Ch. 21 OSPF Network Types and Neighbors
An engineer connects routers R11 and R12 to the same Ethernet LAN and configures them to use OSPFv2. Which answers describe a combination of settings that would prevent the two routers from becoming OSPF neighbors? (Choose two answers.) a. R11's interface uses area 11 while R12's interface uses area 12. b. R11's OSPF process uses process ID 11 while R12 uses process ID 12. c. R11's interface uses OSPF priority 11 while R12's uses OSPF priority 12. d. R11's interface uses an OSPF Hello timer value of 11 while R12's uses 12.
A and D. As worded, the correct answers list a scenario that would prevent the neighbor relationship. One correct answer mentions the use of two different OSPF areas on the potential OSPF neighbors; to become neighbors, the two routers must use the same area number. The other correct answer mentions the use of two different Hello timers, a mismatch that causes two routers to reject each other and to not become neighbors. The two incorrect answers list scenarios that do not cause issues, making them incorrect answers. One mentions mismatched OSPF process IDs; OSPF process IDs do not need to match for two routers to become neighbors. The other incorrect answer (that is, a scenario that does not cause a problem) mentions the use of two different priority values. The priority values give OSPF a means to prefer one router over the other when electing a DR/BDR, so the setting is intended to be set to different values on different routers and does not cause a problem.
Routers R1 and R2, with router IDs 1.1.1.1 and 2.2.2.2, connect over an Ethernet WAN link. The configuration uses all defaults, except giving R1 an interface priority of 11 and changing both routers to use OSPF network type point-to-point. If the WAN link initializes for both routers at the same time, which of the following answers are true? (Choose two answers.) a. Router R1 will become the DR. b. Router R1 will dynamically discover the existence of router R2. c. Router R2 will be neither the DR nor the BDR. d. Router R2's show ip ospf neighbor command will list R1 with a state of "FULL/DR."
B and C. First, the OSPF point-to-point network type causes the two routers to dynamically discover neighbors, making one answer correct. Next, IOS assigns a default OSPF interface priority of 1, so R1's configured priority of 11 would be better in a DR/BDR election. However, the point-to-point network type causes the router to not use a DR/BDR on the interface. As a result, the answer about R1 becoming the DR is incorrect (because no DR exists at all), and the answer listing a state of "FULL/DR" is incorrect for the same reason. However, the answer that claims that R2 will be neither DR nor BDR is true because no DR or BDR is elected.
Routers R1 and R2, with router IDs 1.1.1.1 and 2.2.2.2, connect over an Ethernet WAN link. If using all default OSPF settings, if the WAN link initializes for both routers at the same time, which of the following answers are true? (Choose two answers.) a. Router R1 will become the DR. b. Router R1 will dynamically discover the existence of router R2. c. Router R2 will be neither the DR nor the BDR. d. Router R1's show ip ospf neighbor command will list R2 with a state of "FULL/DR."
B and D. By default, IOS assigns Ethernet interfaces an OSPF network type of broadcast, with an OSPF interface priority of 1. As a result, both routers attempt to discover the other routers on the link (which identifies one correct answer). The broadcast network type means that the routers also attempt to elect a DR and BDR. With a tie-in priority, the routers choose the DR based on the highest router ID (RID) values, meaning that R2 will become the DR and R1 will become the BDR. These facts combine to show why the two incorrect answers are incorrect. The other correct answer is correct because the show ip ospf neighbor command lists the local router's neighbor relationship state (FULL) and the role filled by that neighbor (DR), which would be the output shown on R1 when R2 is acting as DR.
An engineer connects routers R13 and R14 to the same Ethernet LAN and configures them to use OSPFv2. Which answers describe a combination of settings that would prevent the two routers from becoming OSPF neighbors? a. Both routers' interface IP addresses reside in the same subnet. b. Both routers' OSPF process uses process ID 13. c. Both routers' OSPF process uses router ID 13.13.13.13. d. Both routers' interfaces use an OSPF Dead interval of 40.
C. As worded, the correct answers should be a scenario that would prevent the neighbor relationship. The answers all list values that are identical or similar on the two routers. Of those, the use of an identical OSPF router ID (RID) on the two routers prevents them from becoming neighbors, making that one answer correct. Of the incorrect answers, both routers must have the same Dead interval, so both using a Dead interval of 40 causes no issues. The two routers can use any OSPF process ID (the same or different value, it does not matter), making that answer incorrect. Finally, the two routers' IP addresses must be in the same subnet, so again that scenario does not prevent R13 and R14 from becoming neighbors.
Router R15 has been a working part of a network that uses OSPFv2. An engineer then issues the shutdown command in OSPF configuration mode on R15. Which of the following occurs? a. R15 empties its IP routing table of all OSPF routes but keeps its LSDB intact. b. R15 empties its LSDB but keeps OSPF neighbor relationships active. c. R15 keeps OSPF neighbors open but does not accept new OSPF neighbors. d. R15 keeps all OSPF configuration but ceases all OSPF activities (routes, LSDB, neighbors).
D. The OSPF shutdown command tells the OSPF process to stop operating. That process includes removing any OSPF-learned routes from the IP routing table, clearing the router's LSDB, and closing existing OSPF neighbor relationships. In effect, it causes OSPF to stop working on the router, but it does retain the configuration so that a no shutdown command will cause the router to start using OSPF again with no changes to the configuration.
Per the command output, with how many routers is router R9 full adjacent over its Gi0/0 interface? R9# show ip ospf interface brief Interface PID Area IP Address/Mask Cost State Nbrs F/C Gi0/0 1 0 10.1.1.1/24 1 DROTH 2/5 a. 7 b. 0 c. 5 d. 2
D. The show ip ospf interface brief command lists a pair of counters under the heading "Nbrs F/C" on the far right of the output. The first of the two numbers represents the number of fully adjacent neighbors (2 in this case), and the second number represents the total number of neighbors.
