This exam will cover material from Modules 1 – 2 OSPF Concepts and Configuration of the CCNA 3 Enterprise Networking, Security, and Automation v7.0 (ENSA) curriculum. This exam will be scored using the Weighted Model where each MCSA (Multiple-Choice Single-Answer) is worth two points and each MCMA (Multiple-Choice Multiple-Answer) is worth one point for each correct option. Other tasks types such as fill-in-the-blank, drag and drop (matching) and Packet Tracer items may be included in this exam. For Packet Tracer tasks, you must have the latest version of Packet Tracer installed on your machine.
This exam will cover material from Modules 1 – 2 of the CCNA3 Enterprise Networking, Security, and Automation v7.0 (ENSA) curriculum.
This exam will be scored using the Weighted Model where each MCSA (Multiple-Choice Single-Answer) is worth two points and each MCMA (Multiple-Choice Multiple-Answer) is worth one point for each correct option. If more options are selected than required, the student will receive a score of zero.
CCNA3 v7 OSPF Concepts and Configuration Exam Answers
What will an OSPF router prefer to use first as a router ID?
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.
What is a benefit of multiarea OSPF routing?
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.
An administrator is configuring single-area OSPF on a router. One of the networks that must be advertised is 220.127.116.11 255.255.255.128. What wildcard mask would the administrator use in the OSPF network statement?
Which step does an OSPF-enabled router take immediately after establishing an adjacency with another router?
Explanation: 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
What is the format of the router ID on an OSPF-enabled router?
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.
What is a function of OSPF hello packets?
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?
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
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?
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 protocols checks that OSPF is enabled and lists the networks that are advertised.
Refer to the exhibit. What is the OSPF cost to reach the router A LAN 172.16.1.0/24 from B?
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
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?
To advertise only the 172.16.1.0/16 network the wildcard mask used in the network command must match the first 16-bits exactly. To match bits exactly, a wildcard mask uses a binary zero. This means that the first 16-bits of the wildcard mask must be zero. The low order 16-bits can all be set to 1.
An administrator is configuring single-area OSPF on a router. One of the networks that must be advertised is 18.104.22.168 255.255.255.192. What wildcard mask would the administrator use in the OSPF network statement?
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?
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.
In an OSPFv2 configuration, what is the effect of entering the command network 192.168.1.1 0.0.0.0 area 0?
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.
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?
Cisco IOS automatically modifies the dead interval to four times the hello interval.
What command would be used to determine if a routing protocol-initiated relationship had been made with an adjacent router?
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.
By default, what is the OSPF cost for any link with a bandwidth of 100 Mb/s or greater?
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.
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?
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.
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?
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.
What is the default router priority value for all Cisco OSPF routers?
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.
What is the reason for a network engineer to alter the default reference bandwidth parameter when configuring OSPF?
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.
Which OSPF data structure is identical on all OSPF routers that share the same area?
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.
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?
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.
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?
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.
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?
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.
What does an OSPF area contain?
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?
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.
Refer to the exhibit. If no router ID was manually configured, what would router Branch1 use as its OSPF router ID?
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.
Which OPSF packet contains the different types of link-state advertisements?
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?
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.
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?
Which three OSPF states are involved when two routers are forming an adjacency? (Choose three.)
Please select 3 correct answers
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.
Refer to the exhibit. Which command did an administrator issue to produce this output?
Which three statements describe features of the OSPF topology table? (Choose three.)
Please select 3 correct answers
What are the two purposes of an OSPF router ID? (Choose two.)
Please select 2 correct answers
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.
What indicates to a link-state router that a neighbor is unreachable?
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.
Which step in the link-state routing process is described by a router running an algorithm to determine the best path to each destination?
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.)
Please select 3 correct answers
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.
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?
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.
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.)
Please select 2 correct answers
The hello and dead interval timers contained in a hello packet must be the same on neighboring routers in order to form an adjacency.
What is used to facilitate hierarchical routing in OSPF?
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.
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 dead timer (20 seconds) on R1 expires before the next hello packet from R2 (25 seconds).
In an OSPF network when are DR and BDR elections required?
When the routers are interconnected over a common Ethernet network, then a designated router (DR) and a backup DR (BDR) must be elected.
After modifying the router ID on an OSPF router, what is the preferred method to make the new router ID effective?
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.
ENSA | Modules 1 – 2
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