1

OSPF in Multiple AreasOSPF in Multiple Areas

2 22

Scalability Problems in Large Scalability Problems in Large OSPF AreasOSPF Areas

Scalability problems in large OSPF areas Scalability problems in large OSPF areas includeinclude Large routing tablesLarge routing tables Large topology databasesLarge topology databases Frequent recalculation of Shortest Path First Frequent recalculation of Shortest Path First

(SPF) algorithm(SPF) algorithm Slow synchronizationSlow synchronization

3 33

Advantages of Using Multiple Advantages of Using Multiple AreasAreas

Advantages of using multiple OSPF areas Advantages of using multiple OSPF areas within an Autonomous System includewithin an Autonomous System include Smaller routing tablesSmaller routing tables Less frequent SPF recalculationsLess frequent SPF recalculations Less routing update overheadLess routing update overhead Faster synchronizationFaster synchronization

4 44

Connecting OSPF AreasConnecting OSPF Areas

To connect two OSPF areas, use a router To connect two OSPF areas, use a router with at least one interface in both areaswith at least one interface in both areas

OSPF uses different types of routers and OSPF uses different types of routers and areasareas

5 55

Types of RoutersTypes of Routers

Types of OSPF routers include:Types of OSPF routers include: Internal routersInternal routers Backbone routersBackbone routers Areas Border Routers (ABRs)Areas Border Routers (ABRs) Autonomous System Border Routers (ASBRs)Autonomous System Border Routers (ASBRs)

A router may simultaneously be several types of A router may simultaneously be several types of routersrouters Keep in mind the cost of performing each functionKeep in mind the cost of performing each function

6 66

The Backbone AreaThe Backbone Area

Backbone area is most important area in Backbone area is most important area in any OSPF autonomous systemany OSPF autonomous system It behaves like a standard areaIt behaves like a standard area All other areas connect to it to exchange All other areas connect to it to exchange

routing informationrouting information All packets sent between areas go through itAll packets sent between areas go through it

Backbone area must remain stable and Backbone area must remain stable and reachablereachable

7 77

Other Types of AreasOther Types of Areas

Classified by interaction with each otherClassified by interaction with each other

Standard area – accepts link updates and Standard area – accepts link updates and route summaries internally and externallyroute summaries internally and externally

Stub area – does not accept routing Stub area – does not accept routing information from ASBRs; uses a default information from ASBRs; uses a default route to reach external devicesroute to reach external devices

8 88

Other Types of AreasOther Types of Areas

Totally stubby area – does not accept external Totally stubby area – does not accept external routes and refuses summary routes from other routes and refuses summary routes from other areas; uses a default route; Cisco's proprietary areas; uses a default route; Cisco's proprietary extension to OSPF protocolextension to OSPF protocol

Not-so-stubby-area (NSSA) – accepts limited Not-so-stubby-area (NSSA) – accepts limited information about external routes, but refuses information about external routes, but refuses routing information from ASBRs; useful for routing information from ASBRs; useful for connecting stub area to another Autonomous connecting stub area to another Autonomous SystemSystem

9 99

Not-so-stubby AreaNot-so-stubby Area

10 1010

Routing Between AreasRouting Between Areas

Routing between multiple areas adds Routing between multiple areas adds complexity to OSPFcomplexity to OSPF

Must first understand different types of Must first understand different types of LSAs used to describe topology of a LSAs used to describe topology of a networknetwork

11 1111

Types of LSAsTypes of LSAs

• OSPF uses several types of LSAs to OSPF uses several types of LSAs to describe topology of a networkdescribe topology of a network Router link entry LSA (LSA type 1)Router link entry LSA (LSA type 1) Network link entry LSA (LSA type 2)Network link entry LSA (LSA type 2) Summary link entry LSA (LSA types 3 and 4)Summary link entry LSA (LSA types 3 and 4) Autonomous System external link entry LSA Autonomous System external link entry LSA

(type 5)(type 5) NSSA Autonomous System external link entry NSSA Autonomous System external link entry

LSA (type 7)LSA (type 7)

12 1212

Types of LSAsTypes of LSAs

continued

13 1313

Types of LSAsTypes of LSAs

Type 6 LSA is not included because CISCO routers do not support it

14 1414

Routes Between Multiple AreasRoutes Between Multiple Areas

• If packet’s destination is inside same area If packet’s destination is inside same area as source, it passes through internal as source, it passes through internal routersrouters

• If packet’s destination is outside area of If packet’s destination is outside area of source, it goes to an ABR in its own areasource, it goes to an ABR in its own area ABR sends it through backbone to an ABR in ABR sends it through backbone to an ABR in

destination areadestination area

• OSPF Autonomous System uses a variety OSPF Autonomous System uses a variety of types of routesof types of routes

15 1515

Calculating Routing Tables and Calculating Routing Tables and Flooding in Multiple AreasFlooding in Multiple Areas

ABRs must learn about routes and network ABRs must learn about routes and network changes in other areaschanges in other areas Within an area, each router receives LSUs describing Within an area, each router receives LSUs describing

changes and then sends summary LSAs to any changes and then sends summary LSAs to any attached areasattached areas

When an ABR or ASBR receives a summary LSA, it When an ABR or ASBR receives a summary LSA, it adds information to link-state database and floods adds information to link-state database and floods local areas. Totally stubby areas do not receive LSAs. local areas. Totally stubby areas do not receive LSAs.

After all routers receive summary LSAs, they After all routers receive summary LSAs, they recalculate their routing tablesrecalculate their routing tables

See Figure 4-3See Figure 4-3

16 1616

Virtual LinksVirtual Links

Each area in an OSPF Autonomous Each area in an OSPF Autonomous System must be connected to backboneSystem must be connected to backbone

If no physical link exists, OSPF uses If no physical link exists, OSPF uses virtual link or a logical linkvirtual link or a logical link Connects two ABRs via a common area, Connects two ABRs via a common area,

sometimes called the transit area, and one sometimes called the transit area, and one ABR attached to backboneABR attached to backbone

17 1717

Adding an Area with a Virtual Adding an Area with a Virtual LinkLink

18 1818

Purposes of Virtual LinksPurposes of Virtual Links

• Virtual links have three purposesVirtual links have three purposes Connecting areas to backbone when they Connecting areas to backbone when they

don’t have an ABR with an interface in don’t have an ABR with an interface in backbone areabackbone area

Connecting backbone itself in situations Connecting backbone itself in situations where it has been partitionedwhere it has been partitioned

Adding a redundant connection to backbone Adding a redundant connection to backbone to prevent failure of single link or backbone to prevent failure of single link or backbone area itself area itself

19 1919

Designing AreasDesigning Areas

Factors to consider when designing Factors to consider when designing OSPF areas include:OSPF areas include: Size of an area Size of an area Number of neighbors each router hasNumber of neighbors each router has Problem isolationProblem isolation Restriction of routing informationRestriction of routing information Stable backbone designStable backbone design Addressing areaAddressing area

20 2020

Size of an AreaSize of an Area

• Size of area affects number of routes in Size of area affects number of routes in routing table and frequency of SPF routing table and frequency of SPF calculationscalculations Topology, memory, and processors in router Topology, memory, and processors in router

determine appropriate sizes of areasdetermine appropriate sizes of areasWith high-speed links and stable network, may With high-speed links and stable network, may have areas with up to 100 routershave areas with up to 100 routersIf your network has slow unstable WAN links, If your network has slow unstable WAN links, should use smaller areasshould use smaller areas

Cisco recommends ABR have interfaces in no Cisco recommends ABR have interfaces in no more than three areasmore than three areas

21 2121

Number of Neighbors Per Number of Neighbors Per RouterRouter

Number of neighbors affects performance Number of neighbors affects performance of routerof router A router exchanges hello packets with its A router exchanges hello packets with its

neighbors and link-state databases with those neighbors and link-state databases with those neighbors with which it forms adjacenciesneighbors with which it forms adjacencies

Cisco recommends routers have no more Cisco recommends routers have no more than 60 neighborsthan 60 neighbors

22 2222

Isolating Potential Network Isolating Potential Network ProblemsProblems

OSPF allows you to isolate potential OSPF allows you to isolate potential network problems to prevent them from network problems to prevent them from affecting other areas of the networkaffecting other areas of the network Route flapping on WAN links causes Route flapping on WAN links causes

instability on OSPF networksinstability on OSPF networks Use areas to isolate WAN links Use areas to isolate WAN links

23 2323

Isolating WAN SegmentsIsolating WAN Segments

24 2424

Restricting Routing Information Restricting Routing Information From an AreaFrom an Area

Restricting routing information has several Restricting routing information has several advantagesadvantages Reduces routing table size in an areaReduces routing table size in an area Reduces amount of flooding traffic after Reduces amount of flooding traffic after

topology changetopology change Adds stability by isolating routers inside areaAdds stability by isolating routers inside area

Configure an area as a stub area, a totally Configure an area as a stub area, a totally stubby area, or a not-so-stubby area to stubby area, or a not-so-stubby area to restrict routing informationrestrict routing information

25 2525

Selecting a Stub AreaSelecting a Stub Area

Stub areas likely have only one exit pointStub areas likely have only one exit point

Stub areas work well for branch offices or Stub areas work well for branch offices or hub-and-spoke topologieshub-and-spoke topologies Spoke needs to get to hub and does not need to know Spoke needs to get to hub and does not need to know

about rest of networkabout rest of network

Stub area cannot be backbone area or transit Stub area cannot be backbone area or transit area for a virtual linkarea for a virtual link

No need to have ASBRs within a stub network No need to have ASBRs within a stub network since stubs do not accept external routessince stubs do not accept external routes

26 2626

Totally Stubby AreasTotally Stubby Areas

Totally stubby areas reject external routes Totally stubby areas reject external routes and intra-area routesand intra-area routes

Rely entirely on default routes to reach Rely entirely on default routes to reach destination outside totally stubby areadestination outside totally stubby area

If area consists entirely of Cisco routers If area consists entirely of Cisco routers and has a single exit point, using a totally and has a single exit point, using a totally stubby area allows smaller routing tablesstubby area allows smaller routing tables Totally stubby areas are proprietary to CiscoTotally stubby areas are proprietary to Cisco

27 2727

Not-so-stubby AreasNot-so-stubby Areas

Good for service providerGood for service provider Allows service provider’s network to connect Allows service provider’s network to connect

to customer network while preventing to customer network while preventing customer network from incorporating into customer network from incorporating into OSPF Autonomous SystemOSPF Autonomous System

Good for redistributing other routing Good for redistributing other routing protocols into OSPF Autonomous systemprotocols into OSPF Autonomous system

See Figure 4-7See Figure 4-7

28 2828

Example of NSSAExample of NSSA

29 2929

Designating the Backbone AreaDesignating the Backbone Area

Stable backbone is essential Stable backbone is essential Areas exchange routing information through Areas exchange routing information through

the backbone areathe backbone area

Design of the backbone area affects Design of the backbone area affects stability of entire OSPF Autonomous stability of entire OSPF Autonomous SystemSystem

30 3030

Designing the Backbone AreaDesigning the Backbone Area

• Keep backbone area small to stabilize itKeep backbone area small to stabilize it• Keep backbone area contiguousKeep backbone area contiguous• Provide redundant links between backbone Provide redundant links between backbone

routers whenever possiblerouters whenever possible See Figure 4-8See Figure 4-8

• Redundant links add complexity and cost, Redundant links add complexity and cost, but should be balanced against the cost of but should be balanced against the cost of a partitioned networka partitioned network See Figure 4-9See Figure 4-9

31 3131

Connecting Areas to the Connecting Areas to the Backbone with Virtual LinksBackbone with Virtual Links

• Use virtual links to prevent backbone area Use virtual links to prevent backbone area from being partitioned or for redundancyfrom being partitioned or for redundancy

• Three reasons to avoid virtual linksThree reasons to avoid virtual links Virtual links prevent transit area from being Virtual links prevent transit area from being

used as a stub areaused as a stub area Stability of virtual link depends on stability of Stability of virtual link depends on stability of

transit areatransit area Using virtual links adds complexity to network Using virtual links adds complexity to network

design and makes troubleshooting more design and makes troubleshooting more difficultdifficult

32 3232

Using WAN Links in the Using WAN Links in the Backbone AreaBackbone Area

• In star topology, keep WAN links out of In star topology, keep WAN links out of backbone to prevent flappingbackbone to prevent flapping

• In extended star topology, use WAN links In extended star topology, use WAN links to connect hubsto connect hubs Avoid single links between backbone routersAvoid single links between backbone routers Add redundant links to prevent single point of Add redundant links to prevent single point of

failurefailure

33 3333

Avoid Other Devices in Avoid Other Devices in Backbone AreaBackbone Area

• Avoid placing anything but routers in Avoid placing anything but routers in backbone areabackbone area

• Servers and workstations use bandwidth Servers and workstations use bandwidth needed for routing traffic and can cause needed for routing traffic and can cause network to become unstablenetwork to become unstable Malfunctioning network interface card or Malfunctioning network interface card or

IP address can duplicate router’s interface IP address can duplicate router’s interface IP address, causing serious problemsIP address, causing serious problems

Other devices in backbone make Other devices in backbone make troubleshooting more difficulttroubleshooting more difficult

34 3434

Addressing with Multiple AreasAddressing with Multiple Areas

Address selection determines how well Address selection determines how well OSPF Autonomous System scales with OSPF Autonomous System scales with growthgrowth

Without route summarization, non-Without route summarization, non-backbone change requires routers in backbone change requires routers in backbone to recalculate routing tablesbackbone to recalculate routing tables

Minimize number of routes by carefully Minimize number of routes by carefully using route summarizationusing route summarization

35 3535

Route Summarization in OSPFRoute Summarization in OSPF

OSPF requires each router in an area to have OSPF requires each router in an area to have same knowledge of networksame knowledge of network

Two consequences for route summarizationTwo consequences for route summarization Routes cannot be summarized within an Routes cannot be summarized within an

area; would create different views of topologyarea; would create different views of topology Summarization of routes from an area must Summarization of routes from an area must

occur at ABRs since ABRs advertise routes occur at ABRs since ABRs advertise routes to other areasto other areas

36 3636

Summarizing routesSummarizing routes

Summarize external routes with an ASBR or an Summarize external routes with an ASBR or an ABRABR

Summarize only contiguous blocks of IP Summarize only contiguous blocks of IP addressesaddresses

Summarize routes on ABRs connecting non-Summarize routes on ABRs connecting non-backbone areas to backbonebackbone areas to backbone

Route summarization is useful for large Route summarization is useful for large networks,networks, Be sure all summarized routes are in same areaBe sure all summarized routes are in same area

37 3737

Summarizing Routes Summarizing Routes

38 3838

Limits of Route SummarizationLimits of Route Summarization

Route summarization with OSPF has limitsRoute summarization with OSPF has limits

Can lead to poor path selectionCan lead to poor path selection

May generate unnecessary traffic through May generate unnecessary traffic through backbonebackbone

Does not allow you to summarize groups Does not allow you to summarize groups of areasof areas

39 3939

Addressing AreasAddressing Areas

Consider numbering OSPF areas in Consider numbering OSPF areas in contiguous blockscontiguous blocks Allows use of route summarization without Allows use of route summarization without

having to readdress networkhaving to readdress network Basing addressing on topology of network Basing addressing on topology of network

or geographic location works wellor geographic location works well Basing addressing on other schemes, such Basing addressing on other schemes, such

as department structure of organization, can as department structure of organization, can lead lead to problemsto problems

40 4040

Addressing AreasAddressing Areas

• Assigning address blocks at an octet Assigning address blocks at an octet boundary makes them easy to remember boundary makes them easy to remember and configureand configure

• Keep thorough and accurate Keep thorough and accurate documentationdocumentation

• Avoid overlapping subnets or assigning Avoid overlapping subnets or assigning subnets in discontiguous blockssubnets in discontiguous blocks

• Assign IP addresses in contiguous blocks Assign IP addresses in contiguous blocks based on router to which each subnet is based on router to which each subnet is attached to facilitate route summarizationattached to facilitate route summarization

41 4141

Configuring OSPF in Multiple Configuring OSPF in Multiple AreasAreas

Similar to configuring OSPF in a single Similar to configuring OSPF in a single area, but adds layer of complexityarea, but adds layer of complexity

Activating multiple areas on an ABRActivating multiple areas on an ABR Use router ospf statement to activate OSPFUse router ospf statement to activate OSPF Use network statement to put each interface Use network statement to put each interface

on the ABR into appropriate areaon the ABR into appropriate area

42 4242

Activating Multiple Areas on Activating Multiple Areas on an ABRan ABR

43 4343

Configuring Stub AreasConfiguring Stub Areas

Each router in stub area must be Each router in stub area must be specifically configured as stub routerspecifically configured as stub router Sets Stub Flag in hello packetsSets Stub Flag in hello packets If Stub Flag does not match on two If Stub Flag does not match on two

neighboring routers, they cannot become neighboring routers, they cannot become neighborsneighbors

Use area area-number command with stub Use area area-number command with stub keywordkeyword

Router(config-router) area 6 stubRouter(config-router) area 6 stub

44 4444

Configuring Totally Stubby Configuring Totally Stubby AreasAreas

ABR is logical place to configure a totally stubby ABR is logical place to configure a totally stubby areaarea

Use area area-number command with stub and Use area area-number command with stub and no-summary keywordsno-summary keywords No-summary keyword must be used on each ABR in No-summary keyword must be used on each ABR in

a totally stubby areaa totally stubby area

Configure each router in totally stubby area with Configure each router in totally stubby area with area area-number stub commandarea area-number stub command

Router(config-router) area 6 stub no-summaryRouter(config-router) area 6 stub no-summary

45 4545

Configuring Not-so-stubby Configuring Not-so-stubby AreasAreas

Each router in NSSA must be specifically Each router in NSSA must be specifically configured to be part of NSSAconfigured to be part of NSSA Use area area-number command with Use area area-number command with

keyword nssakeyword nssa Router(config-router) area 6 nssaRouter(config-router) area 6 nssa

46 4646

Default RoutesDefault Routes

Configuring default routes varies from one area Configuring default routes varies from one area to anotherto another In normal areas, use default-information originate In normal areas, use default-information originate

commandcommand Can use router with a default route to propagate Can use router with a default route to propagate

default route to rest of OSPF Autonomous Systemdefault route to rest of OSPF Autonomous System Add Add alwaysalways keyword to configure a default route if keyword to configure a default route if

router does not have onerouter does not have oneCan configure type of external link, cost of default route, and Can configure type of external link, cost of default route, and identify a route map as default routeidentify a route map as default route

47 4747

Default-information Originate Default-information Originate CommandCommand

48 4848

Default RoutesDefault Routes

In stub and totally stubby areas, ABRs In stub and totally stubby areas, ABRs automatically generate default route using a automatically generate default route using a summary LSAsummary LSA No need to use default-information originate commandNo need to use default-information originate command

In NSSA, an ABR must be forced to generate a In NSSA, an ABR must be forced to generate a default route when you configure it with area default route when you configure it with area commandcommand Use default-information originate keywordUse default-information originate keyword Use no-redistribution keyword to control route Use no-redistribution keyword to control route

redistributionredistribution

49 4949

Route SummarizationRoute Summarization

Configuring route summarization in OSPF Configuring route summarization in OSPF depends on whether you are summarizing depends on whether you are summarizing intra-area routers or external routersintra-area routers or external routers To summarize intra-area routers, use area To summarize intra-area routers, use area

range commandrange command To summarize an external route, use To summarize an external route, use

summary-address commandsummary-address commandNon-advertise keyword prevents advertising a Non-advertise keyword prevents advertising a routeroute

Tag keyword attaches a numeric tag valueTag keyword attaches a numeric tag value

50 5050

Configuring Virtual LinksConfiguring Virtual Links

Use area virtual-link command to Use area virtual-link command to configure virtual linkconfigure virtual link Routers at each end of virtual link must know Routers at each end of virtual link must know

transit area and router ID at other endtransit area and router ID at other end See Figure 4-14See Figure 4-14

51 5151

Configuring a Virtual LinkConfiguring a Virtual Link

52 5252

Monitoring and Troubleshooting Monitoring and Troubleshooting OSPF in Multiple AreasOSPF in Multiple Areas

• Use Use show ip ospfshow ip ospf command to see what command to see what area a particular interface is in and whether area a particular interface is in and whether the router is properly configured as an ABRthe router is properly configured as an ABR

• Use Use show ip ospf interfaceshow ip ospf interface to identify to identify whether an interface is in proper areawhether an interface is in proper area

• Use Use show ip ospf databaseshow ip ospf database command command to see topology database of routerto see topology database of router

53 5353

Show IP OSPF Database Show IP OSPF Database Command and KeywordsCommand and Keywords

54 5454

Monitoring and Troubleshooting Monitoring and Troubleshooting OSPF in Multiple AreasOSPF in Multiple Areas

• Use Use show ip ospf border-routersshow ip ospf border-routers command to see internal OSPF routing command to see internal OSPF routing table entries to ABRs and ASBRstable entries to ABRs and ASBRs

• Use Use show ip routeshow ip route command to command to troubleshoot route summarization troubleshoot route summarization problemsproblems

• Use Use show ip ospf virtual-linksshow ip ospf virtual-links command to see status of virtual links, its command to see status of virtual links, its cost, and transit areacost, and transit area