PAQ Pre-Assessment Quiz

1. PAQPre-Assessment Quiz Produced by Mohamed BEN HASSINE CNA Instructor The American University of Paris

2. 1. What are the two main ways of classifying dynamic IGP routing protocols? Distance-vector Link-state

3. 2. Which ones are also known as “shortest path first” protocols? Why? Link-state, because that’s exactly what they do: they look for the shortest path to the destination (regardless of whether or not this is the BEST path).

4. 3. Fill in the following table:

27. 4. What are “hello” packets used for? • Link state protocols use them to make sure a link is still active. They’re very small packets.

28. 5. What are the five things that link state protocols use to maintain their tables: • Link-state advertisements (LSAs) • A topological database • The shortest path first (SPF) algorithm • The resulting SPF tree • A routing table of paths and ports to each network to determine the best paths for packets

29. 6. What is meant by a “link” when talking about link state protocols? • A link is an interface on a router.

30. 7. How does a link state protocol build its topological database? What kind of information is in it? • It’s a database of the entire topology of the network and includes each link and how to reach it. It’s built by using the LSAs that have been received by the router.

31. 8. What are the three types of networks recognized by OSPF? • Nonbroadcast multi-access (NBMA), such as frame relay • Broadcast multi-access, such as Ethernet • Point-to-point networks

32. 9. What is the DR and the BDR in an OSPF network? What do they do? • DR – Designated Router acts as the “brains” of the network. This router makes sure that all the LSAs are sent to all the other routers in the Area. • BDR – Backup Designated Router acts as the “second in command” of the network. If the DR goes down, the BDR takes over as DR. However, until the DR goes down, the BDR does not send out LSAs to any other routers in the Area.

33. 10. What is the multicast address used by the DR to send out LSAs to all other OSPF routers? For LSAs just to other designated (and backup) routers? • 224.0.0.5 • 224.0.0.6

34. 11. What is a hello packet used for and what is the multicast address used for it? Why this address? • A hello packet is the way OSPF routers make sure that a link is still active. 224.0.0.5 is the address used so that all links are checked, not just DRs and BDRs.

35. 12. What is the default hello interval? • 10 seconds

36. 13. What is the main area of an OSPF network designated? • Area 0 (zero)

37. 14. What is the command used to start OSPF routing on a router? • Router(config)#router ospf [process ID]

38. 15. What is the processor ID? • It’s an identifier for the OSPF routing process on the router

39. 16. What is the command used to identify networks on an OSPF router? • Router(config-router)#network [network no.] [wildcard-mask] area [area-id]

40. 17. What does an OSPF router use for its router ID? How can you force a different ID? • It uses the highest active IP address on any of the interfaces (where OSPF is running). If you want to change the ID, set a Loopback interface with a higher IP address so it will be forced to be the ID.

41. 18. What is a Loopback? • It is a logical (virtual) interface; not a physical (real) one

42. 19. What is the command used to set a Loopback? • Router(config)#int Loopback[No.] • Router(config-if)#ip addr [IP number] [subnet mask]

43. 20. What is the recommended subnet mask to use on a Loopback interface? • 255.255.255.255

44. 21. What is a “priority” number used for? How can you change it? • If there is more than 1 router in the Area (broadcast multi-access), then there must be a DR and maybe a BDR for the Area. The priority number is used to determine which routers will be the DR and the BDR.You can change it with the following command: Router(config-if)#ip ospf [priority number]

45. 22. Why must you set the bandwidth on an interface running OSPF? What is the default bandwidth? How can you change it? • Cost (the default metric of OSPF) uses bandwidth to determine the best route. The default is 1.544 Mbps. You can change it with the command: • Router(config)#interface [type] [number] • Router(config-if)#bandwidth [Kbps]

46. 23. How can OSPF routers authenticate each other? • They exchange passwords that only other OSPF routers will know. Use the commands below to set authentication: • Router(config-if)#ip ospf authentication-key [password] • Router(config-router)#area [area-number] authentication

47. 24. What is the difference between default authentication and a message-digest authentication procedure? What is the command to do the latter? • The default authentication sends the password in plain text; message-digest encrypts the password that’s sent. • Router(config-if)#ip ospf authentication message-digest-key [key-id] md5 [encryption-type key] • Router(config-router)#area [area-id] authentication message-digest

48. 25. What is the relationship between the “hello” packet interval and the “dead” interval? What is meant by the “dead” interval? • The dead interval is 4 times the hello interval (e.g., hello is 10 seconds, so dead is 40 seconds). • The dead interval is the time used to determine that a link is down, or dead. In other words, if a hello isn’t received from a link for the space of 4 times the normal hello interval, it’s considered dead.

49. 26. What would happen if routers in the OSPF network have different hello intervals configured? • They wouldn’t be able to “talk” to each other, so would be considered inactive. Hello intervals must be the same on all the routers in the network.

50. 27. What is the best way to define a default route on an OSPF router? • Use the “quad zero” command: Router(config)#ip route 0.0.0.0 0.0.0.0 [interface or IP address of next hop]