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Module 3 : OSPF – Part2

Module 3 : OSPF – Part2. By Sang Gon Lee Spring 2009. Contents. 3.6 Multiarea OSPF Operation. Large OSPF Network Issues. With single area calculations Large routing Large LSDB Hierarchical area Reduced frequency of SPF calculations - isolate route flapping

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Module 3 : OSPF – Part2

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  1. Module 3 : OSPF – Part2 By Sang Gon Lee Spring 2009

  2. Contents

  3. 3.6 Multiarea OSPF Operation

  4. Large OSPF Network Issues • With single area • calculations • Large routing • Large LSDB • Hierarchical area • Reduced frequency of SPF calculations - isolate route flapping • Smaller routing tables • Reduced LSU overhead

  5. An advantage of Multiple Areas-FYI • Question: I understand the routing table is recalculated every time the router receives an new version of an LSA. Does OSPF recalculate its routing table when their is a topology change in another area? show ip ospf displays no change in SPF execution, but show ip ospf database shows a change in the topology? • Answer: Good question! OSPF areas are designed to keep issues like flapping links within an area. SPF is not recalculated if the topology change is in another area. The interesting thing is that OSPF distributes inter-area (between areas) topology information using a distance-vector method.OSPF uses link-state principles only within an area. ABRs do not announce topological information between areas, instead, only routing information is injected into other areas. ABRs relay routing information between areas via distance vector technique similar to RIP or IGRP. This is why show ip ospf does not show a change in the number of times SPF has been executed when the topology change is in another area. • Note: It is still a good idea to perform route summarization between areas, announcing multiple routes as a single inter-area route. This will hide any changes in one area from affecting routing tables in other areas.

  6. OSPF LSA Types

  7. OSPF LSA Types(계속)

  8. OSPF LSA Types(계속)

  9. OSPF LSA Types(계속)

  10. OSPF LSA Types(계속)

  11. OSPF LSA Types(계속)

  12. Interpreting the OSPF LSDB and Routing Table • Link ID: Identifies each LSA. • ADV Router: Advertising router • Age: The age in sec, maximum age : 3,600 sec. • Seq#: Sequence number of the LSA. • Checksum: Checksum of the individual LSA . • Link count: Total number of directly attached links,

  13. Interpreting the Routing Table

  14. OSPF LSA Types(계속) • Type 6 – Multicast OSPF • Type 7 – used in not-so-stubby areas (NSSAs), originated by ASBRs, NSSA에 한정 • Type 8 – used in internetworking OSPF and Border Gateway Protocol (BGP) • Types 9, 10, 11 - opaque LSAs, 향후 업그레이드용

  15. Calculating Costs for E1 and E2 Routes • 외부경로 타입 E1 : 다수의 ASBR들이 외부경로를 동일 자치 시스템에 홍보할 때. • 외부경로 타입 E2 : 오직 한 개의 ASBR이 자치 시스템에 외부경로를 홍보할 때.

  16. Configuring OSPF LSDB Overload Protection

  17. Changing the Cost Metric • Cost = 100 Mbps / (bandwidth in Mbps).

  18. 3.7 Stub, Totally Stubby, and Not-So-Stubby Areas

  19. Configuring OSPF Area Types

  20. OSPF Area Types • Standard area: 모든 종류의 LSA를 수신.(default). • Backbone area (transit area): area 간 경로 정보 교환 • Stub area: - AS 외부 경로는 받지 않음. - Type 4 and 5 LSAs are blocked, default 경로 만 알려 줌. • Totally stubby area: - 외부 AS 경로와 다른 area의 summary routes 받지 않음. - blocks type 4 and 5 LSAs as well as all summary LSAs (type 3) - Type 3 LSA (advertise the default route)는 제외. • Not-so-stubby area: - 외부 경로 유입 허용. - ASBR Type 7 LSAs  flood ABR. Type 5 LSA  AS 전파

  21. Stub and Totally Stubby Areas Rules

  22. Configuring Stub Area

  23. OSPF Stub Area Configuration Example

  24. Configuring Totally Stub Areas

  25. Totally Stubby Area Configuration Example

  26. Interpreting Routing Tables

  27. Interpreting Routing Tables(계속)

  28. Interpreting Routing Tables(계속)

  29. Interpreting Routing Tables(계속) • No O IA code.

  30. Configuring Not-So-Stubby Areas • NSSA : 제한된 형태로 외부경로 정보를 stub area에 유입시킨다. • Type 7 LSA in the routing table : • O N1 : an external type 1 metric • O N2 : an external type 2 metric • N stands for NSSA. • The default is O N2. • ASBR : 외부 routes를 받아 들인다. Type 7으로 redistribute. • ABR : Type 7을 Type 5로 변환하여 area 0로 주입. 요약한다.

  31. Configuring Not-So-Stubby Areas

  32. Configuring Not-So-Stubby Areas

  33. Configuring Not-So-Stubby Area Example

  34. Configuring an NSSA Totally Stubby Area Example

  35. Verifying All Stub Area Types

  36. 3.8 Virtual Links

  37. OSPF Virtual Links • Hello : 10sec • LSA : does not age out

  38. Configuring Virtual Links

  39. Verifying a Virtual Link Configuration

  40. 3.9 Route Summarization

  41. ABR and ASBR Implementation • area 간 route • ABR • 연속적 서버 넷 • AS 간 route • ASBR • 연속적 서버 넷

  42. OSPF Route Summarization Example

  43. Configuring OSPF Route Summarization

  44. Configuring Route Summarization at ABR • 확인 해보기

  45. Configuring Route Summarization at ASBR

  46. Benefits of a Default Route in OSPF • 디폴트 경로를 표준 area에 홍보하는 방법 2가지 • (1) advertise 0.0.0.0 into the OSPF domain홍보하는 router은 이미 default route를 라우팅 테이블에 가지고 있어야 함. • (2) advertise 0.0.0.0 (default route 유무에 관계없이) – default –information origination command

  47. Configuring a Default Route in OSPF • 보통: 디폴터 라우터가 라우팅 테이블에 있을때, 0.0.0.0 디폴터 홍보 • Always : 디폴터 경로가 없더라도 0.0.0.0 디폴터 홍보 • Metric type : external type 1 혹은 2, 디폴터는 2.

  48. Example of Configuring a Default Route in OSPF • multihomed to dual Internet service providers (ISPs). • Provider A : preferred, provider B : backup.

  49. 3.10 OSPF Authentication

  50. OSPF Authentication Methods • Simple password authentication (also called plain-text authentication) • Message Digest (MD) 5 authentication

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