CCNA FAQ: OSPF

Q: Which of the following affects the calculation of OSPF routes when all possible default values are used?

A. OSPF calculates metrics based on the cost associated with each interface. OSPF, by default, calculates interface cost based on the bandwidth setting.

Q1. 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. OSPF calculates metrics based on the cost associated with each interface. OSPF, by default, calculates interface cost based on the bandwidth setting.

Q2. OSPF runs an algorithm to calculate the currently best route. Which of the following terms refer to that algorithm? (Choose two answers.)
a. SPF
b. DUAL
c. Feasible successor
d. Dijkstra
e. Good old common sense

Answer: A and D. OSPF uses the SPF algorithm, conceived by a mathematician named Dijkstra.

Q3. 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? (Choose two answers.)
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 and D. Routers must use the same authentication type and, if so, the same authentication key. Additionally, the subnet number and range of addresses, as calculated from the interfaces’ IP addresses and masks, must be the same subnet.

Q4. 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. Neighboring OSPF routers that complete the database exchange are considered fully adjacent and rest in a Full neighbor state.

Q5. Which of the following is true about an existing OSPF designated router? (Choose two answers.)
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 and E. The DR is elected based on the highest OSPF priority. If there is a tie, it is based on the highest OSPF RID. However, after the DR is elected, the DR role cannot be taken over by a better router until the DR and BDR have lost connectivity to the subnet. The DR attempts to be fully adjacent to all other routers on the subnet as part of the optimized database exchange process.

Q6. 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. The network 10.0.0.0 0.255.255.255 area 0 command works, because it matches all interfaces whose first octet is 10. The network 10.0.0.1 0.255.255.0 area 0 command uses matching logic that matches all interfaces whose first octet is 10 and last octet is 1, which matches all three interface IP addresses. However, the wildcard mask used in OSPF network commands can have only one string of consecutive binary 1s, with all other digits as binary 0s, and this wildcard mask breaks that rule.

Q7. 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. The network 0.0.0.0 255.255.255.255 area 0 command matches all IP addresses as a result of the 255.255.255.255 wildcard mask, so this command enables OSPF in Area 0 on all interfaces. The answer with wildcard mask 0.255.255.0 is illegal, because it represents more than one string of binary 0s separated by binary 1s. The answer with x’s is syntactically incorrect. The answer with wildcard mask 255.0.0.0 means “Match all addresses whose last 3 octets are 0.0.0,” so none of the three interfaces are matched.

Q8. Which of the following commands list the OSPF neighbors off interface serial 0/0? (Choose three answers.)
a. show ip ospf neighbor
b. show ip ospf interface
c. show ip neighbor
d. show ip interface
e. show ip ospf neighbor serial 0/0

Answer: A, B, and E

Q9. Routers R1, R2, and R3 connect to the same VLAN using their F0/0 interfaces. All three use OSPF, and all three have a neighbor relationship that reached a FULL state. R1’s configuration shows the ip ospf authentication command under interface F0/0. Which of the following statements is true regarding OSPF in this small part of the internetwork? (Choose two answers.)
a. R1 also has an ip ospf authentication-key command configured on that same interface.
b. R2’s show ip ospf neighbor command shows the use of simple password authentication.
c. R3’s show ip ospf interface f0/0 command shows the use of MD5 authentication.
d. R3 must also have an ip ospf authentication command configured on its F0/0 interface.

Answer: A. The OSPF simple password configuration uses the ip ospf authentication subcommand, which identifies the type as simple password authentication. R1 uses simple password authentication, and forms neighbor relationships with the other two routers, so they also must use the same authentication type. R1’s configuration must define the password with the ip ospf authentication-key value interface subcommand. However, the other routers can enable simple password authentication either by using the same ip ospf authentication subcommand, or by using the area authentication command, so R3 may not have an ip ospf authentication subcommand configured.The show ip ospf interface f0/0 command will list the fact that simple password authentication is used, on any of the three routers, because they all happen to use the same interface number. However, it will not list MD5 authentication, because none of the routers use OSPF MD5 authentication. The show ip ospf neighbor command does not list information about the authentication type.

Q10. 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. The OSPF maximum-paths number router subcommand sets the number of equalcost routes added to the routing table. This command defaults to a setting of 4.

Q11. What is the OSPF router ID, and how is it determined?

Answer: The router uses the OSPF router ID to know the rest of the OSPF routing domain. The highest IP address of the logical loopback interfaces is used at the startup of the OSPF routing process to determine the OSPF router ID. If no loopback interfaces are configured or enabled, the router uses the highest IP address of an active physical interface.

Q12. What is the significance of a designated router, and what circumstances are necessary for a router to become a DR?

Answer: The designated router is elected by broadcast and nonbroadcast multiaccess networks to minimize the amount of routing update overhead. On each network segment, the router with the highest interface priority (default is 1) is elected as the DR, and the router that has the interface with the second-highest priority is the BDR. In cases where the interface priorities are tied, the highest router ID is used as a tiebreaker.

Q13. What is the purpose of an OSPF area?

Answer: OSPF areas are a way to segment an OSPF routing domain into smaller routing systems to reduce the amount of routing overhead and to confine topology changes. Area 0 is known as the backbone area, to which all other areas must connect.

Q14. How does the DUAL algorithm use the contents of the topology table to ensure rapid convergence?

Answer: The DUAL algorithm keeps track of the advertised distance and the feasible distance for each network in the topology table. The lowest feasible distance to a destination is known as the successor route and is the network that is put in the routing table. If the router receives an update that has an advertised distance less than the successor route’s feasible distance, that entry becomes the feasible successor, which is used if the successor route fails.

Q15. What are the basic configuration steps for using OSPF and EIGRP routing protocols?

Answer: To configure OSPF and EIGRP, you must enter the routing process using the router keyword followed by the routing protocol. In the case of OSPF, you must also specify a process ID to locally identify the instance of OSPF. The valid range of OSPF process IDs is between 1 and 65535 and does not need to match in all router configurations.

On the other hand, EIGRP requires that you specify an autonomous system number that does have to match in all router configurations. After you are in the routing process, you advertise the connected networks with the network command. EIGRP configurations require that you enter the directly connected classful network(s) after the network keyword. Conversely, OSPF configurations require that you specify the network ID followed by the wildcard mask. Following the wildcard mask, you must also use the area keyword, followed by the area to which that network belongs.

Q16. David, your Cisco coworker, shows you the following output from a debug ip ospf events command:
ccna-faq-ospf-1
He knows that the Rs stand for received and the Cs stand for configured. However, he can’t figure out why he cannot get OSPF to work. What should you tell him?
A. OSPF needs to have the classful networks advertised.
B. Area 51 is an invalid area number.
C. Check the OSPF timers.
D. 10.1.42.100 needs to be a DR.

Answer: C. For OSPF to form a neighbor relationship, the hello/dead timers, stub flag, authentication password, and area ID must match. Because the output shows that the received timers are different from those that were configured, the timers must be misconfigured. Answer A is incorrect because OSPF is a classless routing protocol. Answer B is incorrect because Area 51 is an area within the acceptable ranges of 0 to 65535. Answer D is incorrect because 10.1.42.100 does not need to be a DR to send hello messages.

Q17. Given the following output, which of the following statements is false?
ccna-faq-ospf-2
A. This router will listen for updates on multicast address 224.0.0.6.
B. 10.1.1.1 is not a loopback IP address.
C. The router interface is not connected to the backbone area.
D. This router will listen for LSA hellos on 224.0.0.5.

Answer: B. The router ID is chosen based on the highest active virtual loopback address. If no loopback is present, the highest physical IP address is chosen when the OSPF process starts. Answer A is true because the output indicates that this router’s interface is the DR, which listens to 224.0.0.6. Answer C is also true because this interface is connected to Area 51, not Area 0. Answer D is true because all OSPF routers (including the DR and BDR) listen to hellos on 224.0.0.5.

Q18. Given the following customer requirements, which routing protocol would you recommend?
Requirements: fast convergence, IP only, large network, VLSM support needed, Cisco and Nortel routers.
A. RIPv2
B. RIPv1
C. EIGRP
D. OSPF

Answer: D. Given the customer requirements, Answer D is the only viable option.

Answers A and B aren’t good choices because they require fast convergence. Answer C is tempting, but the fact that the customer has Nortel routers cancels out EIGRP and IGRP, because they are Cisco-proprietary.

Q19. Based on the following output, which of the following statements are true? (Choose two)
ccna-faq-ospf-4
A. The route for 172.17.0.0 is down and is being queried.
B. Router 192.168.1.31 has a composite metric of 2297856 to get to 172.17.0.0.
C. Router 192.168.1.6 has an administrative distance of 2195456.
D. There is no feasible successor to 172.17.0.0.

Answer: B, D. The topology table shows that 192.168.1.31 is advertising (advertised distance) a metric of 2297856. Because that advertised distance is larger than the feasible distance of the successor route (281600), it cannot be a feasible successor.

Answer A is incorrect because the subnet is not in an active state. Answer C is tricky, because the number with the slash is not the administrative distance, as you would see in a routing table. It is the advertised distance followed by the feasible distance.

20. Given the following output, why is OSPF not working correctly?
ccna-faq-ospf-5
A. The network ID and wildcard mask are incorrect.
B. The area needs to be configured as a stub area.
C. You need the no auto-summary command to make OSPF classless.
D. The autonomous system number doesn’t match other router configurations.

Answer: A. The network ID for Serial 0/0’s subnet should be 192.168.1.4, and the wildcard mask for a 255.255.255.252 subnet mask is 0.0.0.3.

Answer B is incorrect because the type of area has no bearing on this configuration. Answer C is incorrect because OSPF does not require a command to make it classless. Answer D is incorrect because OSPF does not use autonomous system numbers.

Q21. Given the following output, what will the OSPF router ID be for this router if you configure it for OSPF?
ccna-faq-ospf-6
A. OSPF does not require a router ID because this router has a broadcast topology.
B. 172.16.0.1
C. 10.1.42.1
D. 192.168.1.5

Answer: D. Typically, the 10.1.42.1 address would be the correct router ID for OSPF. However, the output shows it as administratively down. Because that interface isn’t active, the highest active physical IP address is used (192.168.1.5).

Answer A is incorrect because all OSPF routers use a Router ID. Answer B is incorrect because it is not the highest active IP address. Answer C would be correct if the interface were not administratively shut down.

Q22. What is the cost of a 512kbps link for OSPF?
A. 156
B. 195
C. 10
D. 64

Answer: B. To calculate OSPF cost, you take 108/bandwidth in bps. Thus, 100000000/512000 = 195. Answer A is the cost of a link speed of 640kbps. Answer C is the cost of 10Mbps Ethernet. Answer D is the cost of a T1.

Q23. What is the effect of the following configuration?
ccna-faq-ospf-8
A. Only interface Fast Ethernet 0/0’s subnet will be associated into OSPF area 10.
B. Only interface Serial 0/0/0’s subnet will be associated into OSPF area 10.
C. Only interface Serial 0/0/1’s subnet will be associated into OSPF area 10.
D. None of the above

Answer: D. Because the wildcard mask is configured to allow any network starting with 10.x.x.x to be in the OSPF routing process, all interfaces are applied in this configuration.

Answers A, B, and C are incorrect because the wildcard mask encompasses all three of the networks assigned to the interfaces.

Q24. Which command enables unequal load balancing over all three links with the metrics Network A-234, Network B-23, and Network C-601?
A. variance 10
B. variance 3
C. variance 30
D. variance 25

Answer: C. Because the lowest metric to the destinations is 23, and the highest is 601, a multiplier of 30 enables metric values from 23 to 690 (30×23). Answer A is incorrect because the multiplier only loadbalances over links with a metric up to 230.

Answer B is incorrect because the multiplier only load-balances over links with a metric up to 69. Answer D is incorrect because the multiplier only load-balances over links with a metric up to 575.

Q25. What would be the result of the following two configurations?
ccna-faq-ospf-10
A. Router A adds the 172.18.0.0 network to its routing table.
B. Router B does not add the 192.168.34.0 network, because it is advertising it to Router A.
C. The routing updates do not contain the subnet masks.
D. Router A and Router B ignore each other’s updates.

Answer: D. The autonomous system number must match in both devices, or the routers ignore each other’s updates. Answer A is incorrect because Router A is not advertising the 172.18.0.0 network.

Answer B is incorrect because Router B already has the 192.168.34.0 network in its routing table because Router B is advertising this directly connected network. Answer C is incorrect because EIGRP is configured as classless because the no auto-summary command is used.

Q26. Which of the following characteristics does not apply to EIGRP and OSPF?
A. The timers for hello/dead differ depending on the topology.
B. Both routing protocols support VLSM, router summarization, and discontiguous networks by default.
C. Both routing protocols have a routing table, neighbor table, and topology table.
D. Both routing protocols discover neighbors by sending hellos to a multicast address.

Answer: B. EIGRP is not classless by default. You must configure the no auto-summary command for EIGRP to support VLSM, router summarization, and discontiguous networks.

Answer A is true because OSPF and EIGRP both have different hello/dead intervals, depending on the topology to which the interfaces are connected. Answer C is true because EIGRP and OSPF both have routing, topology, and neighbor tables. Answer D is true because OSPF and EIGRP both discover neighbors by sending hello messages to a multicast address.

Q27. There are three possible routes for a router to reach a destination network. The first route is from OSPF with a metric of 782. The second route is from RIPv2 with a metric of 4. The third is from EIGRP with a composite metric of 20514560. Which route will be installed by the router in its routing table?
A. RIPv2
B. EIGRP
C. OSPF
D. All three

Answer: B. Only the EIGRP routes will be placed in the routing table because it has the lowest administrative distance (AD), and that is always used before metrics.

Q28. In the accompanying diagram, which of the routers must be ABRs? (Choose all that apply.)

Answer: A, B, C. Any router that is member of two areas must be an area border router or ABR.

Q29. Which of the following describe the process identifier that is used to run OSPF on a router? (Choose two.)
A. It is locally significant.
B. It is globally significant.
C. It is needed to identify a unique instance of an OSPF database.
D. It is an optional parameter required only if multiple OSPF processes are running on the router.
E. All routes in the same OSPF area must have the same process ID if they are to exchange routing information.

Answer: A, C. The process ID for OSPF on a router is only locally signifiant and you can use the same number on each router, or each router can have a different number—it just doesn’t matter. The numbers you can use are from 1 to 65,535. Don’t get this confused with area numbers, which can be from 0 to 4.2 billion.

Q30. All of the following must match for two OSPF routers to become neighbors except which?
A. Area ID
B. Router ID
C. Stub area flag
D. Authentication password if using one

Answer: B. The router ID (RID) is an IP address used to identify the router. It need not and should not match.

Q31. In the diagram, by default what will be the router ID of Lab_B?

A. 10.255.255.82
B. 10.255.255.9
C. 192.168.10.49
D. 10.255.255.81

Answer: C. The router ID (RID) is an IP address used to identify the router. Cisco chooses the router ID by using the highest IP address of all confiured loopback interfaces. If no loopback interfaces are confiured with addresses, OSPF will choose the highest IP address of all active physical interfaces.

Q32. You get a call from a network administrator who tells you that he typed the following into his router:

Router(config)#router ospf 1
Router(config-router)#network 10.0.0.0 255.0.0.0 area 0

He tells you he still can’t see any routes in the routing table. What configuration error did the administrator make?
A. The wildcard mask is incorrect.
B. The OSPF area is wrong.
C. The OSPF process ID is incorrect.
D. The AS configuration is wrong.

Answer: A. The administrator typed in the wrong wildcard mask confiuration. The wildcard should have been 0.0.0.255 or even 0.255.255.255.

Q33. Which of the following statements is true with regard to the output shown?

Corp#sh ip ospf neighbor
Neighbor ID  Pri State    Dead Time Address      Interface
172.31.1.4 1     FULL/BDR 00:00:34  10.10.10.2   FastEthernet0/0
192.168.20.1  0  FULL/ -  00:00:31  172.16.10.6  Serial0/1
192.168.10.1  0  FULL/ -  00:00:32  172.16.10.2  Serial0/0

A. There is no DR on the link to 192.168.20.1.
B. The Corp router is the BDR on the link to 172.31.1.4.
C. The Corp router is the DR on the link to 192.168.20.1.
D. The link to 192.168.10.1 is Active.

Answer: A. A dash (-) in the State column indicates no DR election, because they are not required on a point-to-point link such as a serial connection.

Q34. What is the administrative distance of OSPF?
A. 90
B. 100
C. 120
D. 110

Answer: D. By default the administrative distance of OSPF is 110.

Q35. In OSPF, Hellos are sent to what IP address?
A. 224.0.0.5
B. 224.0.0.9
C. 224.0.0.10
D. 224.0.0.1

Answer: A. Hello packets are addressed to multicast address 224.0.0.5.

Q36. What command generated the following output?

172.31.1.4   1 FULL/BDR  00:00:34  10.10.10.2   FastEthernet0/0
192.168.20.  1 0 FULL/ - 00:00:31  172.16.10.6  Serial0/1
192.168.10.  1 0 FULL/ - 00:00:32  172.16.10.2  Serial0/0

A. show ip ospf neighbor
B. show ip ospf database
C. show ip route
D. show ip ospf interface

Answer: A. The show ip ospf neighbor command displays all interface-related neighbor information. This output shows the DR and BDR (unless your router is the DR or BDR), the RID of all directly connected neighbors and the IP address and name of the directly connected interface.

Q37. Updates addressed to 224.0.0.6 are destined for which type of OSPF router?
A. DR
B. ASBR
C. ABR
D. All OSPF routers

Answer: A. 224.0.0.6 is used on broadcast networks to reach the DR and BDR.

Q38. For some reason, you cannot establish an adjacency relationship on a common Ethernet link between two routers. Looking at this output, what is the cause of the problem?

RouterA#
Ethernet0/0 is up, line protocol is up
Internet Address 172.16.1.2/16, Area 0
Process ID 2, Router ID 172.126.1.2, Network Type BROADCAST, Cost: 10
Transmit Delay is 1 sec, State DR, Priority 1
Designated Router (ID) 172.16.1.2, interface address 172.16.1.1
No backup designated router on this network
Timer intervals configured, Hello 5, Dead 20, Wait 20, Retransmit 5

RouterB#
Ethernet0/0 is up, line protocol is up
Internet Address 172.16.1.1/16, Area 0
Process ID 2, Router ID 172.126.1.1, Network Type BROADCAST, Cost: 10
Transmit Delay is 1 sec, State DR, Priority 1
Designated Router (ID) 172.16.1.1, interface address 172.16.1.2
No backup designated router on this network
Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5

A. The OSPF area is not configured properly.
B. The priority on RouterA should be set higher.
C. The cost on RouterA should be set higher.
D. The Hello and Dead timers are not configured properly.
E. A backup designated router needs to be added to the network.
F. The OSPF process ID numbers must match.

Answer: D. The Hello and Dead timers must be set the same on two routers on the same link or they will not form an adjacency (relationship). The default timers for OSPF are 10 seconds for the Hello timer and 40 seconds for the Dead timer.

Q39. In the work area match each OSPF term (by line) to its definition.
Designated router contains only the best routes
Topological database elected on broadcast networks
Hello protocol contains all routes learned
Routing table provides dynamic neighbor discovery

Answer:

A designated router is elected on broadcast networks. Each OSPF router maintains an identical database describing the AS topology. A Hello protocol provides dynamic neighbor discovery. A routing table contains only the best routes.

Q40. Type the command that will disable OSPF on the Fa0/1 interface under the routing process. Write only the command and not the prompt.

Answer: passive-interface fastEthernet 0/1 The command passive-interface fastEthernet 0/1 will disable OSPF on the specifid interface only.

Q41. Which two of the following commands will place network 10.2.3.0/24 into area 0? (Choose two.)
A. router eigrp 10
B. router ospf 10
C. router rip
D. network 10.0.0.0
E. network 10.2.3.0 255.255.255.0 area 0
F. network 10.2.3.0 0.0.0.255 area0
G. network 10.2.3.0 0.0.0.255 area 0

Answer: B, G. To enable OSPF, you must fist start OSPF using a process ID. The number is irrelevant; just choose a number from 1 to 65,535 and you’re good to go. After you start the OSPF process, you must confiure interfaces on which to activate OSPF using the network command with wildcards and specifiation of an area. Option F is wrong because there must be a space after the parameter area and before you list the area number.

Q42. Given the following output, which statement or statements can be determined to be true? (Choose all that apply.)

RouterA2# show ip ospf neighbor

Neighbor ID Pri State   Dead Time Address   Interface
192.168.23.2 1 FULL/BDR 00:00:29  10.24.4.2 FastEthernet1/0
192.168.45.2 2 FULL/BDR 00:00:24  10.1.0.5  FastEthernet0/0
192.168.85.1 1 FULL/-   00:00:33  10.6.4.10 Serial0/1
192.168.90.3 1 FULL/DR  00:00:32  10.5.5.2  FastEthernet0/1
192.168.67.3 1 FULL/DR  00:00:20  10.4.9.20 FastEthernet0/2
192.168.90.1 1 FULL/BDR 00:00:23  10.5.5.4  FastEthernet0/1
<<output omitted>>

A. The DR for the network connected to Fa0/0 has an interface priority higher than 2.
B. This router (A2) is the BDR for subnet 10.1.0.0.
C. The DR for the network connected to Fa0/1 has a router ID of 10.5.5.2.
D. The DR for the serial subnet is 192.168.85.1.

Answer: A. The default OSPF interface priority is 1, and the highest interface priority determines the designated router (DR) for a subnet. The output indicates that the router with a router ID of 192.168.45.2 is currently the backup designated router (BDR) for the segment, which indicates that another router became the DR. It can be then be assumed that the DR router has an interface priority higher than 2. (The router serving the DR function is not present in the truncated sample output.)

Q43. What are three reasons for creating OSPF in a hierarchical design? (Choose three.)
A. To decrease routing overhead
B. To speed up convergence
C. To confine network instability to single areas of the network
D. To make configuring OSPF easier

Answer: A, B, C. OSPF is created in a hierarchical design, not a flt design like RIP. This decreases routing overhead, speeds up convergence, and confies network instability to a single area of the network.

Q44. Type the command that produced the following output. Write only the command and not the prompt.

FastEthernet0/0 is up, line protocol is up
Internet Address 10.10.10.1/24, Area 0
Process ID 1, Router ID 223.255.255.254, Network Type BROADCAST, Cost: 1
Transmit Delay is 1 sec, State DR, Priority 1
Designated Router (ID) 223.255.255.254, Interface address 10.10.10.1
Backup Designated router (ID) 172.31.1.4, Interface address 10.10.10.2
Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5
oob-resync timeout 40
Hello due in 00:00:08
Supports Link-local Signaling (LLS)
Cisco NSF helper support enabled
IETF NSF helper support enabled
Index 3/3, flood queue length 0
Next 0x0(0)/0x0(0)
Last flood scan length is 1, maximum is 1
Last flood scan time is 0 msec, maximum is 0 msec
Neighbor Count is 1, Adjacent neighbor count is 1
Adjacent with neighbor 172.31.1. Suppress hello for 0 neighbor(s)

Answer: show ip ospf interface The show ip ospf interface command displays all interface-related OSPF information. Data is displayed about OSPF information for all OSPF-enabled interfaces or for specifid interfaces.

Q45. A(n) ____________is an OSPF data packet containing link-state and routing information that are shared among OSPF routers.
A. LSA
B. TSA
C. Hello
D. SPF

Answer: A. LSA packets are used to update and maintain the topological database.

Q46. If routers in a single area are configured with the same priority value, what value does a router use for the OSPF router ID in the absence of a loopback interface?
A. The lowest IP address of any physical interface
B. The highest IP address of any physical interface
C. The lowest IP address of any logical interface
D. The highest IP address of any logical interface

Answer: B. At the moment of OSPF process startup, the highest IP address on any active interface will be the router ID (RID) of the router. If you have a loopback interface confiured (logical interface), then that will override the interface IP address and become the RID of the router automatically.

Q47. Which of the following are scalability issues with single-area OSPF networks? (Choose all that apply.)
A. Size of the routing table
B. Size of the OSPF database
C. Maximum hop-count limitation
D. Recalculation of the OSPF database

Answer: A, B, D. As the size of a single-area OSPF network grows, so does the size of the routing table and OSPF database that have to be maintained. Also, if there is a change in network topology, the OSPF algorithm has to be rerun for the entire network.

Q48. Which of the following describes a router that connects to an external routing process (e.g., EIGRP)?
A. ABR
B. ASBR
C. Type 2 LSA
D. Stub router

Answer: B. An autonomous system boundary router (ASBR) is any OSPF router that is connected to an external routing process (another AS). An ABR, on the other hand, connects one (or more) OSPF areas together to area 0.

Q49. Which of the following must match in order for an adjacency to occur between routers? (Choose three.)
A. Process ID
B. Hello and dead timers
C. Link cost
D. Area
E. IP address/subnet mask

Answer: B, D, E. In order for two OSPF routers to create an adjacency, the hello and dead timers must match, and they must both be confiured into the same area as well as being in the same subnet. Also, if authentication is confiured, that info must match as well.

Q50. Which OSPF state do two routers forming an adjacency appear as in the show ip ospf neighbor output after adding neighbors into the table and exchanging hello information?
A. ATTEMPT
B. INIT
C. 2WAY
D. EXSTART
E. FULL

Answer: C. The process starts by sending out Hello packets. Every listening router will then add the originating router to the neighbor database. The responding routers will reply with all of their Hello information so that the originating router can add them to its own neighbor table. At this point, we will have reached the 2WAY state—only certain routers will advance beyond this to establish adjacencies.

Q51. You need to set up a preferred link that OSPF will use to route information to a remote network. Which command will allow you to set the interface link as preferred over another?
A. ip ospf preferred 10
B. ip ospf priority 10
C. ospf bandwidth 10
D. ip ospf cost 10

Answer: D. If you have multiple links to the same network, you can change the default cost of a link so OSPF will prefer that link over another with the ip ospf cost cost command.

Q52. When would a router’s neighbor table show the FULL/DR state?
A. After the first Hello packets are received by a neighbor
B. When all information is synchronized among adjacent neighbors
C. When the router’s neighbor table is too full of information and is discarding neighbor information
D. After the EXSTART state

Answer: B. In the FULL state, all LSA information is synchronized among adjacent neighbors. OSPF routing can begin only after the FULL state has been reached. The FULL state occurs after the LOADING state fiishes.

Q53. Which is/are true regarding OSPFv3? (Choose all that apply.)
A. You must add network statements under the OSPF process.
B. There are no network statements in OSPFv3 configurations.
C. OSPFv3 uses a 128-bit RID.
D. If you have IPv4 configured on the router, it is not mandatory that you configure the RID.
E. If you don’t have IPv4 configured on the router, it is mandatory that you configure the RID.
F. OSPFv3 doesn’t use LSAs like OSPFv2 does.

Answer: B, D, E. Confiuring OSPFv3 is pretty simple, as long as you know what interfaces you are using on your router. There are no network statements, OSPFv3 is confiured on a per-interface basis. OSPFv2 and OSPFv3 both use a 32-bit RID, have an IPv4 address confiured on at least one interface, you do not need to manually set a RID when confiuring EIGRPv3.

Q54. What is the default cost of a Fast Ethernet interface configured with OSPF?
A. 1
B. 10
C. 100
D. 1,000

Answer: A. Cisco uses a simple calculation of 108/bandwidth of a link to determine the cost. This means anything that is 100 Mbps or higher will have a default cost of 1.

Q55. Which type of LSA is generated by DRs and referred to as a network link advertisement (NLA)?
A. Type 1
B. Type 2
C. Type 3
D. Type 4
E. Type 5

Answer: B. Referred to as a network link advertisement (NLA), Type 2 LSAs are generated by designated routers (DRs). Remember that a designated router is elected to represent other routers in its network, and it establishes adjacencies with them. The DR uses a Type 2 LSA to send out information about the state of other routers that are part of the same network.

Q56. Which type of LSA is generated by ABRs and refers to a summary link advertisement (SLA)?
A. Type 1
B. Type 2
C. Type 3
D. Type 4
E. Type 5

Answer: C. Referred to as summary link advertisements (SLAs), Type 3 LSAs are generated by area border routers. These ABRs send Type 3 LSAs toward the area external to the one where they were generated. The Type 3 LSA advertises networks, and these LSAs advertise inter-area routes to the backbone area (area 0).

Q57. Which command will show all the LSAs known by a router?
A. show ip ospf
B. show ip ospf neighbor
C. show ip ospf interface
D. show ip ospf database

Answer: D. To see all LSAs a router has learned from its neighbors, you need to see the OSPF LSDB, and you can see this with the show ip ospf database command.

Q58. Using the following illustration, what is the cost from R1’s routing table to reach the network with Server 1? Each Gigabit Ethernet link has a cost of 4, and each serial link has a cost of 15.
ccna-faq-ospf--12
A. 100
B. 23
C. 64
D. 19
E. 27

Answer: B. Based on the information in the question, the cost from R1 to R2 is 4, the cost from R2 to R3 is 15, and the cost from R3 to R5 is 4. 15 + 4 + 4 = 23. Pretty simple.

Q59. Using the following illustration, which of the following are true? (Choose all that apply.)
ccna-faq-ospf--13
A. R1 is an internal router.
B. R3 would see the networks connected to the R1 router as an inter-area route.
C. R2 is an ASBR.
D. R3 and R4 would receive information from R2 about the backbone area, and the same LSA information would be in both LSDBs.
E. R4 is an ABR.

Answer: B, D. Since R3 is connected to area 1 and R1 is connected to area 2 and area 0, the routes advertised from R3 would show as OI, or inter-area routes.

Q60. Which of the following could cause two routers to not form an adjacency? (Choose all that apply.)
A. They are configured in different areas.
B. Each router sees the directly connected link as different costs.
C. Two different Process ID’s configured.
D. ACL is configured on the routing protocol.
E. IP address/mask.
F. Passive interface is configured.

Answer: A, D, E, F. For two OSPF routers to form an adjacency, they must be in the same area, must be in the same subnet, and must have authentication information match, if confiured. You need to also check if an ACL is set and if a passive interface is confiured.

Q61. Which of the following IOS commands shows the state of an adjacency with directly connected routers?
A. debug ospf events
B. show ip ospf border-routers
C. show ip ospf neighbor
D. show ip ospf database

Answer: C. The IOS command show ip ospf neighbor shows neighbor router information, such as neighbor ID and the state of adjacency with the neighboring router.

Q62. What command will show you the DR and DBR address of the area you are connected to directly with an interface?
A. show interface s0/0/0
B. show interface fa0/0
C. show ip ospf interface s0/0/0
D. show ip ospf interface fa0/0

Answer: D. The command show ip ospf interface on a default broadcast multi-access network will show you DRs and BDRs on that network.

Q63. Which of the following could be causing a problem with the Corp router not forming an adjacency with its neighbor router? (Choose all that apply.)
ccna-faq-ospf--17
A. The routers are configured with the wrong network statements.
B. They have different maximum paths configured.
C. There is a passive interface configured.
D. There is an ACL set stopping Hellos.
E. The costs of the links between the routers are configured differently.
F. They are in different areas.

Answer: A, C, D, F. It’s hard to tell from this single output what is causing the problem with the adjacency, but we need to check the ACL 10 to see what that is doing, verify that the routers are in the same area and in the same subnet, and see if that passive interface is confiured with the interface we’re using.

Q64. Which of the following is/are true? (Choose all that apply.)
A. The reference bandwidth for OSPF and OSPFv3 is 1.
B. The reference bandwidth for OSPF and OSPFv3 is 100.
C. You change the reference bandwidth from global config with the command auto-cost reference bandwidth number.
D. You change the reference bandwidth under the OSPF router process with the command auto-cost reference bandwidth number.
E. Only one router needs to set the reference bandwidth if it is changed from its default.
F. All routers in a single area must set the reference bandwidth if it is changed from its default.
G. All routers in the AS must set the reference bandwidth if it is changed from its default.

Answer: B, D, G. The default reference bandwidth is 100 by default, and you can change it under the OSPF process with the auto-cost reference bandwidth number command, but if you do, you need to confiure this command on all routers in your AS.

Q65. Which of the following interfaces would have the same default cost? (Choose two.)
A. Fast Ethernet
B. Ethernet
C. Serial
D. Gigabit Ethernet

Answer: A, D. Anything Fast Ethernet or above would have a default cost of 1, and serial interfaces have a default cost of 64.

Q66. What is the default cost of a serial interface with OSPF?
A. 1
B. 10
C. 32
D. 64
E. 100

Answer: D. Anything Fast Ethernet or above would have a default cost of 1, and serial interfaces have a default cost of 64.

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CCNA FAQ: OSPF

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