|Exam 400-101||Question id=765||Layer 3 Technologies|
RouterA and RouterB are connected by their Serial 0/0 interfaces. You are able to successfully ping from one router to the other. However, the routers are not establishing an OSPF neighbor relationship. You issue the debug ip ospf hello command on each router and notice that RouterA is sending hello packets but not receiving them.
Which of the following statements best describes why the routers are unable to form a neighbor relationship?
RouterA should be configured as the DR|
RouterB should be configured as the DR|
The Serial 0/0 interface of RouterA is configured as a passive interface|
The Serial 0/0 interface of RouterB is configured as a passive interface|
RouterA and RouterB are configured with different OSPF process IDs|
RouterA and RouterB are configured with different hello timer or dead timer intervals|
The Serial 0/0 interface of RouterB is configured as a passive interface, so RouterA and RouterB are unable to form a neighbor relationship. A passive interface does not send or receive any routing information, regardless of the routing protocol. Additionally, an Open Shortest Path First (OSPF) passive interface does not send hello packets. OSPF uses the periodic exchange of hello packets to maintain neighbor relationships. If an OSPF router does not receive a hello packet from a neighbor after a specified amount of time, the neighbor relationship is terminated and no further routing information is exchanged. In this scenario, RouterA is sending periodic hello packets; however, RouterA is not receiving hello packets, because RouterB is no longer sending them. RouterB also ignores the incoming hello packets from RouterA.
To configure a single interface to be a passive interface, you should issue the passive-interface interface-type interface-number command from router configuration mode. Alternatively, you can issue the passive-interface default command from routerconfiguration mode to configure all interfaces to be passive? you must then issue the no passive-interface interfacetype interfacenumber command from router configuration mode to allow an interface to participate in the routing protocol and to establish neighbor relationships.
The Serial 0/0 interface of RouterA is not configured as a passive interface. The output of the debug ip ospf hello command indicates that RouterA is sending periodic hello packets. An OSPF passive interface does not send or receive routing information, including hello packets.
There is no designated router (DR) elected on an OSPF pointtopoint network segment. RouterA and RouterB are connected by their Serial 0/0 interfaces and do not attempt to elect a DR. A DR is elected by using hello packets on a multiaccess network, such as a LAN. If RouterA and RouterB were connected by an Ethernet switch, RouterA would become theDR because RouterB is not sending hello packets.
The OSPF process ID is an identifier that is locally significant to the router and is used to distinguish between multiple OSPF processes running on the router. Because the OSPF process ID is locally significant, two routers could have different process IDs and still establish a neighbor relationship. Therefore, the OSPF process ID has no effect on whether RouterA and RouterB establish a neighbor relationship.
The hello timer and dead timer intervals are used to determine how frequently a router should expect a hello packet from a neighbor. If a router does not receive a hello packet from a neighbor within the dead timer interval, the relationship with that neighbor is terminated. If a router receives hello packets with a different hello timer interval or dead timer interval, the hello packets will be ignored and a neighbor relationship will not be established. However, mismatched timer intervals do not prevent a router from sending or receiving hello packets. Because the output of the debug ip ospf hello command reveals that RouterA is not receiving hello packets from RouterB, RouterA cannot compare timer values with RouterB to determine whether a neighbor relationship can be established.