1.4.2 Module Quiz - Single-Area OSPFv2 Concepts (Answers)

1.4.2 Module Quiz – Single-Area OSPFv2 Concepts Answers

1. What is a function of OSPF hello packets?

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

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

2. Which OPSF packet contains the different types of link-state advertisements?

LSU
LSR
hello
DBD
LSAck

Explanation: Link-state update (LSU) packets contain different types of link-state advertisements (LSAs). The LSUs are used to reply to link-state requests (LSRs) and to announce new information.

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

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

4. What does an OSPF area contain?

routers that share the same process ID
routers that share the same router ID
routers that have the same link-state information in their LSDBs
routers whose SPF trees are identical

Explanation: An OSPF area contains one set of link-state information, although each router within the area will process that information individually to form its own SPF tree. OSPF process IDs are locally significant and are created by the administrator. Router IDs uniquely identify each router.

5. 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 4 times the hello interval will start.
SPF will run and determine which neighbor router is “down”.
OSPF will run a new DR/BDR election.
OSPF will remove that neighbor from the router link-state database.

Explanation: On Cisco routers the default dead interval is 4 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; this depends on the type of network and on whether or not the router no longer up was a DR or BDR.

6. What is the order of packet types used by an OSPF router to establish convergence?

LSAck, Hello, DBD, LSU, LSR
LSU, LSAck, Hello, DBD, LSR
Hello, DBD, LSR, LSU, LSAck
Hello, LSAck, LSU, LSR, DBD

Explanation: An OSPF router progresses in this order to convergence, using the following packets:

Hello packet, used for OSPF election and establishing neighbor adjacencies
DBD packet, used to synchronize databases with neighbors
LSR packet, used to request more information in synchronizing databases
LSU packet, used to send link-state updates to neighbors
LSAck packet, used to acknowledge receipt of an LSU

7. What is a feature of the OSPF routing protocol?

OSPF authentication is configured in the same way on IPv4 and IPv6 networks.
Routers can be grouped into autonomous systems to support a hierarchical system.
It scales well in both small and large networks.
The SPF algorithm chooses the best path based on 30-second updates.

Explanation: OSPF uses the SPF algorithm to choose the best path. Routing changes trigger routing updates (no 30-second updates). In IPv4, OSPF uses MD5 authentication between two neighboring OSPF routers. In IPv6, OSPFv3 does not include any authentication capabilities of its own. Instead it relies entirely on IPsec to secure communications between neighbors. Routers can be grouped into areas to support a hierarchical system.

8. What is used to create the OSPF neighbor table?

link-state database
forwarding database
routing table
adjacency database

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.

9. What is identical on all OSPF routers within a single area?

neighbor table
routing table
link-state database
static routes

Explanation: When the LSP flooding process completes, all OSPF routers will learn the same link-state information in the routing area. This information is used to build a complete link-state database, which will be the same on all OSPF routers within that specific area.

10. What function is performed by the OSPF designated router?

summarizing routes between areas
maintaining the link-state database
redistribution of external routes into OSPF
dissemination of LSAs

Explanation: OSPF designated routers are elected on multiaccess networks to disseminate LSAs to other OSPF routers. By having a single router disseminate LSAs, the exchanging of LSAs is more efficient.

11. What are two reasons for creating an OSPF network with multiple areas? (Choose two.)

to simplify configuration
to ensure that an area is used to connect the network to the Internet
to reduce use of memory and processor resources
to reduce SPF calculations
to provide areas in the network for routers that are not running OSPF

Explanation: If a router is not running OSPF, it is not configurable with an OSPF area. OSPF areas have no direct relationship with the Internet. Routers that run OSPF can connect to the Internet, but multiple OSPF areas are not required for this purpose. OSPF areas help to decrease the demand for router memory and processing power by limiting OSPF protocol traffic, keeping link-state databases small, and requiring fewer SPF recalculations. Multiarea OSPF requires additional steps to configure and therefore does not simplify the configuration process.

12. At which OSPF state are neighbor routers converged and able to exchange routing updates?

Exchange
Two-Way
ExStart
Full

Explanation: OSPF neighbors that reach the Full state are converged and can exchange routing information.

13. 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?

30 seconds
60 seconds
45 seconds
15 seconds

Explanation: By default, the dead interval is calculated as 4 times the hello interval.

14. What happens immediately after two OSPF routers have exchanged hello packets and have formed a neighbor adjacency?

They exchange abbreviated lists of their LSDBs.
They negotiate the election process if they are on a multiaccess network.
They exchange DBD packets in order to advertise parameters such as hello and dead intervals.
They request more information about their databases.

Explanation: During the exchange of hello packets, OSPF routers negotiate the election process and set the OSPF parameters. DBD packets are exchanged after that step has been completed. DBD packets contain abbreviated lists of link-state information. After that information has been exchanged, OSPF routers exchange Type 3 LSR packets to request further information.

15. Which statement is correct about multiarea OSPF?

OSPF can consolidate a fragmented OSPF area into one large area.
Arranging routers into areas partitions a large autonomous system in order to lighten the load on routers.
All routers are in one area called the backbone area (area 0).
OSPF multiarea increases the frequency of SPF calculation.

Explanation: A company with one large autonomous system or 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.

16. Which OSPF data structure 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. Regardless of which OSPF area a router resides in, the adjacency database, routing table, and forwarding database are unique for each router. The link-state database lists information about all other routers within an area and is identical across all OSPF routers participating in that area.

17. What are the 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: The 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.

18. Which statement describes a multiarea OSPF network?

It consists of multiple network areas that are daisy-chained together.
It has a core backbone area with other areas connected to the backbone area.
It has multiple routers that run multiple routing protocols simultaneously, and each protocol consists of an area.
It requires a three-layer hierarchical network design approach.

Explanation: A multiarea OSPF network requires hierarchical network design (with two levels). The main area is called the backbone area, and all other areas must connect to the main area.

19. What are the advantages of using multiarea OSPF? (Choose two.)

A backbone area is not required.
It allows OSPFv2 and OSPFv3 to run together.
It enables multiple routing protocols to run in a large network.
It improves routing efficiency by reducing the routing table and linkstate update overhead.
It improves routing performance by dividing the neighbor table into separate smaller ones.
Topology changes in one area do not cause SPF recalculations in other areas.

Explanation: A multiarea OSPF network improves routing performance and efficiency in a large network. As the network is divided into smaller areas, each router maintains a smaller routing table because routes between areas can be summarized. Also, fewer updated routes means fewer LSAs are exchanged, thus reducing the need for CPU resources. Running multiple routing protocols simultaneously and implementing both IPv4 and IPv6 are not primary considerations for a multiarea OSPF network. With multiarea OSPF, only routers within an area share the same linkstate database. Changes to the network topology in one area do not impact other areas, which reduces the number of SPF algorithm calculations and the number of link-state databases.

20. Which command can be used to verify the contents of the LSDB in an OSPF area?

show ip ospf database
show ip ospf interface
show ip ospf neighbor
show ip route ospf

Explanation: The show ip ospf database command is used to verify the contents of the LSDB. The show ip ospf interface command is used to verify the configuration information of OSPF-enabled interfaces. The show ip ospf neighbor command is used to gather information regarding OSPF neighbor routers. The show ip route ospf command displays OSPF-related information in the routing table.

21. Which of the following facilitates hierarchical routing in OSPF?

1. Auto-summarization
2. Frequent SPF calculations
3. The election of designated routers
4. The use of multiple areas

Explanation: OSPF supports the concept of areas to prevent larger routing tables, excessive SPF calculations, and large LSDBs. Only routers within an area share link-state information. This allows OSPF to scale in a hierarchical fashion with all areas that connect to a backbone area.

22. Which step does an OSPF-enabled router take immediately after the OSPF router builds the topology table?

Chooses the best path
Establishes an adjacency with another router
Exchanges link-state advertisements
Executes the SPF algorithm

Explanation: The OSPF operation steps are establish neighbor adjacencies, exchange link-state advertisements, build the topology table, execute the SPF algorithm, and choose the best route.

23. Which type of OSPFv2 packet contains an abbreviated list of the LSDB of a sending router and is used by receiving routers to check against the local LSDB?

Database Description
Link-State Acknowledgment
Link-State Request
Link-State Update

Explanation: The Type 2 Database Description (DBD) packet contains an abbreviated list of the LSDB of the sending router and is used by receiving routers to check against the local LSDB. The LSDB must be identical on all link-state routers within an area to construct an accurate SPF tree.

24. Which OSPF states are performed prior to two routers forming a neighbor adjacency? (Choose three.)

Down
Exchange
ExStart
Init
Loading
Two-Way

Explanation: OSPF operation progresses through seven states in establishing neighboring router adjacency, exchanging routing information, calculating the best routes, and reaching convergence. The Down, Init, and Two-Way states are involved in the phase of neighboring router adjacency establishment.

25. In an OSPF network, when are DR and BDR elections required?

When all the routers in an OSPF area cannot form adjacencies
When the routers are interconnected over a common Ethernet network
When the two adjacent neighbors are in two different networks
When the two adjacent neighbors are interconnected over a point-topoint link

Explanation: When the routers are interconnected over a common Ethernet network, a designated router (DR) and a backup DR (BDR) must be elected.

26. When an OSPF network is converged and no network topology change has been detected by a router, how often are LSU packets sent to neighboring routers?

Every 10 seconds
Every 40 seconds
Every 15 minutes
Every 30 minutes

Explanation: After all LSRs have been satisfied for a given router, the adjacent routers are considered synchronized and in a Full state. Updates (LSUs) are sent to neighbors only under the following conditions:

When a network topology change is detected (incremental updates)
Every 30 minutes