into type-5 external network LSAs ASBR

External Networks

1.DTokenRing

TokenRing

Default 2&3

1,2X.1

X.1, X.2

X.1, X.2X.1

2.D2.C

3.DExternalNetworks

AddressingAddressing

Area 1network 131.108.0.0subnets 17-31range 255.255.240.0

Area 0network 192.117.49.0range 255.255.255.0

Assign contiguous ranges of subnets per area to facilitate summarisation

SummarySummary

• Scalable OSPF Network Design

Area hierarchy

Stub areas

Contiguous addressing

Route summarisation

Redistributing Routes intoRedistributing Routes intoOSPFOSPF

ROUTER OSPF <pid#x>

REDISTRIBUTE {protocol} <as#y>

<metric-type (1 or 2)

Router Sub-commandsRouter Sub-commands

• NETWORK <n.n.n.n> <mask> AREA <area-id>

• AREA <area-id> STUB {no-summary}

• AREA <area-id> AUTHENTICATION

• AREA <area-id> DEFAULT_COST <cost>

• AREA <area-id> VIRTUAL-LINK <router-id>...

• AREA <area-id> RANGE <address mask>

Interface SubcommandsInterface Subcommands

• IP OSPF COST <cost>

• IP OSPF PRIORITY <8-bit-number>

• IP OSPF HELLO-INTERVAL <number-of-seconds>

• IP OSPF DEAD-INTERVAL <number-of-seconds>

• IP OSPF AUTHENTICATION-KEY <8-bytes-of-password>

Introduction to OSPF - Internet Societyws.edu.isoc.org/workshops/2004/CEDIA2/material/b2-1up.pdf ·...

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