Chapter 6: Quiz – OSPF (Answers) CCNPv8 ENARSI

1. Refer to the exhibit. Which conclusion can be drawn from this OSPF multiaccess network?​

  • When a DR is elected all other non-DR routers become DROTHER.
  • All DROTHER routers will send LSAs to the DR and BDR to multicast
  • If the DR stops producing Hello packets, a BDR will be elected, and then it promotes itself to assume the role of DR.
  • With an election of the DR, the number of adjacencies is reduced from 6 to 3.

Explanation: On OSPF multiaccess networks, a DR is elected to be the collection and distribution point for LSAs sent and received. A BDR is also elected in case the DR fails. All other non-DR or BDR routers become DROTHER. Instead of flooding LSAs to all routers in the network, DROTHERs only send their LSAs to the DR and BDR using the multicast address If there is no DR/BDR election, the number of required adjacencies is n(n-1)/2 => 4(4-1)/2 = 6. With the election, this number is reduced to 3.​

2. When OSPFv2 neighbors are establishing adjacencies, in which state do they elect a DR and BDR router?

  • Exchange state
  • Init state
  • 2-Way state
  • Loading state

Explanation: Down state: No hello packets are received.
Attempt: No recent information received by NBMA router, but still trying to communicate.
Init state: Hello packets received, but bidirectional communication not established.
2-way state: Bidirectional communication is established and if needed, a DR/BDR election occurs.
ExStart state: Negotiate master/slave role for LSDB synchronization.
Exchange state: Exchange of DBD packets occurs.
Loading state: Request for additional information is sent.
Full state: Routers converged and neighbors are fully adjacent.

3. Which OSPF router type is used in mutliarea OSPF to connect each area to the backbone?

  • DR
  • ABR
  • BDR
  • ASBR

Explanation: There are four OSPF router types:

  1. Internal routers, which have all interfaces in a single area
  2. Backbone routers, which have all interfaces in Area 0
  3. Area Border Routers (ABRs), which have interfaces in multiple areas
  4. Autonomous System Boundary Routers (ASBRs), which have an interface in the OSPF network and an interface in an external autonomous system

4. Which type of routing table entry would indicate that an external route was redistributed into the multiarea OSPF process?

  • O
  • O IA
  • O E2
  • S
  • C

Explanation: An O routing table entry indicates a route, from within the same area, that was learned through OSPF updates. An O IA route is a route from another OSPF area. An O E2 or O E1 route is an external route, such as one redistributed from RIP, EIGRP, or a static route, that was redistributed into the OSPF process. An S entry is for a static route. A C entry represents a directly connected network.

5. What is one reason to use the ip ospf priority command when the OSPF routing protocol is in use?

  • to activate the OSPF neighboring process
  • to influence the DR/BDR election process
  • to provide a backdoor for connectivity during the convergence process
  • to streamline and speed up the convergence process

Explanation: The OSPF priority can be set to a number between 0 and 255. The higher the number set, the more likely the router becomes the DR. A priority 0 stops a router from participating in the election process and the router does not become a DR or a BDR.

6. A network administrator is verifying a multi-area OSPF configuration by checking the routing table on a router in area 1. The administrator notices a route to a network that is connected to a router in area 2. Which code appears in front of this route in the routing table within area 1?

  • C
  • O
  • O E2
  • O IA

Explanation: In a routing table, a route with the code O IA indicates a network that is learned from another area and received by the ABR as an external LSA. The ABR has flooded the route into its area so that internal routers may add it to their databases. Label C would indicate a network that is directly connected to an interface on the router. Label O would indicate a network that is advertised by another router in the same area. Label O E2 would indicate an external network (non-OSPF network) that is advertised by an ASBR.

7. A network administrator has recently implemented OSPFv2 across the entire network topology. Which command can be implemented to prevent the forwarding of OSPF messages to Layer 2 switches and hosts on interface Fast Ethernet 0/1, while maintaining network connectivity?

  • R1(config-if)# no ipv6 ospf 10
  • R1(config-if)# no cdp enable
  • R1(config-if)# no cdp run
  • R1(config-router)# passive-interface FastEthernet 0/1

Explanation: To prevent OSPF from sending messages to a LAN interface that is activated for OSPF, in this case interface Fast Ethernet 0/1, the passive-interface FastEthernet 0/1command must be implemented in the R1(config-router)# mode.

8. Refer to the exhibit. Based on this configuration, which two OSPF router roles are assumed by R2? (Choose two.)

  • backbone router
  • internal router
  • designated router
  • area border router
  • autonomous system boundary router

Explanation: R2 is a backbone router because one of its interfaces (S0/0/1) is in area 0. It is also an area border router (ABR) because it has one interface in area 0 and another interface in area 1.

9. A network engineer is testing OSPF routing and using loopback interfaces to represent future networks. The engineer enters the following commands on a router in OSPF Area 1:

RS29(config)# interface loopback 0
RS29(config-if)# ip address
RS29(config-if)# router ospf 1
RS29(config-router)# network area 1

How will the route appear in each routing table of other area 1 routers?

  • is subnetted, 1 subnets
    L [110/65] via, 00:37:15, Serial 0/0/1
  • is subnetted, 1 subnets
    O [110/65] via, 00:37:15, Serial 0/0/1
  • is subnetted, 1 subnets
    O [110/65] via, 00:37:15, Serial 0/0/1
  • is subnetted, 1 subnets
    O IA [110/65] via, 00:37:15, Serial 0/0/1

Explanation: When a loopback interface is advertised through OSPF, it is always advertised with a /32 prefix length and shown in the routing table of other routers as an OSPF route with a /32 prefix. If the show ip ospf interface command is shown on the router with the loopback interface configured, the loopback interface shows as a loopback network type instead of a point-to-point, point-to-multipoint, broadcast, or nonbroadcast type.

10. 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

Explanation: 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.

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

  • It changes the router ID of the router to
  • 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 networks to be advertised.

Explanation: Entering the command network 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.

12. Why is MD5 authentication more secure than simple authentication for OSPF updates?

  • MD5 does not send the password to the neighbor router.
  • MD5 requires passwords that are at least 8 characters long.
  • MD5 uses both a username and a password to authenticate the neighbor.
  • MD5 employs IPsec to keep the updates from being intercepted.

Explanation: MD5 does not send the configured password across the network. MD5 generates a special hash, or signature, that is attached to the messages and sent to the neighbor. This signature is used to validate the neighbor instead of the password. MD5 does not use a username and does not enforce a minimum length on the password. While IPsec tunnels can be used to keep updates secure, they are not used specifically by MD5.

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


Explanation: 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.


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