1 Module 4: Implementing OSPF

2 Lessons OSPF OSPF Areas and Hierarchical Routing OSPF Operation OSPF Routing Tables Designing an OSPF Network

3 Lesson 4-1: OSPF

4 RIP vs. OSPF RIP Problems –Limited metric –Next-hop view –Limited network diameter –Slow convergence –Non-hierarchical routing OSPF Solutions –Arbitrary, 16-bit metric –Complete map –Theoretical unlimited network diameter –Fast convergence –Hierarchical routing structure

5 RIP vs. OSPF Figure 4-1

6 OSPF Features Authentication Classless Routing Arbitrary Metric Hierarchical Routing Structure Equal-Load Balancing Multicast/Unicast Packets

7 OSPF and Link State Routing Neighbor Discovery/Maintenance Virtual Links Link State Advertisements (LSAs) Link State Database (LSDB) Adjacencies Route Generation Algorithm Routing Tables

8 Adjacent Routers Figure 4-2

9 OSPF Sub-Protocols Table 4-1

10 Non-linear Phases of OSPF Operation Figure 4-3

11 RIP vs. OSPF Table 4-2

12 Lesson 4-2: OSPF Areas and Hierarchical Routing

13 Concepts Types of OSPF Areas Types of OSPF Routers Types of Traffic Route Summarization Area Partitioning Virtual Links

14 OSPF Areas, Routers, and Traffic Figure 4-4

15 Route Summarization Figure 4-5

16 Sample Area Figure 4-6

17 Router X Fails and Area X is Partitioned Figure 4-7

18 Connecting an Area Partition to the Backbone through a Normal Area Figure 4-8

19 Connecting a Partitioned Backbone through a Normal Area Figure 4-9

20 Lesson 4-3: OSPF Operation

21 Format of an OSPF Packet Figure 4-10

22 Types of OSPF Packets Table 4-3

23 Function of Hello Packets Neighbor Discovery / Maintenance –announce the presence of a router –act as keepalives to verify the continued participation of a router Election Process –determine the designated router (DR) and backup designated router (BDR) of a multi-access network segment

24 Only Router A Forwards LSAs from the Multi-Access Network Figure 4-11

25 Exchange Protocol: Phase 2 Operation Exchange DD Packets –establish master-slave relationship Exchange DD Packets –exchange information about LSDBs to build a link state request list Transmit LSRequest Packets –request specific LSAs that are missing from the LSDB of a router

26 OSPF LSA Packet Format Figure 4-12

27 Flooding Protocol Concepts 6 Types of LSAs Sequencing of LSAs Aging of LSAs Guaranteed Delivery of LSAs

28 Types of LSAs Table 4-4

29 Sequence Numbers Figure 4-13

30 Link State Acknowledgements Implicit Acknowledgement –router sends back a copy of the LSA in an LSUpdate packet Explicit Acknowledgement –router sends back an LSAck packet that contains the 20-byte header of the LSA

31 Lesson 4-4: OSPF Routing Tables

32 OSPF Metrics OSPF uses a 16-bit, arbitrary metric Metrics usually based on bandwidth Metric of a route = sum of all outgoing interfaces to the destination OSPF provides equal-load balancing

33 OSPF Path Types Intra-Area Paths Inter-Area Paths E1 Paths E2 Paths

34 Dijkstra Algorithm Also called the Shortest Path First (SPF) algorithm Converts the LSDB of a router into a shortest path tree (the router is the root of the tree) Run twice: –first to create the internal routing table –second to create the standard routing table

35 Sample Network Figure 4-14

36 Shortest Path Tree of Router J as Determined by the First SPF Calculation Figure 4-15

37 Shortest Path Tree of Router J as Determined by the Second SPF Calculation Figure 4-16

38 Lesson 4-5: Designing an OSPF Network

39 OSPF Routing Concepts Hierarchical Routing Structure –three-tiered model Route Summarization –summarize at area boundaries –conserve bandwidth and router resources

40 Figure 4-17

41 Route Summarization Figure 4-18

42 Topology Considerations Minimum/Maximum Routers per OSPF Network Minimum/Maximum Routers per OSPF Area Maximum Number of OSPF Areas

43 Scalability Considerations Plan for growth Ensure routers have the appropriate memory and processing power Place routers appropriately in the network

44 Area Design Considerations Designing the Backbone Area Designing Stub Areas –normal stub areas –not-so-stubby areas Avoiding Partitions and Virtual Links Providing for Route Summarization

45 Commonly Configurable OSPF Parameters Table 4-5

46 Commonly Configurable OSPF Parameters (cont.) Table 4-5

47 Module 4 Lab Exercise Designing OSPF Solutions

48 Lab Exercise 4-1: Designing OSPF Solutions

49 Lab Exercise 4-1: Overview Setup –discussion lab Format –answer questions as a group –share proposed solutions with class –discuss the different strengths and weaknesses of each proposed solution

50 Module 4: Review Questions... Summary...

51 1. Which of the following are link state routing protocols? A. Link Service Protocol B. Open Shortest Path First (OSPF) C. Routing Information Protocol (RIP) D. Intermediate System to Intermediate System (IS-IS) (Choose three.)

52 1. Which of the following are link state routing protocols? A. Link Service Protocol B. Open Shortest Path First (OSPF) C. Routing Information Protocol (RIP) D. Intermediate System to Intermediate System (IS-IS) (Choose three.)

53 2. What piece of information regarding network topology is available to a link state router, but not to a distance vector router? A. the network ID of all reachable destinations B. the next hop along the path to each destination C. the distance (or metric) from the router to the destination D. the status of the links between the router and any router in the network (Choose one.)

54 2. What piece of information regarding network topology is available to a link state router, but not to a distance vector router? A. the network ID of all reachable destinations B. the next hop along the path to each destination C. the distance (or metric) from the router to the destination D. the status of the links between the router and any router in the network (Choose one.)

55 3. Why do link state routers require more processing power than distance vector routers? A. because link state routers use the Dijkstra algorithm to compute paths B. because link state routers maintain more complex routing tables than distance vector routers C. because link state routers can make connections to more destination networks and other routers than distance vector routers D. because link state routers utilize a flooding process that requires them to transmit information about themselves and their links to every other router in their routing domains (Choose one.)

56 3. Why do link state routers require more processing power than distance vector routers? A. because link state routers use the Dijkstra algorithm to compute paths B. because link state routers maintain more complex routing tables than distance vector routers C. because link state routers can make connections to more destination networks and other routers than distance vector routers D. because link state routers utilize a flooding process that requires them to transmit information about themselves and their links to every other router in their routing domains (Choose one.)

57 4. In terms of link state routing protocols, what are neighboring routers? A. routers that share a common link B. routers located on adjacent subnets of an IP network C. routers that communicate with low latency because of their physical proximity D. link state routers that can communicate without routing their packets through any distance vector routers (Choose one.)

58 4. In terms of link state routing protocols, what are neighboring routers? A. routers that share a common link B. routers located on adjacent subnets of an IP network C. routers that communicate with low latency because of their physical proximity D. link state routers that can communicate without routing their packets through any distance vector routers (Choose one.)

59 5. What feature of link state routing protocols enables link state routing domains to converge more quickly than distance vector routing domains? A. Hello packets B. Dijkstra algorithm C. link state database D. link state flooding (Choose one.)

60 5. What feature of link state routing protocols enables link state routing domains to converge more quickly than distance vector routing domains? A. Hello packets B. Dijkstra algorithm C. link state database D. link state flooding (Choose one.)

61 6. What are the advantages of the hierarchical routing structure used by link state routers? A. reduces amount of time necessary to build adjacencies B. reduces load on router memory, router processors, and network bandwidth C. reduces number of fields required in routers' link state databases (LSDBs) D. reduces number of link state advertisements (LSAs) that must be flooded to a routing domain (Choose two.)

62 6. What are the advantages of the hierarchical routing structure used by link state routers? A. reduces amount of time necessary to build adjacencies B. reduces load on router memory, router processors, and network bandwidth C. reduces number of fields required in routers' link state databases (LSDBs) D. reduces number of link state advertisements (LSAs) that must be flooded to a routing domain (Choose two.)

63 7. What are the three categories of OSPF design considerations? A. topology considerations B. reliability considerations C. scalability considerations D. bandwidth considerations E. availability considerations F. area design considerations (Choose three.)

64 7. What are the three categories of OSPF design considerations? A. topology considerations B. reliability considerations C. scalability considerations D. bandwidth considerations E. availability considerations F. area design considerations (Choose three.)

65 8. What is the Internet Engineering Task Force (IETF) recommendation for the maximum number of routers in an OSPF network? A. 200 B. 700 C D (Choose one.)

66 8. What is the Internet Engineering Task Force (IETF) recommendation for the maximum number of routers in an OSPF network? A. 200 B. 700 C D (Choose one.)

67 9. In an OSPF network, what is the best location for routers that are relatively low in memory and processing power? A. backbone B. stub areas C. ISP interfaces D. subnet interfaces (Choose one.)

68 9. In an OSPF network, what is the best location for routers that are relatively low in memory and processing power? A. backbone B. stub areas C. ISP interfaces D. subnet interfaces (Choose one.)

Why should virtual links be reserved for emergencies in an OSPF network and not used as a permanent part of the network's topology? A. Virtual links are prone to errors. B. Virtual links require extra bandwidth. C. Virtual links are difficult to configure. D. Virtual links are slower than physical links. E. Virtual links place increased loads on routers. (Choose two.)

Why should virtual links be reserved for emergencies in an OSPF network and not used as a permanent part of the network's topology? A. Virtual links are prone to errors. B. Virtual links require extra bandwidth. C. Virtual links are difficult to configure. D. Virtual links are slower than physical links. E. Virtual links place increased loads on routers. (Choose two.)

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72 Assumptions You understand the distance vector algorithm.You understand the distance vector algorithm. You know RIP.You know RIP. –Routing tables –Next-hop router –Convergence process Module 4: Implementing OSPF