Module 4 Implementing OSPF 1 Lessons OSPF Areas

Module 4: Implementing OSPF 1

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

Lesson 4 -1: OSPF 3

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 4

RIP vs. OSPF Figure 4 -1 5

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

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

Adjacent Routers Figure 4 -2 8

OSPF Sub-Protocols Table 4 -1 9

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

RIP vs. OSPF Table 4 -2 11

Lesson 4 -2: OSPF Areas and Hierarchical Routing 12

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

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

Route Summarization Figure 4 -5 15

Sample Area Figure 4 -6 16

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

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

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

Lesson 4 -3: OSPF Operation 20

Format of an OSPF Packet Figure 4 -10 21

Types of OSPF Packets Table 4 -3 22

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 23

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

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 25

OSPF LSA Packet Format Figure 4 -12 26

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

Types of LSAs Table 4 -4 28

Sequence Numbers Figure 4 -13 29

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 30

Lesson 4 -4: OSPF Routing Tables 31

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 32

OSPF Path Types • • Intra-Area Paths Inter-Area Paths E 1 Paths E 2 Paths 33

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 34

Sample Network Figure 4 -14 35

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

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

Lesson 4 -5: Designing an OSPF Network 38

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

Figure 4 -17 40

Route Summarization Figure 4 -18 41

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

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

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 44

Commonly Configurable OSPF Parameters Table 4 -5 45

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

Module 4 Lab Exercise • Designing OSPF Solutions 47

Lab Exercise 4 -1: Designing OSPF Solutions 48

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 49

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

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. ) 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

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. ) 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

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. ) 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

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. ) 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

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. ) 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

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. ) 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

7. What are 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. ) 63

7. What are 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

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. 1000 D. 1200 (Choose one. ) 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. 1000 D. 1200 (Choose one. ) 66

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. ) 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

10. 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. ) 69

10. 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. ) 70

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