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Any Questions?. Chapter 9 OSPF. OSPF Protocols and Operation OSPF Configuration. Do I know this?. Go through the Quiz- 5 minutes. 1. Which of the following affects the calculation of OSPF routes when all possible default values are used? a. Bandwidth b. Delay c. Load d. Reliability

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  1. Any Questions?

  2. Chapter 9 OSPF • OSPF Protocols and Operation • OSPF Configuration

  3. Do I know this? Go through the Quiz- 5 minutes

  4. 1. Which of the following affects the calculation of OSPF routes when all possible default values are used? a. Bandwidth b. Delay c. Load d. Reliability e. MTU f. Hop count

  5. 1. Which of the following affects the calculation of OSPF routes when all possible default values are used? a. Bandwidth b. Delay c. Load d. Reliability e. MTU f. Hop count Answer: A

  6. 2. OSPF runs an algorithm to calculate the currently best route. Which of the following terms refer to that algorithm? a. SPF b. DUAL c. Feasible successor d. Dijkstra e. Good old common sense

  7. 2. OSPF runs an algorithm to calculate the currently best route. Which of the following terms refer to that algorithm? a. SPF b. DUAL c. Feasible successor d. Dijkstra e. Good old common sense Answer: A & D

  8. 3. Two OSPF routers connect to the same VLAN using their Fa0/0 interfaces. Which of the following settings on the interfaces of these two potentially neighboring routers would prevent the two routers from becoming OSPF neighbors? a. IP addresses of 10.1.1.1/24 and 10.1.1.254/25, respectively b. The addition of a secondary IP address on one router's interface, but not the other c. Both router interfaces assigned to area 3 d. One router is configured to use MD5 authentication, and the other is not configured to use authentication

  9. 3. Two OSPF routers connect to the same VLAN using their Fa0/0 interfaces. Which of the following settings on the interfaces of these two potentially neighboring routers would prevent the two routers from becoming OSPF neighbors? a. IP addresses of 10.1.1.1/24 and 10.1.1.254/25, respectively b. The addition of a secondary IP address on one router's interface, but not the other c. Both router interfaces assigned to area 3 d. One router is configured to use MD5 authentication, and the other is not configured to use authentication Answer: A &D

  10. 4. Which of the following OSPF neighbor states is expected when the exchange of topology information is complete so that neighboring routers have the same LSDB? a. Two-way b. Full c. Exchange d. Loading

  11. 4. Which of the following OSPF neighbor states is expected when the exchange of topology information is complete so that neighboring routers have the same LSDB? a. Two-way b. Full c. Exchange d. Loading Answer: B

  12. 5. Which of the following is true about an existing OSPF designated router? a. A newly connected router in the same subnet, with a higher OSPF priority, preempts the existing DR to become the new DR. b. A newly connected router in the same subnet, with a lower OSPF priority, preempts the existing DR to become the new DR. c. The DR may be elected based on the lowest OSPF Router ID. d. The DR may be elected based on the highest OSPF Router ID. e. The DR attempts to become fully adjacent with every other neighbor on the subnet.

  13. 5. Which of the following is true about an existing OSPF designated router? a. A newly connected router in the same subnet, with a higher OSPF priority, preempts the existing DR to become the new DR. b. A newly connected router in the same subnet, with a lower OSPF priority, preempts the existing DR to become the new DR. c. The DR may be elected based on the lowest OSPF Router ID. d. The DR may be elected based on the highest OSPF Router ID. e. The DR attempts to become fully adjacent with every other neighbor on the subnet. Answer: D&E

  14. 6. Which of the following network commands, following the command router ospf 1, tells this router to start using OSPF on interfaces whose IP addresses are 10.1.1.1, 10.1.100.1, and 10.1.120.1? a. network 10.0.0.0 255.0.0.0 area 0 b. network 10.0.0.0 0.255.255.255 area 0 c. network 10.0.0.1 255.0.0.255 area 0 d. network 10.0.0.1 0.255.255.0 area 0

  15. 6. Which of the following network commands, following the command router ospf 1, tells this router to start using OSPF on interfaces whose IP addresses are 10.1.1.1, 10.1.100.1, and 10.1.120.1? a. network 10.0.0.0 255.0.0.0 area 0 b. network 10.0.0.0 0.255.255.255 area 0 c. network 10.0.0.1 255.0.0.255 area 0 d. network 10.0.0.1 0.255.255.0 area 0 Answer: B

  16. 7. Which of the following network commands, following the command router ospf 1, tells this router to start using OSPF on interfaces whose IP addresses are 10.1.1.1, 10.1.100.1, and 10.1.120.1? a. network 0.0.0.0 255.255.255.255 area 0 b. network 10.0.0.0 0.255.255.0 area 0 c. network 10.1.1.0 0.x.1x.0 area 0 d. network 10.1.1.0 255.0.0.0 area 0 e. network 10.0.0.0 255.0.0.0 area 0

  17. 7. Which of the following network commands, following the command router ospf 1, tells this router to start using OSPF on interfaces whose IP addresses are 10.1.1.1, 10.1.100.1, and 10.1.120.1? a. network 0.0.0.0 255.255.255.255 area 0 b. network 10.0.0.0 0.255.255.0 area 0 c. network 10.1.1.0 0.x.1x.0 area 0 d. network 10.1.1.0 255.0.0.0 area 0 e. network 10.0.0.0 255.0.0.0 area 0 Answer: A

  18. 8. Which of the following commands list the OSPF neighbors off interface serial 0/0? a. show ip ospf neighbor b. show ip ospf interface c. show ip neighbor d. show ip interface e. show ip ospf neighbor interface serial 0/0

  19. 8. Which of the following commands list the OSPF neighbors off interface serial 0/0? a. show ip ospf neighbor b. show ip ospf interface c. show ip neighbor d. show ip interface e. show ip ospf neighbor interface serial 0/0 Answer: A, B & E

  20. 9. OSPF routers R1, R2, and R3 attach to the same VLAN. R2 has been configured with the ip ospf authentication message-digest interface subcommand on the LAN interface connected to the common VLAN. The show ip ospf neighbor command lists R1 and R3 as neighbors, in an Init and Full state, respectively. Which of the following are true? a. R3 must have an ip ospf authentication message-digest interface subcommand configured. b. R3 must have an ip ospf authentication message-digest-key interface subcommand configured. c. R1's failure must be because of having configured an incorrect OSPF authentication type. d. R1's failure may or may not be related to authentication.

  21. 9. OSPF routers R1, R2, and R3 attach to the same VLAN. R2 has been configured with the ip ospf authentication message-digest interface subcommand on the LAN interface connected to the common VLAN. The show ip ospf neighbor command lists R1 and R3 as neighbors, in an Init and Full state, respectively. Which of the following are true? a. R3 must have an ip ospf authentication message-digest interface subcommand configured. b. R3 must have an ip ospf authentication message-digest-key interface subcommand configured. c. R1's failure must be because of having configured an incorrect OSPF authentication type. d. R1's failure may or may not be related to authentication. Answer: B & D

  22. 10. An OSPF router learns about six possible routes to reach subnet 10.1.1.0/24. All six routes have a cost of 55, and all six are interarea routes. By default, how many of these routes are placed in the routing table? a. 1 b. 2 c. 3 d. 4 e. 5 f. 6

  23. 10. An OSPF router learns about six possible routes to reach subnet 10.1.1.0/24. All six routes have a cost of 55, and all six are interarea routes. By default, how many of these routes are placed in the routing table? a. 1 b. 2 c. 3 d. 4 e. 5 f. 6 Answer: D

  24. Any Questions?

  25. OSPF Operation • Neighbors • How do I get to be a neighbor • Database Exchange • Only done with neighbors • Route Calculation • Once I have details, use SPF Pg 347

  26. OSPF Neighbor • Another router that connects to the same data link with which the first router can exchange OSPF information • Match OSPF settings on both routers • OSPF Hello lets us find neighbors Pg 347

  27. OSPF Router ID • Router ID is used to control OSPF • Uniquely identify each router • 32 bit number • IP Address • Manual config if you want Pg 348

  28. HELLO • Can be neighbors if on the same subnet • Router sends multicast OSPF Hello • Listens for other Router Hellos • Keep sending based on a timer • Hello contains: • RID • Area ID • Hello Interval • Dead Interval • Router priority • Designated RID • Backup Designated RID Pg 349

  29. Any Questions?

  30. Neighbors • Getting a HELLO does not mean you become a neighbor • Subnet mask used on the subnet • Subnet number (as derived using the subnet mask and each router's interface IP address) • Hello interval • Dead interval • OSPF area ID • Must pass authentication checks (if used) • Value of the stub area flag Pg 349

  31. Neighbor States • Once a router see’s its own ID in HELLO, and meets the checks, it becomes neighbor • States • Down-interface down • Init-sending Hello • 2-way-Router see’s its ID in hello from next router Pg 350

  32. OSPF Topology Sharing • Step 1 Based on the OSPF interface type, the routers may or may not collectively elect a Designated Router (DR) and Backup Designated Router (BDR). • Step 2 For each pair of routers that need to become fully adjacent, mutually exchange the contents of their respective LSDBs. • Step 3 When completed, the neighbors monitor for changes and periodically reflood LSAs while in the Full (fully adjacent) neighbor state. Pg 352

  33. Choosing Designated ROUTER • OSPF knows two kinds of networks • Broadcast-Like Ethernet or point to multipoint • Needs DR • DR is hub of topology sharing. Everyone shares to DR who sumamrizess and sends out details • Point to point • No DR • All routers share information Pg 352

  34. Designated Router Pg 353

  35. Electing Designated Routers • The router sending the Hello with the highest OSPF priority setting becomes the DR. • If two or more routers tie with the highest priority setting, the router sending the Hello with the highest RID wins. • It's not always the case, but typically the router with the second-highest priority becomes the BDR. • A priority setting of 0 means that the router does not participate in the election and can never become the DR or BDR. • The range of priority values that allow a router to be a candidate are 1 through 255. • If a new, better candidate comes along after the DR and BDR have been elected, the new candidate does not preempt the existing DR and BDR. Pg 354

  36. Any Questions?

  37. Database Exchange • Send a list of LSAs • Not full details • If a router needs more details, then the full LSA is sent from neighbor • Then they are fully completed • converged Pg 354

  38. Keeping up to date • Hellos sent on interval • LSAs updated every 30 minutes • Individual routes, not all of them • If there is a change, LSA is triggered Pg 355

  39. Summary of OSPF states Pg 355

  40. Routing Tables from OSPF • The OSPF database lists all routers and links • Each router uses Dijkstra SPF algorithm to find best path to destination networks • Best path is based on OSPF cost Pg 356

  41. Any Questions?

  42. OSPF and Large Networks • A larger topology database requires more memory on each router. • Processing the larger-topology database with the SPF algorithm requires processing power that grows exponentially with the size of the topology database. • A single interface status change (up to down or down to up) forces every router to run SPF again! Pg 358

  43. OSPF Areas • Divide larger networks into Areas • Limit how much detail each router needs to know • Routers in different areas just need to know to send a packet to an area, not the details of the area Pg 358

  44. OSPF Areas Pg 359

  45. OSPF Terminology Pg 359

  46. Configuring OSPF • Enabling OSPF • Router OSPF [process #] • Process number doesn’t matter-between 1 and 65, 535 • Configure Router ID (optional) • Router-id [id value] • Configuring OSPF Areas • Network 10.0.0.0 0.255.255.255 area # • Wildcards Pg 361

  47. Wildcards • Used in OSPF and Access Lists • 0 means that the corresponding octet must match exactly • 255 means that corresponding octet can be anything • 10.0.0.0 0.255.255.255 • Anything starting with 10. • 192.168.10.0 0.0.255.255 • Anything starting with 192.168 Pg 361

  48. OSPF Configuration • Change interval times (optional) • Ip ospf hello-interval time • ip ospf dead-interval time interface • Change Costs or Bandwidth • ip ospf cost value interface subcommand • bandwidth value interface subcommand • auto-cost reference-bandwidth value router subcommand Pg 361

  49. OSPF Configuration • OSPF Authentication (Optional) • ip ospf authentication interface subcommand. • area authentication router subcommand. • Configure support for multiple equal-cost routes using • maximum-paths number router subcommand Pg 362

  50. Any Questions?

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