CCNA 2 Routing and Switching Essentials v5 Chapter 8: Single-Area OSPF – Check Your Understanding Questions Answers
1. What will an OSPF router prefer to use first as a router ID?
- Any IP address that is configured using the router-id command
- A loopback interface that is configured with the highest IP address on the router
- The highest active interface IP that is configured on the router
- The highest active interface that participates in the routing process because of a specifically configured network statement
Explanation: The first preference for an OSPF router ID is an explicitly configured 32-bit address. This address is not included in the routing table and is not defined by the network command. If a router ID that is configured through the router-id command is not available, OSPF routers next use the highest IP address available on a loopback interface because loopbacks used as router IDs are also not routable addresses. Lacking either of these alternatives, an OSPF router will use the highest IP address from its active physical interfaces.
2. At which OSPF state are neighbor routers converged and can exchange routing updates?
- Two-Way
- ExStart
- Exchange
- Full
Explanation: OSPF neighbors that reach the Full state are converged and can exchange routing information.
3. Which OSPF wildcard mask would be appropriate to use for the given network prefix?
- /30 and 0.0.0.2
- /13 and 0.7.255.255
- /23 and 0.0.2.255
- /18 and 0.0.64.255
Explanation: To calculate wildcard masks, follow these steps:
1. Change the subnet mask into dotted-decimal format.
2 Subtract the resulting mask from 255.255.255.255.
3. The result is the wildcard mask.
Example: A mask of /18 equals 255.255.192.0. A mask of 255.255.192.0 subtracted from
255.255.255.255 yields a wildcard mask of 0.0.63.255.
4. Which statement is correct about multiarea OSPF?
- OSPF can consolidate a fragmented OSPF area into one large area.
- All routers are in one area called the backbone area (area 0).
- Arranging routers into areas partitions a large autonomous system to lighten the load on routers.
- OSPF multiarea increases the frequency of SPF calculation.
Explanation: A company with one large autonomous system (AS) can be divided into smaller areas. When this occurs and the OSPF routing protocol is implemented, the design is called multi-area OSPF. Multi-area OSPF decreases the frequency of the SPF calculation, thus lightening the load on the router. In a single area OSPF design, all the routers are located in area 0 or the backbone area.
5. A network technician issues the following commands when configuring a router:
R1(config)# router ospf 11 R1(config-router)# network 10.10.10.0 0.0.0.255 area 0
What does the number 11 represent?
- The autonomous system number to which R1 belongs
- The area number where R1 is located
- The cost of the link to R1
- The OSPF process ID on R1
- The administrative distance that is manually assigned to R1
Explanation: There is no autonomous system number to configure on OSPF. The area number is located at the end of the network statement. The cost of a link can be modified in the interface configuration mode. The process ID is local to the router.
6. The OSPF Hello timer has been set to 15 seconds on a router in a point-to-point network. By default, what is the dead interval on this router?
- 15 seconds
- 30 seconds
- 45 seconds
- 60 seconds
Explanation: By default, the dead interval is calculated as 4 times the Hello interval.
7. A network administrator configures a loopback interface as the OSPF router ID with the IP address of 192.168.1.1/30. What could be the consequence of using this 30-bit mask for the loopback interface?
- Older routers do not recognize the router-id command.
- The interface is not enabled for OSPF.
- OSPF routers must also be configured with a router priority value.
- This loopback interface may be advertised as a reachable network.
Explanation: A loopback interface used as an OSPF router ID normally uses a 32-bit mask that creates a host route. This route is not advertised as a route to other routers unless a network statement including this interface is added.
8. What are three entries that are displayed by the show ip ospf neighbor command? (Choose three.)
- The route metric and neighbor next-hop address
- The router ID of the neighboring routers
- The OSPF state of each interface
- The OSPF process ID used to establish the adjacency
- The OSPF area number shared by the neighbor routers
- The IP address of the neighbor router interface to which this router is directly connected
Explanation: The route metric and neighbor next-hop address are routing table entries that are displayed by the show ip route command. The show ip ospf neighbor command does not display the OSPF process ID or the OSPF area number.
9. Which statement describes a difference or similarity between OSPFv2 and OSPFv3?
- OSPFv2 requires the DR/BDR election to occur on multiaccess networks only, whereas OSPv3 requires DR/BDR elections for all network types.
- Both OSPFv2 and OSPFv3 use the router configuration network command to advertise networks.
- Both OSPFv2 and OSPFv3 use multicast destination addresses for link-state packets.
- OSPFv2 uses a 32-bit router ID and OSPFv3 uses a 128 bit router ID.
Explanation: OSPFv2 uses the router configuration network command to advertise networks. OSPFv3 uses the ipv6 ospf process-id area area-id interface configuration command. Both OSPFv2 and OSPFv3 use a 32 bit router ID. Both OSPFv2 and OSPFv3 require the DR/BDR election to occur on multiaccess networks. Neither protocol requires a DR/BDR election for point-to-point networks.
10. Refer to the exhibit. With the default metric settings, the OSPF cost for R1 to reach the network 172.16.1.0 is
- 64
- 1563
- 65
- 1564
- 10
- 1
Explanation: The OSPF cost metric is the accumulated value from R1 to network 172.16.1.0. The cost for the first link is 100,000,000/64,000 (1562). The cost for the second link is 100,000,000/100,000,000 (1). The OSPF cost metric from R1 to 172.16.1.0 is therefore 1563.