CCNA 3 v6.0 (Scaling Networks v6) Chapter 10: OSPF Tuning and Troubleshooting: Check Your Understanding Questions Answers
1. What information can be gathered from the routing table entry O*E2 0.0.0.0/0 [110/1] via 192.168.16.3, 00:20:22, Serial0/0/0?
- The edge of the OSPF area 0 is the interface that is addressed 192.168.16.3.
- The metric for this route is 110.
- The route is located two hops away.
- This route is a propagated default route.
Explanation: The metric toward this external route is 1, and 192.168.16.3 is the address of the next interface toward the destination.
2. Which command can 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 fastethernet 0/1
- show ip ospf interface serial 0/0/0
- show ipv6 ospf interface serial 0/0/0
Explanation: The show ip ospf interface serial 0/0/0 command displays 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 displays the configured hello and dead timer intervals on a pointto-point serial link between two OSPFv3 routers. The show ip ospf interface fastethernet 0/1 command displays the configured hello and dead timer intervals on a multiaccess link between two (or more) OSPFv2 routers. The show ip ospf neighbor command displays the dead interval elapsed time since the last hello message was received but does not show the configured value of the timer.
3. A network engineer suspects that OSPFv3 routers are not forming neighbor adjacencies because there are interface timer mismatches. Which two commands can be issued on the interface of each OSFPv3 router to resolve all timer mismatches? (Choose two.)
- ip ospf dead-interval 40
- ip ospf hello-interval 10
- no ipv6 ospf cost 10
- no ipv6 ospf dead-interval
- no ipv6 ospf hello-interval
- no ipv6 router ospf 10
Explanation: The no ipv6 ospf hello-interval and no ipv6 ospf dead-interval commands issued on each OSPFv3 interface reset the intervals to the respective default periods. This ensures that the timers on all routers match and, provided that other appropriate configurations are correct, the routers form adjacencies. The ip ospf hello-interval 10 and ip ospf dead-interval 40 commands are OPSFv2 commands that are used for IPv4 routing. If the ipv6 ospf hello-interval and ipv6 ospf dead-interval commands are used, the interval has to be specified in seconds. The parameter default is not valid in these commands.
4. A network engineer is troubleshooting convergence and adjacency issues in an OSPFv2 network and has noted that some expected network route entries are not displayed in the routing table. Which two commands provide additional information about the state of router adjacencies, timer intervals, and the area ID? (Choose two.)
- show ip ospf interface
- show ip ospf neighbor
- show ip protocols
- show ip route ospf
- show running-configuration
Explanation: The show ip ospf interface command displays routing table information that is already known. The show ip ospf neighbors command displays adjacency information on neighboring OSPF routers. The show running-configuration and show ip protocols commands display aspects of the OSPF configuration on the router but do not display adjacency state details or timer interval details.
5. When OSPFv2 neighbors are establishing adjacencies, in which state do they elect a DR and BDR router?
- Exchange state
- Init state
- Loading state
- Two-Way state
Explanation: The states are as follows: Down state, no Hello packets are received; Init state, Hello packets are received; Two-way state, DR and BDR election; ExStart state, negotiate master/slave and DBD packet sequence number; Exchange state, exchange of DBD packets; Loading state, additional information is sent; and Full state, routers converged.
6. A network engineer is troubleshooting OSPFv2 routing issues on two connected routers. Which two requirements to form an adjacency need to be verified? (Choose two.)
- Verify that both routers are using the same OSPFv2 process ID.
- Verify that one of the interfaces that connects the two routers is active and the other passive.
- Verify that one of the routers is the DR or BDR, and the other router is a DROTHER.
- Verify that the interfaces connecting the two routers are in the same area.
- Verify that the interfaces connecting the two routers are in the same subnet.
Explanation: The OSPFv2 process ID is local to each router and does not have to be common. Neither interface on the link connecting the two routers can be passive. Both must be participating in the OSPF area. The DR, BDR, and DROTHER status of the router has no bearing on the adjacency relationship.
7. 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 interface brief
- show ip ospf interface
- show ip protocols
- show ip route ospf
Explanation: 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.
8. During verification or troubleshooting of the OPSFv3 configuration on a router, which three parameters are displayed by the show ipv6 ospf interface command? (Choose three.)
- The global unicast IPv6 address of the interface
- The hello and dead intervals
- The metric of the route that is attached to the interface
- The number of interfaces in the area
- The OSPFv3 area that the interface is in
- The process ID that is assigned to the interface
Explanation: The number of interfaces in the area is displayed by the show ipv6 ospf command. The metric of the route that is attached to the interface is displayed by the show ipv6 route command. The interface global unicast IPv6 address is displayed in the show running-configuration command output.
9. A network engineer is troubleshooting an OSPFv2 network and discovers that two routers connected by a point-to-point WAN serial link are not establishing an adjacency. The OSPF routing process, network commands, and area ID are all confirmed as correct, and the interfaces are not passive. Testing shows that the cabling is correct, the link is up, and pings between the interfaces are successful. What is most likely the problem?
- A clock rate has not been set on the DCE interface of the serial link.
- A DR election has not taken place.
- The OSPFv2 process IDs on the routers do not match.
- The subnet masks on the two connected serial interfaces do not match.
Explanation: The establishment of an OSPF adjacency between connected routers requires all the interfaces of the link to be in the same subnet. Each interface must be configured with correct IP addresses and the same subnet mask. Pings across serial links can be successful with correct IP addresses and different subnet masks. Successful pings verify that a clock rate has been set on the DCE interface of the serial link. The OSPFv2 process IDs on the routers are local and do not need to match. A DR election does not take place across point-to-point serial links between OSPF routers.