CCNA 3 v6 Chapter 8: Check Your Understanding Questions Answers

CCNA 3 v6.0 (Scaling Networks v6) Chapter 8: Single-Area OSPF: Check Your Understanding Questions Answers

1. A router is participating in an OSPFv2 domain. What will always happen if the dead interval expires before the router receives a Hello packet from an adjacent DROTHER OSPF router?

  • A new dead interval timer of four times the hello interval will start.
  • OSPF will remove that neighbor from the router link-state database.
  • OSPF will run a new DR/BDR election.
  • SPF will run and determine which neighbor router is down.

Explanation: On Cisco routers, the default dead interval is four times the hello interval, and this timer has expired in this case. SPF does not determine the state of neighbor routers; it determines which routes become routing table entries. A DR/DBR election will not always automatically run; whether it runs depends on the type of network and on whether the router that is no longer up was a DR or BDR.

2. Which three statements describe the similarities between OSPFv2 and OSPFv3? (Choose three.)

  • They both are link-state protocols.
  • They both have unicast routing enabled by default.
  • They both share the concept of multiple areas.
  • They both support IPsec for authentication.
  • They both use the global address as the source address when sending OSPF messages.
  • They both use the same DR/BDR election process.

Explanation: Only OSPFv2 messages are sourced from the IP address of the exit interface; OSPFv3 uses the link-local address of the exit interface. Only OSPFv3 uses IPsec; OSPFv2 uses plaintext or MD5 authentication. Unicast routing is enabled by default only with OSPFv2.

3. Which OSPF component is identical in all routers in an OSPF area after convergence?

  • adjacency database
  • link-state database
  • routing table
  • SPF tree

Explanation: Each OSPF router views the network differently, as the root of a unique SPF tree. Each router builds adjacencies based on its own position in the topology. Each routing table in the area is developed individually through the application of the SPF algorithm. The link-state database for an area, however, must reflect the same information for all routers.

4. Which three statements describe features of the OSPF topology table? (Choose three.)

  • After convergence, the table contains only the lowest-cost route entries for all known networks.
  • It is a link-state database that represents the network topology.
  • Its contents are the result of running the SPF algorithm.
  • The table can be viewed via the show ip ospf database command.
  • The topology table contains feasible successor routes.
  • When converged, all routers in an area have identical topology tables.

Explanation: The topology table on an OSPF router is a link-state database (LSDB) that lists information about all other routers in the network and represents the network topology. All routers within an area have identical link-state databases, and the table can be viewed using the show ip ospf database command. The EIGRP topology table contains feasible successor routes. This concept is not used by OSPF. The SPF algorithm uses the LSDB to produce the unique routing table for each router, which contains the lowest cost route entries for known networks.

5. Which of the following is used to create the OSPF neighbor table?

  • Adjacency database
  • Link-state database
  • Forwarding database
  • Routing table

Explanation: The adjacency database is used to create the OSPF neighbor table. The link-state database is used to create the topology table, and the forwarding database is used to create the routing table.

6. What is a function of OSPF Hello packets?

  • to discover neighbors and build adjacencies between them
  • to ensure database synchronization between routers
  • to request specific link-state records from neighbor routers
  • to send specifically requested link-state records

Explanation: The OSPF Hello packet serves three primary functions: discover OSPF neighbors and establish adjacencies, advertise parameters that OSPF neighbors must agree on, and, when necessary, elect the DR and BDR.

7. Which OSPF packet contains the different types of link-state advertisements?

  • Hello
  • DBD
  • LSAck
  • LSR
  • LSU

Explanation: Link-State Update (LSU) packets contain different types of link-state advertisements (LSA). The LSUs are used to reply to Link-State Request (LSR) packets and to announce new information.

8. What are the two purposes of an OSPF router ID? (Choose two.)

  • To enable the SPF algorithm to determine the lowest-cost path to remote networks
  • To facilitate router participation in the election of the designated router
  • To facilitate the establishment of network convergence
  • To facilitate the transition of the OSPF neighbor state to Full
  • To uniquely identify the router within the OSPF domain

Explanation: OSPF router ID does not contribute to SPF algorithm calculations, nor does it facilitate the transition of the OSPF neighbor state to Full. Although the router ID is contained within OSPF messages when router adjacencies are being established, it has no bearing on the convergence process.

9. What is the first criterion used by OSPF routers to elect a DR?

  • Highest priority
  • Highest IP address
  • Highest router ID
  • Highest MAC address

Explanation: When electing a DR, the router with the highest OSPF priority becomes the DR. If all routers have the same priority, then the router with the highest router ID is elected.

10. Which wildcard mask would be used to advertise the network as part of an OSPF configuration?


Explanation: The wildcard mask can be found by subtracting the subnet mask from

11. What are two factors that will prevent two routers from forming an OSPFv2 adjacency? (Choose two.)

  • Mismatched Cisco IOS versions are used
  • Mismatched Ethernet interface (for example, Fa0/0 to G0/0)
  • Mismatched OSPF hello or dead timers
  • Mismatched subnet masks on the link interfaces
  • Use of private IP addresses on the link interfaces

Explanation: C and D. There may be several reasons why routers running OSPF will fail to form an OSPF adjacency, including subnet masks not matching, OSPF hello or dead timers not matching, OSPF network types not matching, or a missing or incorrect OSPF network command. Mismatched IOS versions, the use of private IP addresses, and different types of interface ports do not cause an OSPF adjacency to fail to form between two routers.

12. What command would be used to determine whether a routing protocol–initiated relationship had been made with an adjacent router?

  • ping
  • show ip interface brief
  • show ip ospf neighbor
  • show ip protocols

Explanation: While the show ip interface brief and ping commands can be used to determine whether Layer 1, 2, and 3 connectivity exists, neither command can be used to determine whether a particular OSPF or EIGRP-initiated relationship has been made. The show ip protocols command is useful in determining routing parameters such as timers, router ID, and metric information associated with a specific routing protocol. The show ip ospf neighbor command shows whether two adjacent routers have exchanged OSPF messages in order to form a neighbor relationship.

13. Which function works differently in OSPFv3 than in OSPFv2?

  • Authentication
  • Election process
  • Hello mechanism
  • Metric calculation
  • OSPF packet types

Explanation: Both versions of OSPF use the same five basic packet types, cost metric, and DR/BDR election process. Hello packets are used in both versions to build adjacencies. OSPFv3, however, uses advanced encryption and authentication features that are provided by IPsec, while OSPFv2 uses either plaintext or MD5 authentication.

14. Which three addresses could be used as the destination address for OSPFv3 messages? (Choose three.)

  • FE80::1
  • FF02::5
  • FF02::6
  • FF02::A
  • FF02::1:2
  • 2001:db8:cafe::1

Explanation: OSPFv6 messages can be sent to either the OSPF router multicast FF02::5, the OSPF DR/BDR multicast FF02::6, or the link-local address.

15. What does a Cisco router use automatically to create link-local addresses on serial interfaces when OSPFv3 is implemented?

  • An Ethernet interface MAC address available on the router, the FE80::/10 prefix, and the EUI-64 process
  • The FE80::/10 prefix and the EUI-48 process
  • The highest MAC address available on the router, the FE80::/10 prefix, and the EUI-48 process
  • The MAC address of the serial interface, the FE80::/10 prefix, and the EUI64 process

Explanation: Because serial interfaces do not have MAC addresses, OSPFv3 automatically assigns a link-local address to them, based on the first available MAC address from the pool of Ethernet interface addresses on the router. A FE80::/10 prefix is added. The router then applies the EUI-64 process to the MAC address by inserting FFFE into the middle of the existing 48-bit address and flipping the seventh bit.

16. A network administrator enters the command ipv6 router ospf 64 in global configuration mode. What is the result of this command?

  • The router is assigned an autonomous system number of 64.
  • The router is assigned a router ID of 64.
  • The reference bandwidth is set to 64 Mbps.
  • The OSPFv3 process is assigned an ID of 64.

Explanation: The basic command to implement OSPFv3 on a router uses the same process-id parameter as OSPFv2 to assign a locally significant number to the OSPF process. OSPF does not use autonomous system numbers. Following the assignment of the process ID, a prompt directs the user to manually assign a router ID. After the router ID is assigned, the reference bandwidth can be set.

17. Single-area OSPFv3 has been enabled on a router via the ipv6 router ospf 20 command. Which command enables this OSPFv3 process on an interface of that router?

  • ipv6 ospf 0 area 0
  • ipv6 ospf 0 area 20
  • ipv6 ospf 20 area 0
  • ipv6 ospf 20 area 20

Explanation: The command to enable an OSPFv3 process on a router interface is ipv6 ospf process-id area area-id. In the case, the process ID is 20, and the area ID is 0.

18. Which command verifies that a router that is running OSPFv3 has formed an adjacency with other routers in its OSPF area?

  • show ipv6 interface brief
  • show ipv6 ospf neighbor
  • show ipv6 route ospf
  • show running-configuration

Explanation: The show ipv6 ospf neighbor command verifies neighbor adjacencies for OSPFv3 routers. The other options do not provide neighbor information.

19. Which command provides information specific to OSPFv3 routes in the routing table?

  • show ip route
  • show ip route ospf
  • show ipv6 route
  • show ipv6 route ospf

Explanation: The show ipv6 route ospf command gives specific information that is related to OSPFv3 routes in the routing table. The show ipv6 route command shows the entire routing table. The show ip route and show ip route ospf commands are used with OSPFv2.

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