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 PracticeAn 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 network engineer is configuring OSPF on a Cisco Nexus switch for a data center network. The requirement is to ensure that the switch does not become the Designated Router (DR) on a multi-access segment. Which OSPF configuration achieves this?
Explanation: Setting the OSPF priority to 0 on the interface prevents the switch from participating in the DR/BDR election process, ensuring it will never become the Designated Router (DR) or Backup Designated Router (BDR) on a multi-access segment. This is the standard method per RFC 2328 to make a router ineligible for DR/BDR status while still allowing it to form full adjacencies with the DR and BDR.
A network engineer is configuring OSPF on a Nexus switch. To ensure fast convergence in case of a link failure, which OSPF feature should be enabled?
Explanation: OSPF Fast Hello (B) is the correct feature to enable for fast convergence on a Nexus switch because it reduces the dead interval to less than one second by sending Hello packets at sub-second intervals (e.g., every 333 ms for a 1-second dead interval). This allows OSPF neighbors to detect a link failure much faster than the default 40-second dead interval on broadcast networks, triggering quicker SPF recalculation and route convergence.
A network engineer is troubleshooting an OSPF adjacency issue between two Nexus switches. The neighbors are stuck in the EXSTART state. What is the most likely cause?
Explanation: When OSPF neighbors are stuck in the EXSTART state, the most common cause is an MTU mismatch. In EXSTART, the routers exchange Database Description (DBD) packets, which must be no larger than the interface MTU. If one interface has a smaller MTU, the DBD packet is dropped, preventing the adjacency from progressing to the Exchange state. Cisco Nexus switches default to an MTU of 1500 bytes, but any mismatch will cause this issue.
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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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