OSPF neighbour adjacencies, route advertisements, and DR/BDR elections appear consistently on the CCNA. These questions test whether you can read OSPF state from show commands and identify why two routers fail to reach FULL adjacency or why a route isn't being learned.
Start Scenario PracticeDuring a network upgrade, an engineer applies a new OSPF configuration on a Nexus 9000 spine. After the change, several leaf switches lose connectivity to each other. The engineer examines the logs and sees OSPF adjacency flapping. What is the most likely cause?
Explanation: Duplicate router IDs cause OSPF adjacency flapping because OSPF uses the Router ID (RID) to uniquely identify each router in the OSPF domain. When two routers share the same RID, they reject each other's Hello packets, leading to repeated adjacency resets. In a Nexus 9000 spine-leaf topology, this often occurs when the spine's RID is accidentally configured to match an existing leaf's RID, disrupting the entire fabric's OSPF convergence.
An engineer is designing a Layer 3 network for a data center using OSPF. The core switches are connected to aggregation switches. To optimize convergence, which OSPF network type should be used on the links between core and aggregation?
Explanation: In a data center spine-leaf architecture, the links between core (spine) and aggregation (leaf) switches are typically point-to-point Layer 3 links. Configuring OSPF network type point-to-point (option D) on these interfaces eliminates the need for DR/BDR election, reduces hello and dead timers (default 10s/40s vs 30s/120s for broadcast), and allows faster convergence because OSPF immediately forms a neighbor adjacency without waiting for election delays.
Sequence the steps for configuring OSPF on a Cisco Nexus switch in a data center fabric.
Explanation: OSPF configuration involves creating the process, setting router ID, enabling on interfaces, and verification.
A Nexus switch experiences high CPU utilization due to excessive ICMP traffic. An engineer applies a CoPP policy that includes a class matching ICMP with a drop action. After applying, legitimate OSPF hello packets are also being dropped. What is the most likely cause?
Explanation: When a CoPP policy is applied, traffic that does not match any explicit class-map falls into the default class. If the default class has a drop action, all unmatched traffic—including OSPF hello packets (which use IP protocol 89)—will be dropped. The class-map matching ICMP (typically based on protocol or DSCP) does not match OSPF, so OSPF packets are handled by the default class, causing the observed behavior.
An engineer is configuring OSPF on a Cisco Nexus switch in a data center. The network consists of multiple point-to-point links. To improve convergence, the engineer wants to reduce the dead interval to 10 seconds. Which command sets the dead interval correctly?
Explanation: The 'ip ospf dead-interval 10' command directly sets the OSPF dead interval to 10 seconds on a Cisco Nexus switch. The dead interval is the time a router waits without receiving a hello packet before declaring the neighbor down, and reducing it to 10 seconds speeds up convergence on point-to-point links.
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Practice all OSPF Troubleshooting ScenariosOSPF neighbour adjacencies, route advertisements, and DR/BDR elections appear consistently on the CCNA. These questions test whether you can read OSPF state from show commands and identify why two routers fail to reach FULL adjacency or why a route isn't being learned. These appear throughout the 350-601 and require you to apply your knowledge, not just recall facts.
Cisco doesn't publish an exact breakdown, but scenario-based questions (especially exhibit and command-output formats) make up a significant portion of the 350-601. Practicing each scenario type ensures you're ready for any format.
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