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CCNP Route FAQ: OSPF Overview and Neighbor Relationships

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CCNP Route FAQ: OSPF Overview and Neighbor Relationships


Figure: Typical Hierarchical OSPF Design

Q1. A router has been configured with the commands router ospf 9, network 172.16.1.0 0.0.0.255 area 8, and network 172.16.0.0 0.0.255.255 area 9, in that order. No other OSPF-related commands have been configured. The answers list the IP addresses that could be assigned to this router’s Fa0/0 interface. Which answers list an IP address/prefix length that would cause the router to put Fa0/0 into area 9? (Choose two.)
a. 172.16.0.1/23
b. 172.16.1.1/26
c. 172.16.1.1/24
d. 172.16.0.255/23
e. None of the other answers is correct.

Answer: A and D. The wildcard mask is used for matching the prefix only, and not the prefix length. As such, 172.16.1.0 0.0.0.255 matches all addresses that begin with 172.16.1, and 172.16.0.0 0.0.255.255 matches all addresses that begin 172.16. Also, OSPF reviews the network command with the most specific wildcard masks (wildcard masks with the most binary 0’s) first, so an interface IP address beginning with 172.16.1 matches the command that references area 8.

Q2. Which of the following is true about an OSPF area border router (ABR)?
a. The ABR must have multiple interfaces connected to the backbone area.
b. An ABR is a router with two interfaces, each connected to a different nonbackbone area.
c. The only requirement to be considered an ABR is at least one interface connected to the backbone area.
d. An ABR must have at least one interface in the backbone area plus at least one other interface in a nonbackbone area.

Answer: D. ABRs, by definition, connect the backbone area to one or more nonbackbone areas. To perform this function, a router must have at least one interface assigned to the backbone area, and at least one interface assigned to a nonbackbone area.

Q3. Which of the following can either directly or indirectly identify all the interfaces for which both 1) OSPF has been enabled and 2) OSPF is not passive? (Choose two.)
a. show ip ospf database
b. show ip ospf interface brief
c. show ip protocols
d. show ip route ospf
e. show ip ospf neighbors

Answer: B and C. First, for the two correct answers: show ip ospf interface brief explicitly lists all OSPF-enabled interfaces that are not passive. show ip protocols lists either the details of the configured network commands, or if configured using the ip ospf area command, it lists the interfaces on which OSPF is enabled. This command also lists the passive interfaces, so armed with interface IP address information, the list of OSPF-enabled nonpassive interfaces could be derived. Of the three wrong answers, show ip ospf database does not list enough detail to show the OSPF-enabled interfaces. show ip route ospf lists only routes learned with OSPF, so if no routes use a particular OSPF-enabled interface as an outgoing interface, this command would not indirectly identify the interface. Finally, an interface may be OSPF-enabled but with no neighbors reachable on the interface, so the show ip ospf neighbor command may not identify all OSPF-enabled interfaces.

Q4. Router R1 directly connects to subnet 10.1.1.0/24 with its Fa0/0 interface. R1 can ping four other working OSPF routers in that subnet. R1 is neither the designated router (DR) nor backup DR (BDR). OSPF is working correctly on all five routers. Which of the following is true on R1? (Choose two.)
a. The show ip ospf neighbors command lists two neighbors off Fa0/0.
b. The show ip ospf neighbors command lists four neighbors off Fa0/0.
c. The show ip ospf neighbors command lists two neighbors off Fa0/0 in the FULL state.
d. The show ip ospf neighbors command lists two neighbors off Fa0/0 in the DISCO state.

Answer: B and C. On a LAN, the non-DR routers form fully adjacent neighborships with only the DR and BDR, giving R1 two neighbors in the FULL state. The other two neighbors settle into the 2WAY state.

Q5. Routers R1 and R2 are OSPF neighbors using their Fa0/0 interfaces, respectively, using default settings for all timers. An engineer adds the ip ospf hello-interval 6 command to R1’s Fa0/0 configuration. Which of the following is true regarding the results from this change? (Choose 2)
a. The show ip ospf neighbor command on R1 lists the revised Hello timer.
b. The show ip ospf interface brief command on R1 lists the revised Hello timer.
c. The R1-R2 neighborship fails due to Hello timer mismatch.
d. The show ip ospf interface command on R1 lists the revised Hello timer.

Answer: C and D. The show ip ospf interface command displays a router’s OSPF Hello Interval setting for each enabled interface. The other listed commands do not display the timer. Also, OSPF routers do need to have matching Hello timers to become neighbors, so the neighborship would fail.

Q6. Routers R1 and R2, OSPF neighbors in area 0 over their Fa0/0 interfaces (respectively), currently both successfully use OSPF MD5 authentication. The OSPF configuration includes the area 0 authentication command under the router ospf 1 command. Which of the following commands must have been configured on R1’s Fa0/0 interface? (Choose two.)
a. ip ospf authentication null
b. ip ospf authentication message-digest
c. ip ospf authentication-key whatever-it-is
d. ip ospf message-digest-key 1 md5 whatever-it-is
e. ip ospf md5 1 key whatever-it-is

Answer: B and D. The area 0 authentication command tells R1 to use simple text password authentication on all interfaces in area 0 unless overridden by an interface subcommand. So, R1 must have configured the ip ospf authentication message-digest on its Fa0/0 interface, enabling MD5 authentication instead. The other correct answer is the command that correctly configures the MD5 authentication key. Of the two incorrect answers that list an authentication key, ip ospf authentication-key defines the clear-text key, and the other is not a valid IOS command.

Q7. Which of the following settings do not prevent two potential OSPF neighbors from becoming neighbors?
a. The interface used to connect to that neighbor being passive in the OSPF process
b. Duplicate OSPF router IDs
c. Mismatched dead timers
d. IP addresses of 10.1.1.1/24 and 10.2.2.2/24
e. Mismatched OSPF process IDs

Answer: E. For OSPF, Router IDs must be unique; the interfaces must not be passive; the dead timers must match; and the primary IP addresses must be in the same subnet, with the same subnet mask. However, the process IDs, found on the router ospf process-id command, do not have to match.

Q8. A company has a Frame Relay WAN with one central-site router and 100 branch office routers. A partial mesh of PVCs exists: one PVC between the central site and each of the 100 branch routers. All routers use point-to-point subinterfaces and one subnet per PVC. Which of the following is true about OSPF in this design?
a. The central site router has 100 fully adjacent neighborships with the 100 branches.
b. The central site router has neighborships with all branch routers, but fully
adjacent neighbors with only two branches.
c. The central site router has a neighborship with the Frame Relay switch.
d. None of the other answers is correct.

Answer: A. Frame Relay is a Layer 2 service and as such does not participate in customer routing protocols. Because the design uses a separate subnet per PVC, and one point-topoint subinterface per PVC/subnet, OSPF will use a point-to-point network type. That means that the two routers on either end of a PVC will become neighbors, and become fully adjacent, meaning the central site router will have 100 fully adjacent neighborships.