JN0-106 Networking Fundamentals Practice Question

Answer analysis

Option-by-option breakdown

For each option: why learners choose it and why it is or isn't the right answer here.

• ✗

  The physical link is flapping

  Why it's wrong here

  Link flapping would cause the physical interface to transition between up and down, generating syslog messages such as "Physical link down" and "Physical link up," and the OSPF adjacency would drop and immediately restart based on interface state changes. The exhibit shows the neighbor adjacency repeatedly cycling through non-full states without any interface state change, which is inconsistent with a flapping physical link. Additionally, link flapping affects all routed protocols, whereas an MTU mismatch selectively impacts adjacency formation while keeping the interface operational.

• ✓

  MTU mismatch between the two routers

  Why this is correct

  MTU mismatch is the correct answer because OSPF uses Database Description (DBD) packets to exchange link-state summaries, and these packets can exceed the interface MTU. When a router receives a DBD packet larger than its configured MTU, it silently drops the packet, causing the neighbor to remain stuck or repeatedly restart the ExStart/Exchange process. The router that is the slave in the exchange will detect the mismatch via the MTU field in the DBD packet and cycle back to ExStart, which produces the repeated adjacency behavior shown in the exhibit.

• ✗

  OSPF authentication is misconfigured

  Why it's wrong here

  Authentication misconfiguration would cause OSPF hello packets to be discarded or rejected, producing "authentication failed" log messages at each hello interval. The neighbor would remain in the Down or Attempt state and never progress to ExStart, because no hello packets would successfully pass the authentication check. In the exhibit, the adjacency is repeatedly being attempted and reaching the database exchange phase, which would be impossible if authentication were failing on every hello packet.

• ✗

  The routers are in different OSPF areas

  Why it's wrong here

  If the routers were in different OSPF areas, the area ID is carried in the OSPF header, and a mismatch causes incoming Hello packets to be silently dropped. As a result, the neighbor would never leave the Down state, and no adjacency would be attempted. Because the exhibit explicitly shows the neighbor adjacency repeatedly forming and then resetting across multiple states, an area mismatch cannot explain the behavior—it would prevent any progression beyond Down.

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