CCNA 3 v7 – ENSA v7.02 – Modules 1 – 2: OSPF Concepts and Configuration Exam Answers

CCNA 3 v7 – ENSA v7.02 – Modules 1 – 2: OSPF Concepts and Configuration Exam Answers Full 100% 2023 2024

This is NetAcad Cisco CCNA 3 v7 ENSA v7.02 Modules 1 – 4 Exam Answers 2023 2024 and Enterprise Networking, Security, and Automation ( Version 7.00) – OSPF Concepts and Configuration Exam Answers Full 100%. All answers have been verified by experts.

Cisco Netacad ENSA Version 7.00 CCNA 3 v7 Modules 1 – 2: OSPF Concepts and Configuration Exam Answers 2023 2024 – Enterprise Networking, Security, and Automation

1. What is used to facilitate hierarchical routing in OSPF?

   • autosummarization
   • the use of multiple areas
   • frequent SPF calculations
   • the election of designated routers
     

     Answers Explanation & Hints: 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.

2. What is a benefit of multiarea OSPF routing?

   • Topology changes in one area do not cause SPF recalculations in other areas.
   • Automatic route summarization occurs by default between areas.
   • Routers in all areas share the same link-state database and have a complete picture of the entire network.
   • A backbone area is not required.
     

     Answers Explanation & Hints: With multiarea OSPF, only routers within an area share the same link-state database. Changes to the network topology in one area do not impact other areas, which reduces the number of SPF algorithm calculations and the of link-state databases.

3. Which OSPF data structure is identical on all OSPF routers that share the same area?

   • adjacency database
   • link-state database
   • routing table
   • forwarding database
     

     Answers Explanation & Hints: 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.

4. Which step does an OSPF-enabled router take immediately after establishing an adjacency with another router?

   • chooses the best path
   • builds the topology table
   • executes the SPF algorithm
   • exchanges link-state advertisements
     

     Answers Explanation & Hints: The OSPF operation steps are as follows: Establish neighbor adjacencies Exchange link-state advertisements Build the topology table Execute the SPF algorithm Choose the best route

5. A network engineer has manually configured the hello interval to 15 seconds on an interface of a router that is running OSPFv2. By default, how will the dead interval on the interface be affected?

   • The dead interval will now be 15 seconds.
   • The dead interval will now be 30 seconds.
   • The dead interval will now be 60 seconds.
   • The dead interval will not change from the default value.
     

     Answers Explanation & Hints: Cisco IOS automatically modifies the dead interval to four times the hello interval.

6. Refer to the exhibit. A network administrator has configured the OSPF timers to the values that are shown in the graphic. What is the result of having those manually configured timers?

   

   • The R1 dead timer expires between hello packets from R2.
   • R1 automatically adjusts its own timers to match the R2 timers.
   • The hello timer on R2 expires every ten seconds.
   • The neighbor adjacency has formed.
     

     Answers Explanation & Hints: The dead timer (20 seconds) on R1 expires before the next hello packet from R2 (25 seconds).

7. Match each OSPF packet type to how it is used by a router. (Not all options are used.)

   

   Explanation & Hint: Based on OSPF protocol operations, here’s how each packet type is used by a router: Hello Packet: Used to establish and maintain adjacencies between OSPF routers. OSPF routers send Hello packets to discover and maintain neighbor relationships. Database Description Packet: Used to compare local topology to that sent by another router. These packets describe the contents of the router’s topological database and are exchanged when an adjacency is being initialized. Link-State Update Packet: Used to advertise new routing information. These packets are used to convey information about the state of the router’s links, and they are flooded out all OSPF interfaces when there is a change in the network topology. Link-State Request Packet: Used to query another router for additional information. If a router discovers, through a Database Description exchange, that it is missing information, it sends a Link-State Request packet to request the missing link-state advertisements. Link-State Acknowledgment Packet: This type is used to confirm receipt of an update, but it doesn’t seem to be pictured in the image you provided. These packets are sent to acknowledge the receipt of Link-State Update packets. These OSPF packet types are crucial for the protocol’s operation, enabling routers to establish neighbor relationships, exchange routing information, and ensure network convergence.

8. To establish a neighbor adjacency two OSPF routers will exchange hello packets. Which two values in the hello packets must match on both routers? (Choose two.)

   • router ID
   • hello interval
   • dead interval
   • router priority
   • list of neighbors
     

     Answers Explanation & Hints: The hello and dead interval timers contained in a hello packet must be the same on neighboring routers in order to form an adjacency.

9. What is the default router priority value for all Cisco OSPF routers?

   • 0
   • 1
   • 10
   • 255
     

     Answers Explanation & Hints: The router priority value is used in a DR/BDR election. The default priority for all OSPF routers is 1 but it can be manually altered to any value 0 to 255.

10. 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?

    • link-state update
    • link-state request
    • database description
    • link-state acknowledgment
      

      Answers Explanation & Hints: The database description (DBD) packet contains an abbreviated list of the LSDB sent by a neighboring router and is used by receiving routers to check against the local LSDB.

11. Match the OSPF state with the order in which it occurs. (Not all options are used.)

    

    Answers Explanation & Hints: The active and passive states are used by EIGRP.

12. What indicates to a link-state router that a neighbor is unreachable?

    • if the router no longer receives routing updates
    • if the router no longer receives hello packets
    • if the router receives an update with a hop count of 16
    • if the router receives an LSP with previously learned information
      

      Answers Explanation & Hints: OSPF routers send hello packets to monitor the state of a neighbor. When a router stops receiving hello packets from a neighbor, that neighbor is considered unreachable and the adjacency is broken.

13. Which three OSPF states are involved when two routers are forming an adjacency? (Choose three.)

    • Init
    • Down
    • ExStart
    • Loading
    • Two-way
    • Exchange
      

      Answers Explanation & Hints: OSPF operation progresses through 7 states for 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.

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

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

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

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

    • every 5 minutes
    • every 10 minutes
    • every 30 minutes
    • every 60 minutes
      

      Answers Explanation & Hints: 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

16. 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
      

      Answers Explanation & Hints: 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, as 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.

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

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

      Answers Explanation & Hints: 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 actual convergence process.

18. Refer to the exhibit. If no router ID was manually configured, what would router Branch1 use as its OSPF router ID?

    

    • 10.0.0.1
    • 10.1.0.1
    • 192.168.1.100
    • 209.165.201.1
      

      Answers Explanation & Hints: In OSPFv2, a Cisco router uses a three-tier method to derive its router ID. The first choice is the manually configured router ID with the router-id command. If the router ID is not manually configured, the router will choose the highest IPv4 address of the configured loopback interfaces. Finally if no loopback interfaces are configured, the router chooses the highest active IPv4 address of its physical interfaces.

19. 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
      

      Answers Explanation & Hints: 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.

20. An OSPF router has three directly connected networks; 172.16.0.0/16, 172.16.1.0/16, and 172.16.2.0/16. Which OSPF network command would advertise only the 172.16.1.0 network to neighbors?

    • router(config-router)# network 172.16.1.0 0.0.255.255 area 0
    • router(config-router)# network 172.16.0.0 0.0.15.255 area 0
    • router(config-router)# network 172.16.1.0 255.255.255.0 area 0
    • router(config-router)# network 172.16.1.0 0.0.0.0 area 0
      

      Answers Explanation & Hints: To advertise only the 172.16.1.0/24 network the wildcard mask used in the network command must match the first 24-bits exactly. To match bits exactly, a wildcard mask uses a binary zero. This means that the first 24-bits of the wildcard mask must be zero. The low order 8-bits can all be set to 1.

21. Which command will a network engineer issue to verify the configured hello and dead timer intervals on a point-to-point WAN link between two routers that are running OSPFv2?

    • show ip ospf neighbor
    • show ip ospf interface serial 0/0/0
    • show ipv6 ospf interface serial 0/0/0
    • show ip ospf interface fastethernet 0/1
      

      Answers Explanation & Hints: The show ip ospf interface serial 0/0/0 command will display the configured hello and dead timer intervals on a point-to-point serial WAN link between two OSPFv2 routers. The show ipv6 ospf interface serial 0/0/0 command will display the configured hello and dead timer intervals on a point-to-point serial link between two OSPFv3 routers. The show ip ospf interface fastethernet 0/1 command will display the configured hello and dead timer intervals on a multiaccess link between two (or more) OSPFv2 routers. The show ip ospf neighbor command will display the dead interval elapsed time since the last hello message was received, but does not show the configured value of the timer.

22. Refer to the exhibit. Which three statements describe the results of the OSPF election process of the topology that is shown in the exhibit? (Choose three.)

    

    • R2 will be elected DR.
    • R1 will be elected BDR.
    • R3 will be elected BDR.
    • The R4 router ID is 172.16.1.1.
    • The R4 FastEthernet 0/0 priority is 128.
    • The router ID on R2 is the loopback interface.
      

      Answers Explanation & Hints: R2 will be elected DR because it has the highest priority of 255, all of the others have a priority of 1. R3 will be elected BDR because it has the numerically highest router-ID of 192.168.1.4. The R4 router-ID is 172.16.1.1 because it is the IPv4 address attached to the loopback 0 interface.

23. Refer to the exhibit. Suppose that routers B, C, and D have a default priority, and router A has a priority 0. Which conclusion can be drawn from the DR/BDR election process?​

    

    • Router A will become the DR and router D will become the BDR.​
    • If the DR fails, the new DR will be router B.
    • If a new router with a higher priority is added to this network, it will become the DR.
    • If the priority of router C is changed to 255, then it will become the DR.
      

      Answers Explanation & Hints: If the priority is set to 0, the router is not capable of becoming the DR, so router A cannot be the DR. OSPF DR and BDR elections are not preemptive. If a new router with a higher priority or higher router ID is added to the network after the DR and BDR election, the newly added router does not take over the DR or the BDR role.​

24. Refer to the exhibit. If the switch reboots and all routers have to re-establish OSPF adjacencies, which routers will become the new DR and BDR?

    

    • Router R4 will become the DR and router R1 will become the BDR.
    • Router R2 will become the DR and router R3 will become the BDR.
    • Router R1 will become the DR and router R2 will become the BDR.
    • Router R4 will become the DR and router R3 will become the BDR.
      

      Answers Explanation & Hints: OSPF elections of a DR are based on the following in order of precedence:highest pritority from 1 -255 (0 = never a DR) highest router ID highest IP address of a loopback or active interface in the absence of a manually configured router ID. Loopback IP addresses take higher precedence than other interfaces. In this case routers R4 and R1 have the highest router priority. Between the two, R3 has the higher router ID. Therefore, R4 will become the DR and R1 will become the BDR.

25. By default, what is the OSPF cost for any link with a bandwidth of 100 Mb/s or greater?

    • 1
    • 100
    • 10000
    • 100000000
      

      Answers Explanation & Hints: OSPF uses the formula: Cost = 100,000,000 / bandwidth. Because OSPF will only use integers as cost, any bandwidth of 100 Mb/s or greater will all equal a cost of 1.

26. Refer to the exhibit. What is the OSPF cost to reach the router A LAN 172.16.1.0/24 from B?

    

    • 782
    • 74
    • 128
    • 65
      

      Answers Explanation & Hints: The formula used to calculate the OSPF cost is as follows: Cost = reference bandwidth / interface bandwidth The default reference bandwidth is 10^8 (100,000,000); therefore, the formula is Cost = 100,000,000 bps / interface bandwidth in bps Thus the cost to reach the A LAN 172.16.1.0/24 from B is as follows: Serial link (1544 Kbps) from B to A cost => 100,000,000 / 1,544,000 = 64 Gigabit Ethernet link on A cost => 100,000,000 / 1,000,000,000 = 1 Total cost to reach 172.16.1.0/24 = 64 + 1 = 65

27. Refer to the exhibit. On which router or routers would a default route be statically configured in a corporate environment that uses single area OSPF as the routing protocol?

    

    • ISP
    • R0-A
    • ISP and R0-A
    • R0-A, R0-B, and R0-C
    • R0-B and R0-C
    • ISP, R0-A, R0-B, and R0-C
      

      Answers Explanation & Hints: The default route is applied to the router that connects to the Internet, or R0-A. R0-A then distributes that default route using the OSPF routing protocol.

28. What command would be used to determine if a routing protocol-initiated relationship had been made with an adjacent router?

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

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

29. Refer to the exhibit. Which command did an administrator issue to produce this output?

    

    • R1# show ip ospf
    • R1# show ip ospf neighbor
    • R1# show ip ospf interface serial0/0/1
    • R1# show ip route ospf

    • Explanation & Hint: The output shown in the exhibit is from an OSPF (Open Shortest Path First) interface on a router. The information includes details about the interface’s IP address, OSPF area, network type, cost, timers, and neighbor count, among other things. This type of detailed interface information is typically displayed by the command: R1# show ip ospf interface serial0/0/1 This command shows OSPF-related information specifically for the interface Serial0/0/1 on the router.

30. Which command is used to verify that OSPF is enabled and also provides a list of the networks that are being advertised by the network?​

    • show ip protocols
    • show ip ospf interface
    • show ip interface brief
    • show ip route ospf
      

      Answers Explanation & Hints: The command show ip ospf interface verifies the active OSPF interfaces. The command show ip interface brief is used to check that the interfaces are operational. The command show ip route ospf displays the entries that are learned via OSPF in the routing table. The command show ip protocol s checks that OSPF is enabled and lists the networks that are advertised.

31. Refer to the exhibit. A network administrator has configured OSPFv2 on the two Cisco routers but PC1 is unable to connect to PC2. What is the most likely problem?

    

    • Interface S0/0 is configured as a passive-interface on router R2.
    • Interface s0/0 has not been activated for OSPFv2 on router R2.
    • Interface Fa0/0 is configured as a passive-interface on router R2.
    • Interface Fa0/0 has not been activated for OSPFv2 on router R2.
      

      Answers Explanation & Hints: If a LAN network is not advertised using OSPFv2, a remote network will not be reachable. The output displays a successful neighbor adjacency between router R1 and R2 on the interface S0/0 of both routers.

32. What is the recommended Cisco best practice for configuring an OSPF-enabled router so that each router can be easily identified when troubleshooting routing issues?

    • Use the highest IP address assigned to an active interface participating in the routing process.
    • Configure a value using the router-id command.
    • Use the highest active interface IP address that is configured on the router.
    • Use a loopback interface configured with the highest IP address on the router.
      

      Answers Explanation & Hints: A Cisco router is assigned a router ID to uniquely identify it. It can be automatically assigned and take the value of the highest configured IP address on any interface, the value of a specifically-configured loopback address, or the value assigned (which is in the exact form of an IP address) using the router-id command. Cisco recommends using the router-id command.

33. Which step in the link-state routing process is described by a router running an algorithm to determine the best path to each destination?

    • executing the SPF algorithm
    • choosing the best route
    • load balancing equal-cost paths
    • declaring a neighbor to be inaccessible

    • Explanation & Hint: The step in the link-state routing process described by a router running an algorithm to determine the best path to each destination is executing the SPF algorithm. SPF stands for Shortest Path First, which is an algorithm used by link-state routing protocols like OSPF (Open Shortest Path First) to calculate the shortest path to all known destinations. The SPF algorithm constructs a tree of shortest paths from the starting vertex (the router running the algorithm) to all other vertices (networks) in the graph (network).

34. Which step in the link-state routing process is described by a router inserting best paths into the routing table?

    • choosing the best route
    • executing the SPF algorithm
    • load balancing equal-cost paths
    • declaring a neighbor to be inaccessible

    • Explanation & Hint: Choosing the Best Route: This is the correct step described by a router inserting the best paths into the routing table. After the router has computed the shortest paths to all nodes in the network using the Shortest Path First (SPF) algorithm, it needs to select the most efficient routes. The chosen routes are then inserted into the routing table, which is used to direct traffic to various network destinations. Executing the SPF Algorithm: This step is incorrect in the context of inserting paths into the routing table. The execution of the SPF algorithm is a preceding step. It involves calculating the shortest paths from the router to all other nodes in the network. This step is crucial for determining potential routes, but it’s distinct from the actual selection and insertion of these routes into the routing table. Load Balancing Equal-Cost Paths: This option is incorrect for the specific action of inserting paths into the routing table. Load balancing involves distributing traffic across multiple paths that have the same cost. While it’s an important aspect of optimizing network performance, it’s a separate process from the insertion of routes into the routing table. Load balancing decisions may influence which paths are chosen, but the act of balancing itself occurs after routes have been inserted into the table. Declaring a Neighbor to be Inaccessible: This choice is also incorrect for the specific action of inserting paths into the routing table. Declaring a neighbor as inaccessible is part of the network monitoring and update process. It involves updating the router’s knowledge of the network, particularly when a previously reachable node becomes unreachable. While important for maintaining accurate routing information, this process does not directly involve inserting paths into the routing table. In summary, the action of inserting the best paths into a router’s routing table is specifically aligned with the step of choosing the best route after computing paths with the SPF algorithm. The other options, while integral to the link-state routing protocol, pertain to different aspects of network routing and management.

35. Which step in the link-state routing process is described by a router sending Hello packets out all of the OSPF-enabled interfaces?

    • establishing neighbor adjacencies
    • exchanging link-state advertisements
    • electing the designated router
    • injecting the default route

    • Explanation & Hint: The step in the link-state routing process described by a router sending Hello packets out all of the OSPF-enabled interfaces is “establishing neighbor adjacencies.” Let’s explain this and why the other options are not correct: Establishing Neighbor Adjacencies: This is the correct answer. In the OSPF (Open Shortest Path First) protocol, routers send Hello packets to discover and maintain neighbor relationships. When a router sends Hello packets out of its OSPF-enabled interfaces, it is attempting to discover other OSPF routers on those networks and establish adjacency with them. This adjacency is necessary for the routers to exchange routing information. Exchanging Link-State Advertisements (LSAs): This is an incorrect choice for this specific action. Exchanging LSAs is a later step in OSPF, where routers share information about the state of their links (like network types, metrics, and neighbors). This step occurs after neighbor adjacencies have been established through the exchange of Hello packets. Electing the Designated Router (DR): This is also an incorrect answer for the action described. The election of a Designated Router is a process that happens among OSPF routers on multi-access networks, like Ethernet. The DR helps reduce network traffic by serving as the point of contact for routers outside the network and for LSAs. While the Hello packets do play a role in the DR election process, the primary purpose of sending Hello packets is to establish neighbor adjacencies, not specifically to elect a DR. Injecting the Default Route: This is incorrect for this specific action. Injecting a default route into OSPF is an administrative action where a router is configured to advertise a default route to other OSPF routers. This process is independent of the sending of Hello packets, which is focused on neighbor discovery and maintaining neighbor relationships.

36. Which step in the link-state routing process is described by a router flooding link-state and cost information about each directly connected link?

    • exchanging link-state advertisements
    • building the topology table
    • injecting the default route
    • selecting the router ID

    • Explanation & Hint: The step in the link-state routing process described by a router flooding link-state and cost information about each directly connected link is “exchanging link-state advertisements.” Let’s explain this and why the other options are not correct: Exchanging Link-State Advertisements: This is the correct answer. In link-state routing protocols like OSPF (Open Shortest Path First), routers exchange link-state advertisements (LSAs) to share information about their directly connected links. LSAs contain information about the state of each link (up or down), the cost of using the link, and other data necessary for building a complete view of the network topology. Building the Topology Table: This is an incorrect choice for this specific action. Building the topology table is a step that occurs after the LSAs have been exchanged. The router uses the information from the LSAs to construct a topology table, which represents the network’s structure. While related to the exchange of LSAs, this step is about processing and organizing the received information rather than sending it. Injecting the Default Route: This is also an incorrect answer for the action described. Injecting a default route into a routing protocol like OSPF is an administrative task where a network administrator configures a router to advertise a default route to other routers in the OSPF network. This process does not involve the router autonomously flooding link-state and cost information. Selecting the Router ID: This is incorrect for this specific action. The selection of the router ID is an initial configuration step in OSPF. Each OSPF router has an ID that uniquely identifies it in the network, often chosen based on the highest IP address of the router’s interfaces or set manually. This step is separate from and precedes the process of flooding link-state and cost information.

37. Which step in the link-state routing process is described by a router flooding link-state and cost information about each directly connected link?

    • exchanging link-state advertisements
    • establishing neighbor adjacencies
    • electing the designated router
    • injecting the default route

    • Explanation & Hint: The step in the link-state routing process described by a router flooding link-state and cost information about each directly connected link is “exchanging link-state advertisements.” Here’s why this is correct and the other options are not: Exchanging Link-State Advertisements: This is the correct step. In link-state routing protocols like OSPF (Open Shortest Path First), routers use link-state advertisements (LSAs) to communicate information about their own directly connected links to other routers. This includes details about the existence of each link, its state (up or down), and the cost associated with using that link. Flooding this information ensures that all routers in the network receive an up-to-date view of the network topology. Establishing Neighbor Adjacencies: This is incorrect for the action described. Establishing neighbor adjacencies involves routers sending Hello packets to discover and communicate with other OSPF routers on directly connected networks. This is a preliminary step to exchanging link-state advertisements and is essential for establishing a reliable communication channel between neighboring routers. Electing the Designated Router: This is also incorrect for the specific action described. In OSPF, on multi-access networks (like Ethernet networks), the routers elect a Designated Router (DR) and a Backup Designated Router (BDR) to optimize the LSA exchange process. While the election process depends on the exchange of Hello packets and the establishment of adjacencies, it is a separate process from the flooding of link-state and cost information. Injecting the Default Route: This option is incorrect in this context. Injecting a default route into a routing protocol like OSPF is a configuration step where a router is set to distribute a default route to the other routers in the network. This action is more about network design and route distribution policy than about the dynamic process of exchanging link-state information.

38. Which step in the link-state routing process is described by a router building a link-state database based on received LSAs?

    • building the topology table
    • executing the SPF algorithm
    • selecting the router ID
    • declaring a neighbor to be inaccessible

    • Explanation & Hint: The step in the link-state routing process described by a router building a link-state database based on received LSAs (Link-State Advertisements) is “building the topology table.” Here’s an explanation for this and the other options: Building the Topology Table: This is the correct step. In link-state routing protocols like OSPF (Open Shortest Path First), once a router receives LSAs from its neighbors, it compiles this information into a link-state database. This database essentially forms a topology table that represents the network’s structure. The router uses this table to understand the network’s layout, including which routers are connected to which others and the cost associated with each link. Executing the SPF Algorithm: This is an incorrect choice for this specific action. Executing the SPF (Shortest Path First) algorithm is a subsequent step that occurs after the topology table has been built. Using the information in the topology table, the SPF algorithm calculates the shortest path to each node in the network for efficient routing. Selecting the Router ID: This is also incorrect for this specific action. Selecting the router ID is an initial configuration step in OSPF. The router ID is a unique identifier for each router in the OSPF network and is usually determined by the highest IP address of the router’s interfaces or can be manually set. This step occurs before the process of exchanging LSAs and building the topology table. Declaring a Neighbor to be Inaccessible: This option is incorrect in this context. Declaring a neighbor to be inaccessible is part of the network monitoring and update process. It involves recognizing and responding to changes in the network, such as when a previously reachable neighbor becomes unreachable. While this might lead to updates in the link-state database and the topology table, it is not about building the topology table based on received LSAs.

39. Which step in the link-state routing process is described by a router building a link-state database based on received LSAs?

    • building the topology table
    • executing the SPF algorithm
    • load balancing equal-cost paths
    • declaring a neighbor to be inaccessible

    • Explanation & Hint: The step in the link-state routing process described by a router building a link-state database based on received LSAs (Link-State Advertisements) is “building the topology table.” Here’s an explanation for each of the given options: Building the Topology Table: This is the correct step. In link-state routing protocols like OSPF (Open Shortest Path First), routers receive LSAs from other routers in the network. These LSAs contain information about network topology, such as the links and their states. The router uses this information to build a link-state database, which effectively represents the network’s topology. This topology table is essential for the router to understand the network layout and to make routing decisions. Executing the SPF Algorithm: This step is incorrect for the action described but is closely related. After the topology table is built, the router executes the Shortest Path First (SPF) algorithm using the information in the topology table. The SPF algorithm calculates the shortest path to all nodes in the network, which is crucial for determining the best routes for packet forwarding. Load Balancing Equal-Cost Paths: This is incorrect for the specific action of building a link-state database. Load balancing across equal-cost paths is a strategy used in routing to distribute traffic evenly across multiple paths that have the same cost. This occurs after the SPF algorithm has identified multiple equal-cost paths to a destination, which is a subsequent step following the construction of the topology table. Declaring a Neighbor to be Inaccessible: This is also incorrect for the action described. Declaring a neighbor to be inaccessible is part of network monitoring and maintenance. It involves recognizing when a previously reachable neighbor becomes unreachable, often due to network failures or changes. This may result in updates to the link-state database and, subsequently, the topology table, but it is not the process of building the database itself based on received LSAs.

40. Which step in the link-state routing process is described by a router running an algorithm to determine the best path to each destination?

    • executing the SPF algorithm
    • choosing the best route
    • declaring a neighbor to be inaccessible
    • electing the designated router

    • Explanation & Hint: The step in the link-state routing process described by a router running an algorithm to determine the best path to each destination is executing the SPF algorithm. SPF stands for Shortest Path First, which is an algorithm used by link-state routing protocols like OSPF (Open Shortest Path First) to calculate the shortest path to all known destinations. The SPF algorithm constructs a tree of shortest paths from the starting vertex (the router running the algorithm) to all other vertices (networks) in the graph (network).

41. Which step in the link-state routing process is described by a router building a link-state database based on received LSAs?

    • building the topology table
    • exchanging link-state advertisements
    • selecting the router ID
    • injecting the default route

    • Explanation & Hint: The step in the link-state routing process described by a router building a link-state database based on received LSAs (Link-State Advertisements) is “building the topology table.” Here’s why this is the correct answer and the other options are not: Building the Topology Table: This is the correct step. In link-state routing protocols such as OSPF (Open Shortest Path First), when a router receives LSAs from other routers, it compiles this information into a link-state database. This database effectively forms the topology table of the network, which includes detailed information about all the routers, their links, and the state of those links. The topology table is crucial for the router to understand the network’s layout and to make informed routing decisions. Exchanging Link-State Advertisements: This is incorrect for the specific action described. The exchange of link-state advertisements is a separate step that occurs before the building of the topology table. During this step, routers send and receive LSAs to and from their neighbors. These LSAs contain information about the state of each router’s links, which is then used to build the topology table. Selecting the Router ID: This is also incorrect for the action described. Selecting the router ID is an initial configuration step in OSPF. The router ID is a unique identifier for each router in the OSPF network, typically the highest IP address on the router’s interfaces or a manually assigned value. This step is necessary for the router to participate in the OSPF process but is unrelated to the building of the topology table. Injecting the Default Route: This is incorrect for this specific action. Injecting a default route into a routing protocol like OSPF is a configuration action where a router is set to distribute a default route to other routers in the OSPF network. This process is about network design and route distribution policy, not about the dynamic process of building a topology table based on received LSAs.

42. Which step in the link-state routing process is described by a router running an algorithm to determine the best path to each destination?

    • executing the SPF algorithm
    • building the topology table
    • declaring a neighbor to be inaccessible
    • selecting the router ID

    • Explanation & Hint: The step in the link-state routing process described by a router running an algorithm to determine the best path to each destination is executing the SPF algorithm. SPF stands for Shortest Path First, which is an algorithm used by link-state routing protocols like OSPF (Open Shortest Path First) to calculate the shortest path to all known destinations. The SPF algorithm constructs a tree of shortest paths from the starting vertex (the router running the algorithm) to all other vertices (networks) in the graph (network).

43. An administrator is configuring single-area OSPF on a router. One of the networks that must be advertised is 64.102.0.0 255.255.255.128. What wildcard mask would the administrator use in the OSPF network statement?

    • 0.0.0.127
    • 0.0.0.63
    • 0.0.0.31
    • 0.0.0.15

    • Explanation & Hint: To determine the correct wildcard mask for the OSPF network statement, you need to understand the concept of a wildcard mask in OSPF configuration. A wildcard mask is used to indicate which bits of the IP address should be considered when evaluating a network for inclusion in an OSPF area. It’s essentially the inverse of a subnet mask. In this case, the given subnet mask is 255.255.255.128. In binary, this mask is 11111111.11111111.11111111.10000000. A wildcard mask inverts this, so each ‘1’ becomes a ‘0’ and each ‘0’ becomes a ‘1’. Therefore, the wildcard mask for 255.255.255.128 would be 0.0.0.127 in decimal format, which corresponds to 00000000.00000000.00000000.01111111 in binary. The correct wildcard mask to use in the OSPF network statement for the network 64.102.0.0 with a subnet mask of 255.255.255.128 is 0.0.0.127.

44. An administrator is configuring single-area OSPF on a router. One of the networks that must be advertised is 64.102.0.0 255.255.255.128. What wildcard mask would the administrator use in the OSPF network statement?

    • 0.0.0.127
    • 0.0.0.63
    • 0.0.63.255
    • 0.0.31.255

    • Explanation & Hint: To determine the correct wildcard mask for the OSPF network statement, you need to understand the concept of a wildcard mask in OSPF configuration. A wildcard mask is used to indicate which bits of the IP address should be considered when evaluating a network for inclusion in an OSPF area. It’s essentially the inverse of a subnet mask. In this case, the given subnet mask is 255.255.255.128. In binary, this mask is 11111111.11111111.11111111.10000000. A wildcard mask inverts this, so each ‘1’ becomes a ‘0’ and each ‘0’ becomes a ‘1’. Therefore, the wildcard mask for 255.255.255.128 would be 0.0.0.127 in decimal format, which corresponds to 00000000.00000000.00000000.01111111 in binary. The correct wildcard mask to use in the OSPF network statement for the network 64.102.0.0 with a subnet mask of 255.255.255.128 is 0.0.0.127.

45. An administrator is configuring single-area OSPF on a router. One of the networks that must be advertised is 128.107.0.0 255.255.252.0. What wildcard mask would the administrator use in the OSPF network statement?

    • 0.0.3.255
    • 0.0.0.3
    • 0.0.0.7
    • 0.0.63.255

    • Explanation & Hint: To find the correct wildcard mask for the OSPF network statement, we need to calculate the inverse of the subnet mask. The given subnet mask is 255.255.252.0. In binary, this mask is 11111111.11111111.11111100.00000000. A wildcard mask is the inverse of the subnet mask, where each ‘1’ in the subnet mask becomes a ‘0’ in the wildcard mask, and each ‘0’ in the subnet mask becomes a ‘1’ in the wildcard mask. Let’s convert the given subnet mask to a wildcard mask: Subnet Mask: 255.255.252.0 (in binary: 11111111.11111111.11111100.00000000) Wildcard Mask: 0.0.3.255 (in binary: 00000000.00000000.00000011.11111111) Therefore, the correct wildcard mask to use in the OSPF network statement for the network 128.107.0.0 with a subnet mask of 255.255.252.0 is 0.0.3.255.

46. An administrator is configuring single-area OSPF on a router. One of the networks that must be advertised is 192.168.223.0 255.255.254.0. What wildcard mask would the administrator use in the OSPF network statement?

    • 0.0.1.255
    • 0.0.7.255
    • 0.0.15.255
    • 0.0.31.255

    • Explanation & Hint: To find the correct wildcard mask for the OSPF network statement, we need to calculate the inverse of the subnet mask. The given subnet mask is 255.255.254.0. In binary, this mask is 11111111.11111111.11111110.00000000. A wildcard mask is the inverse of the subnet mask, where each ‘1’ in the subnet mask becomes a ‘0’ in the wildcard mask, and each ‘0’ in the subnet mask becomes a ‘1’ in the wildcard mask. Let’s convert the given subnet mask to a wildcard mask: Subnet Mask: 255.255.254.0 (in binary: 11111111.11111111.11111110.00000000) Wildcard Mask: 0.0.1.255 (in binary: 00000000.00000000.00000001.11111111) Therefore, the correct wildcard mask to use in the OSPF network statement for the network 192.168.223.0 with a subnet mask of 255.255.254.0 is 0.0.1.255.

47. An administrator is configuring single-area OSPF on a router. One of the networks that must be advertised is 192.168.181.0 255.255.254.0. What wildcard mask would the administrator use in the OSPF network statement?

    • 0.0.1.255
    • 0.0.31.255
    • 0.0.63.255
    • 0.0.15.255

    • Explanation & Hint: To determine the correct wildcard mask for the OSPF network statement, you calculate the inverse of the given subnet mask. The subnet mask provided is 255.255.254.0. In binary, this mask looks like: 11111111.11111111.11111110.00000000. A wildcard mask inverts the subnet mask, so each ‘1’ becomes a ‘0’ and each ‘0’ becomes a ‘1’. Let’s convert the given subnet mask to a wildcard mask: Subnet Mask: 255.255.254.0 (in binary: 11111111.11111111.11111110.00000000) Wildcard Mask: 0.0.1.255 (in binary: 00000000.00000000.00000001.11111111) Thus, the correct wildcard mask to use in the OSPF network statement for the network 192.168.181.0 with a subnet mask of 255.255.254.0 is 0.0.1.255.

48. An administrator is configuring single-area OSPF on a router. One of the networks that must be advertised is 198.19.0.0 255.255.252.0. What wildcard mask would the administrator use in the OSPF network statement?

    • 0.0.3.255
    • 0.0.31.255
    • 0.0.63.255
    • 0.0.15.255

    • Explanation & Hint: To determine the correct wildcard mask for the OSPF network statement, we need to calculate the inverse of the given subnet mask. The subnet mask provided is 255.255.252.0. In binary, this mask is represented as 11111111.11111111.11111100.00000000. A wildcard mask inverts the subnet mask, converting each ‘1’ to a ‘0’ and each ‘0’ to a ‘1’. Let’s convert the given subnet mask to a wildcard mask: Subnet Mask: 255.255.252.0 (in binary: 11111111.11111111.11111100.00000000) Wildcard Mask: 0.0.3.255 (in binary: 00000000.00000000.00000011.11111111) Therefore, the correct wildcard mask to use in the OSPF network statement for the network 198.19.0.0 with a subnet mask of 255.255.252.0 is 0.0.3.255.

49. An administrator is configuring single-area OSPF on a router. One of the networks that must be advertised is 198.19.0.0 255.255.252.0. What wildcard mask would the administrator use in the OSPF network statement?

    • 0.0.3.255
    • 0.0.31.255
    • 0.0.63.255
    • 0.0.0.255

    • Explanation & Hint: To find the correct wildcard mask for the OSPF network statement, we need to calculate the inverse of the subnet mask. The given subnet mask is 255.255.252.0. In binary, this mask is represented as 11111111.11111111.11111100.00000000. A wildcard mask inverts the subnet mask, converting each ‘1’ to a ‘0’ and each ‘0’ to a ‘1’. Let’s convert the given subnet mask to a wildcard mask: Subnet Mask: 255.255.252.0 (in binary: 11111111.11111111.11111100.00000000) Wildcard Mask: 0.0.3.255 (in binary: 00000000.00000000.00000011.11111111) Therefore, the correct wildcard mask to use in the OSPF network statement for the network 198.19.0.0 with a subnet mask of 255.255.252.0 is 0.0.3.255.

50. An administrator is configuring single-area OSPF on a router. One of the networks that must be advertised is 64.100.0.0 255.255.255.0. What wildcard mask would the administrator use in the OSPF network statement?

    • 0.0.0.255
    • 0.0.0.127
    • 0.0.0.63
    • 0.0.0.31

    • Explanation & Hint: To determine the correct wildcard mask for the OSPF network statement, we need to calculate the inverse of the given subnet mask. The subnet mask provided is 255.255.255.0. In binary, this mask is represented as 11111111.11111111.11111111.00000000. A wildcard mask inverts the subnet mask, converting each ‘1’ to a ‘0’ and each ‘0’ to a ‘1’. Let’s convert the given subnet mask to a wildcard mask: Subnet Mask: 255.255.255.0 (in binary: 11111111.11111111.11111111.00000000) Wildcard Mask: 0.0.0.255 (in binary: 00000000.00000000.00000000.11111111) Therefore, the correct wildcard mask to use in the OSPF network statement for the network 64.100.0.0 with a subnet mask of 255.255.255.0 is 0.0.0.255.

51. An administrator is configuring single-area OSPF on a router. One of the networks that must be advertised is 128.107.0.0 255.255.255.192. What wildcard mask would the administrator use in the OSPF network statement?

    • 0.0.0.63
    • 0.0.0.3
    • 0.0.0.7
    • 0.0.63.255

    • Explanation & Hint: To determine the correct wildcard mask for the OSPF network statement, we need to calculate the inverse of the given subnet mask. The subnet mask provided is 255.255.255.192. In binary, this mask is represented as 11111111.11111111.11111111.11000000. A wildcard mask inverts the subnet mask, converting each ‘1’ to a ‘0’ and each ‘0’ to a ‘1’. Let’s convert the given subnet mask to a wildcard mask: Subnet Mask: 255.255.255.192 (in binary: 11111111.11111111.11111111.11000000) Wildcard Mask: 0.0.0.63 (in binary: 00000000.00000000.00000000.00111111) Therefore, the correct wildcard mask to use in the OSPF network statement for the network 128.107.0.0 with a subnet mask of 255.255.255.192 is 0.0.0.63.

52. An administrator is configuring single-area OSPF on a router. One of the networks that must be advertised is 128.107.0.0 255.255.255.192. What wildcard mask would the administrator use in the OSPF network statement?

    • 0.0.0.63
    • 0.0.0.15
    • 0.0.0.31
    • 0.0.0.7

    • Explanation & Hint: To determine the correct wildcard mask for the OSPF network statement, we need to calculate the inverse of the given subnet mask. The subnet mask provided is 255.255.255.192. In binary, this mask is represented as 11111111.11111111.11111111.11000000. A wildcard mask inverts the subnet mask, converting each ‘1’ to a ‘0’ and each ‘0’ to a ‘1’. Let’s convert the given subnet mask to a wildcard mask: Subnet Mask: 255.255.255.192 (in binary: 11111111.11111111.11111111.11000000) Wildcard Mask: 0.0.0.63 (in binary: 00000000.00000000.00000000.00111111) Therefore, the correct wildcard mask to use in the OSPF network statement for the network 128.107.0.0 with a subnet mask of 255.255.255.192 is 0.0.0.63.

53. What is the format of the router ID on an OSPF-enabled router?

    • a character string with no space
    • a 32-bit number formatted like an IPv4 address
    • a unique phrase with no more than 16 characters
    • a unique router host name that is configured on the router
    • an 8-bit number with a decimal value between 0 and 255
      

      Answers Explanation & Hints: A router ID is a 32-bit number formatted like an IPv4 address and assigned in order to uniquely identify a router among OSPF peers.

54. Match the description to the term. (Not all options are used.)

    

    Answers Explanation & Hints: DUAL is the algorithm used by EIGRP. In multiarea OSPF, OSPF is implemented using multiple areas, and all of them must be connected to the backbone area.

55. After modifying the router ID on an OSPF router, what is the preferred method to make the new router ID effective?

    • HQ# copy running-config startup-config
    • HQ# clear ip ospf process
    • HQ# clear ip route *
    • HQ# resume
      

      Answers Explanation & Hints: To modify a router-id on an OSPF-enabled router, it is necessary to reset the OSPF routing process by entering either the clear ip ospf process command or the reload command.

56. In an OSPFv2 configuration, what is the effect of entering the command network 192.168.1.1 0.0.0.0 area 0 ?

    • It changes the router ID of the router to 192.168.1.1.
    • It enables OSPF on all interfaces on the router.
    • It tells the router which interface to turn on for the OSPF routing process.
    • It allows all 192.168.1.0 networks to be advertised.
      

      Answers Explanation & Hints: Entering the command network 192.168.1.1 0.0.0.0 area 0 will turn on only the interface with that IP address for OSPF routing. It does not change the router ID. Instead, OSPF will use the network that is configured on that interface.

57. What is the reason for a network engineer to alter the default reference bandwidth parameter when configuring OSPF?

    • to more accurately reflect the cost of links greater than 100 Mb/s
    • to increase the speed of the link
    • to force that specific link to be used in the destination route
    • to enable the link for OSPF routing
      

      Answers Explanation & Hints: By default, Fast Ethernet, Gigabit, and 10 Gigabit Ethernet interfaces all have a cost of 1. Altering the default reference bandwidth alters the cost calculation, allowing each speed to be more accurately reflected in the cost.

58. Open the PT Activity. Perform the tasks in the activity instructions and then answer the question.

    Which task has to be performed on Router 1 for it to establish an OSPF adjacency with Router 2?

    • Issue the clear ip ospf process command.
    • Remove the passive interface command from interface FastEthernet 0/0.
    • Add the network 10.0.1.0 0.0.0.255 area 0 command to the OSPF process.
    • Change the subnet mask of interface FastEthernet 0/0 to 255.255.255.0.
      

      Answers Explanation & Hints: Each interface on the link connecting the OSPF routers must be in the same subnet for an adjacency to be established. The IP address subnet mask on FastEthernet interface 0/0 must be changed to 255.255.255.0. The FastEthernet interface 0/0 is not passive. The 10.0.1.0/24 network is only connected to Router2 so should not be advertised by Router1. The clear ip ospf process command will start the OPSF process on Router1 but will not cause an adjacency to be established if the subnet mask mismatch on the connecting interfaces still exists.