1. 1 OSPF in Multiple Areas

2. 2 2 Scalability Problems in Large OSPF Areas Scalability problems in large OSPF areas include Large routing tables Large routing tables Large topology databases Large topology databases Frequent recalculation of Shortest Path First (SPF) algorithm Frequent recalculation of Shortest Path First (SPF) algorithm Slow synchronization Slow synchronization

3. 3 3 Advantages of Using Multiple Areas Advantages of using multiple OSPF areas within an Autonomous System include Smaller routing tables Smaller routing tables Less frequent SPF recalculations Less frequent SPF recalculations Less routing update overhead Less routing update overhead Faster synchronization Faster synchronization

4. 4 4 Connecting OSPF Areas To connect two OSPF areas, use a router with at least one interface in both areas OSPF uses different types of routers and areas

5. 5 5 Types of Routers Types of OSPF routers include: Internal routers Internal routers Backbone routers Backbone 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 routers Keep in mind the cost of performing each function Keep in mind the cost of performing each function

6. 6 6 The Backbone Area Backbone area is most important area in any OSPF autonomous system It behaves like a standard area It behaves like a standard area All other areas connect to it to exchange routing information All other areas connect to it to exchange routing information All packets sent between areas go through it All packets sent between areas go through it Backbone area must remain stable and reachable

7. 7 7 Other Types of Areas Classified by interaction with each other Standard area – accepts link updates and route summaries internally and externally Stub area – does not accept routing information from ASBRs; uses a default route to reach external devices

8. 8 8 Other Types of Areas Totally stubby area – does not accept external routes and refuses summary routes from other areas; uses a default route; Cisco's proprietary extension to OSPF protocol Not-so-stubby-area (NSSA) – accepts limited information about external routes, but refuses routing information from ASBRs; useful for connecting stub area to another Autonomous System

9. 9 9 Not-so-stubby Area

10. 10 Routing Between Areas Routing between multiple areas adds complexity to OSPF Must first understand different types of LSAs used to describe topology of a network

11. 11 Types of LSAs OSPF uses several types of LSAs to describe topology of a networkOSPF uses several types of LSAs to describe 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 (type 5) Autonomous System external link entry LSA (type 5) NSSA Autonomous System external link entry LSA (type 7) NSSA Autonomous System external link entry LSA (type 7)

12. 12 Types of LSAs continued

13. 13 Types of LSAs Type 6 LSA is not included because CISCO routers do not support it

14. 14 Routes Between Multiple Areas If packet’s destination is inside same area as source, it passes through internal routersIf packet’s destination is inside same area as source, it passes through internal routers If packet’s destination is outside area of source, it goes to an ABR in its own areaIf packet’s destination is outside area of source, it goes to an ABR in its own area ABR sends it through backbone to an ABR in destination area ABR sends it through backbone to an ABR in destination area OSPF Autonomous System uses a variety of types of routesOSPF Autonomous System uses a variety of types of routes

15. 15 Calculating Routing Tables and Flooding in Multiple Areas ABRs must learn about routes and network changes in other areas Within an area, each router receives LSUs describing changes and then sends summary LSAs to any attached areas Within an area, each router receives LSUs describing changes and then sends summary LSAs to any attached areas When an ABR or ASBR receives a summary LSA, it adds information to link-state database and floods local areas. Totally stubby areas do not receive LSAs. When an ABR or ASBR receives a summary LSA, it adds information to link-state database and floods local areas. Totally stubby areas do not receive LSAs. After all routers receive summary LSAs, they recalculate their routing tables After all routers receive summary LSAs, they recalculate their routing tables See Figure 4-3

16. 16 Virtual Links Each area in an OSPF Autonomous System must be connected to backbone If no physical link exists, OSPF uses virtual link or a logical link Connects two ABRs via a common area, sometimes called the transit area, and one ABR attached to backbone Connects two ABRs via a common area, sometimes called the transit area, and one ABR attached to backbone

17. 17 Adding an Area with a Virtual Link

18. 18 Purposes of Virtual Links Virtual links have three purposesVirtual links have three purposes Connecting areas to backbone when they don’t have an ABR with an interface in backbone area Connecting areas to backbone when they don’t have an ABR with an interface in backbone area Connecting backbone itself in situations where it has been partitioned Connecting backbone itself in situations where it has been partitioned Adding a redundant connection to backbone to prevent failure of single link or backbone area itself Adding a redundant connection to backbone to prevent failure of single link or backbone area itself

19. 19 Designing Areas Factors to consider when designing OSPF areas include: Size of an area Size of an area Number of neighbors each router has Number of neighbors each router has Problem isolation Problem isolation Restriction of routing information Restriction of routing information Stable backbone design Stable backbone design Addressing area Addressing area

20. 20 Size of an Area Size of area affects number of routes in routing table and frequency of SPF calculationsSize of area affects number of routes in routing table and frequency of SPF calculations Topology, memory, and processors in router determine appropriate sizes of areas Topology, memory, and processors in router determine appropriate sizes of areas With high-speed links and stable network, may have areas with up to 100 routers If your network has slow unstable WAN links, should use smaller areas Cisco recommends ABR have interfaces in no more than three areas Cisco recommends ABR have interfaces in no more than three areas

21. 21 Number of Neighbors Per Router Number of neighbors affects performance of router A router exchanges hello packets with its neighbors and link-state databases with those neighbors with which it forms adjacencies A router exchanges hello packets with its neighbors and link-state databases with those neighbors with which it forms adjacencies Cisco recommends routers have no more than 60 neighbors

22. 22 Isolating Potential Network Problems OSPF allows you to isolate potential network problems to prevent them from affecting other areas of the network Route flapping on WAN links causes instability on OSPF networks Route flapping on WAN links causes instability on OSPF networks Use areas to isolate WAN links Use areas to isolate WAN links

23. 23 Isolating WAN Segments

24. 24 Restricting Routing Information From an Area Restricting routing information has several advantages Reduces routing table size in an area Reduces routing table size in an area Reduces amount of flooding traffic after topology change Reduces amount of flooding traffic after topology change Adds stability by isolating routers inside area Adds stability by isolating routers inside area Configure an area as a stub area, a totally stubby area, or a not-so-stubby area to restrict routing information

25. 25 Selecting a Stub Area Stub areas likely have only one exit point Stub areas work well for branch offices or hub-and-spoke topologies Spoke needs to get to hub and does not need to know about rest of network Spoke needs to get to hub and does not need to know about rest of network Stub area cannot be backbone area or transit area for a virtual link No need to have ASBRs within a stub network since stubs do not accept external routes

26. 26 Totally Stubby Areas Totally stubby areas reject external routes and intra-area routes Rely entirely on default routes to reach destination outside totally stubby area If area consists entirely of Cisco routers and has a single exit point, using a totally stubby area allows smaller routing tables Totally stubby areas are proprietary to Cisco Totally stubby areas are proprietary to Cisco

27. 27 Not-so-stubby Areas Good for service provider Allows service provider’s network to connect to customer network while preventing customer network from incorporating into OSPF Autonomous System Allows service provider’s network to connect to customer network while preventing customer network from incorporating into OSPF Autonomous System Good for redistributing other routing protocols into OSPF Autonomous system See Figure 4-7

28. 28 Example of NSSA

29. 29 Designating the Backbone Area Stable backbone is essential Areas exchange routing information through the backbone area Areas exchange routing information through the backbone area Design of the backbone area affects stability of entire OSPF Autonomous System

30. 30 Designing 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 routers whenever possibleProvide redundant links between backbone routers whenever possible See Figure 4-8 See Figure 4-8 Redundant links add complexity and cost, but should be balanced against the cost of a partitioned networkRedundant links add complexity and cost, but should be balanced against the cost of a partitioned network See Figure 4-9 See Figure 4-9

31. 31 Connecting Areas to the Backbone with Virtual Links Use virtual links to prevent backbone area from being partitioned or for redundancyUse virtual links to prevent backbone area from being partitioned or for redundancy Three reasons to avoid virtual linksThree reasons to avoid virtual links Virtual links prevent transit area from being used as a stub area Virtual links prevent transit area from being used as a stub area Stability of virtual link depends on stability of transit area Stability of virtual link depends on stability of transit area Using virtual links adds complexity to network design and makes troubleshooting more difficult Using virtual links adds complexity to network design and makes troubleshooting more difficult

32. 32 Using WAN Links in the Backbone Area In star topology, keep WAN links out of backbone to prevent flappingIn star topology, keep WAN links out of backbone to prevent flapping In extended star topology, use WAN links to connect hubsIn extended star topology, use WAN links to connect hubs Avoid single links between backbone routers Avoid single links between backbone routers Add redundant links to prevent single point of failure Add redundant links to prevent single point of failure

33. 33 Avoid Other Devices in Backbone Area Avoid placing anything but routers in backbone areaAvoid placing anything but routers in backbone area Servers and workstations use bandwidth needed for routing traffic and can cause network to become unstableServers and workstations use bandwidth needed for routing traffic and can cause network to become unstable Malfunctioning network interface card or IP address can duplicate router’s interface IP address, causing serious problems Malfunctioning network interface card or IP address can duplicate router’s interface IP address, causing serious problems Other devices in backbone make troubleshooting more difficult Other devices in backbone make troubleshooting more difficult

34. 34 Addressing with Multiple Areas Address selection determines how well OSPF Autonomous System scales with growth Without route summarization, non- backbone change requires routers in backbone to recalculate routing tables Minimize number of routes by carefully using route summarization

35. 35 Route Summarization in OSPF OSPF requires each router in an area to have same knowledge of network Two consequences for route summarization Routes cannot be summarized within an area; would create different views of topology Routes cannot be summarized within an area; would create different views of topology Summarization of routes from an area must occur at ABRs since ABRs advertise routes to other areas Summarization of routes from an area must occur at ABRs since ABRs advertise routes to other areas

36. 36 Summarizing routes Summarize external routes with an ASBR or an ABR Summarize only contiguous blocks of IP addresses Summarize routes on ABRs connecting non- backbone areas to backbone Route summarization is useful for large networks, Be sure all summarized routes are in same area Be sure all summarized routes are in same area

37. 37 Summarizing Routes

38. 38 Limits of Route Summarization Route summarization with OSPF has limits Can lead to poor path selection May generate unnecessary traffic through backbone Does not allow you to summarize groups of areas

39. 39 Addressing Areas Consider numbering OSPF areas in contiguous blocks Allows use of route summarization without having to readdress network Allows use of route summarization without having to readdress network Basing addressing on topology of network or geographic location works well Basing addressing on topology of network or geographic location works well Basing addressing on other schemes, such as department structure of organization, can lead to problems Basing addressing on other schemes, such as department structure of organization, can lead to problems

40. 40 Addressing Areas Assigning address blocks at an octet boundary makes them easy to remember and configureAssigning address blocks at an octet boundary makes them easy to remember and configure Keep thorough and accurate documentationKeep thorough and accurate documentation Avoid overlapping subnets or assigning subnets in discontiguous blocksAvoid overlapping subnets or assigning subnets in discontiguous blocks Assign IP addresses in contiguous blocks based on router to which each subnet is attached to facilitate route summarizationAssign IP addresses in contiguous blocks based on router to which each subnet is attached to facilitate route summarization

41. 41 Configuring OSPF in Multiple Areas Similar to configuring OSPF in a single area, but adds layer of complexity Activating multiple areas on an ABR Use router ospf statement to activate OSPF Use router ospf statement to activate OSPF Use network statement to put each interface on the ABR into appropriate area Use network statement to put each interface on the ABR into appropriate area

42. 42 Activating Multiple Areas on an ABR

43. 43 Configuring Stub Areas Each router in stub area must be specifically configured as stub router Sets Stub Flag in hello packets Sets Stub Flag in hello packets If Stub Flag does not match on two neighboring routers, they cannot become neighbors If Stub Flag does not match on two neighboring routers, they cannot become neighbors Use area area-number command with stub keyword Router(config-router) area 6 stub Router(config-router) area 6 stub

44. 44 Configuring Totally Stubby Areas ABR is logical place to configure a totally stubby area Use area area-number command with stub and no-summary keywords No-summary keyword must be used on each ABR in a totally stubby area No-summary keyword must be used on each ABR in a totally stubby area Configure each router in totally stubby area with area area-number stub command Router(config-router) area 6 stub no-summary Router(config-router) area 6 stub no-summary

45. 45 Configuring Not-so-stubby Areas Each router in NSSA must be specifically configured to be part of NSSA Use area area-number command with keyword nssa Use area area-number command with keyword nssa Router(config-router) area 6 nssa Router(config-router) area 6 nssa

46. 46 Default Routes Configuring default routes varies from one area to another In normal areas, use default-information originate command In normal areas, use default-information originate command Can use router with a default route to propagate default route to rest of OSPF Autonomous System Can use router with a default route to propagate default route to rest of OSPF Autonomous System Add always keyword to configure a default route if router does not have one Add always keyword to configure a default route if router does not have one Can configure type of external link, cost of default route, and identify a route map as default route

47. 47 Default-information Originate Command

48. 48 Default Routes In stub and totally stubby areas, ABRs automatically generate default route using a summary LSA No need to use default-information originate command No need to use default-information originate command In NSSA, an ABR must be forced to generate a default route when you configure it with area command Use default-information originate keyword Use default-information originate keyword Use no-redistribution keyword to control route redistribution Use no-redistribution keyword to control route redistribution

49. 49 Route Summarization Configuring route summarization in OSPF depends on whether you are summarizing intra-area routers or external routers To summarize intra-area routers, use area range command To summarize intra-area routers, use area range command To summarize an external route, use summary-address command To summarize an external route, use summary-address command Non-advertise keyword prevents advertising a route Tag keyword attaches a numeric tag value

50. 50 Configuring Virtual Links Use area virtual-link command to configure virtual link Routers at each end of virtual link must know transit area and router ID at other end Routers at each end of virtual link must know transit area and router ID at other end See Figure 4-14 See Figure 4-14

51. 51 Configuring a Virtual Link

52. 52 Monitoring and Troubleshooting OSPF in Multiple Areas Use show ip ospf command to see what area a particular interface is in and whether the router is properly configured as an ABRUse show ip ospf command to see what area a particular interface is in and whether the router is properly configured as an ABR Use show ip ospf interface to identify whether an interface is in proper areaUse show ip ospf interface to identify whether an interface is in proper area Use show ip ospf database command to see topology database of routerUse show ip ospf database command to see topology database of router

53. 53 Show IP OSPF Database Command and Keywords

54. 54 Monitoring and Troubleshooting OSPF in Multiple Areas Use show ip ospf border-routers command to see internal OSPF routing table entries to ABRs and ASBRsUse show ip ospf border-routers command to see internal OSPF routing table entries to ABRs and ASBRs Use show ip route command to troubleshoot route summarization problemsUse show ip route command to troubleshoot route summarization problems Use show ip ospf virtual-links command to see status of virtual links, its cost, and transit areaUse show ip ospf virtual-links command to see status of virtual links, its cost, and transit area